Error handling in C code

Question

What do you consider  best practice  when it comes to error handling errors in a consistent way in a C library   There are two ways I ve been thinking of   Always return error code  A typical function would look like this   MYAPI ERROR getObjectSize MYAPIHandle h  int  returnedSize     The always provide an error pointer approach   int getObjectSize MYAPIHandle h  MYAPI ERROR  returnedError     When using the first approach it s possible to write code like this where the error handling check is directly placed on the function call   int size  if getObjectSize h   amp size     MYAPI SUCCESS         Error handling     Which looks better than the error handling code here    MYAPIError error  int size  size   getObjectSize h   amp error   if error    MYAPI SUCCESS           Error handling     However  I think using the return value for returning data makes the code more readable  It s obvious that something was written to the size variable in the second example   Do you have any ideas on why I should prefer any of those approaches or perhaps mix them or use something else  I m not a fan of global error states since it tends to make multi threaded use of the library way more painful   EDIT  C   specific ideas on this would also be interesting to hear about as long as they are not involving exceptions since it s not an option for me at the moment

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I have done a lot of C programming in the past  And I really apreciated the error code return value  But is has several possible pitfalls    Duplicate error numbers  this can be solved with a global errors h file  Forgetting to check the error code  this should be solved with a cluebat and long debugging hours  But in the end you will learn  or you will know that someone else will do the debugging

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EDIT If you need access only to the last error  and you don t work in multithreaded environment   You can return only true false  or some kind of  define if you work in C and don t support bool variables   and have a global Error buffer that will hold the last error   int getObjectSize MYAPIHandle h  int  returnedSize   MYAPI ERROR LastError  MYAPI ERROR  getLastError    return LastError     define FUNC SUCCESS 1  define FUNC FAIL 0  if getObjectSize h   amp size     FUNC SUCCESS         MYAPI ERROR  error   getLastError           error handling

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I prefer error handling in C using the following technique    struct lnode  insert char  data  int len  struct lnode  list        struct lnode  p   q      uint8 t good      struct               uint8 t alloc node   1              uint8 t alloc str   1        cleanup     0  0           allocate node      p    struct lnode   malloc sizeof struct lnode        good   cleanup alloc node    p    NULL          good  then allocate str     if  good                p- gt str    char   malloc sizeof char  len               good   cleanup alloc str    p- gt str    NULL                good  copy data     if good                memcpy   p- gt str  data  len                 still good  insert in list     if good                if NULL    list                        p- gt next   NULL                      list   p                else                       q   list                      while q- gt next    NULL  amp  amp  good                                   duplicate found--not good                             good    strcmp q- gt str p- gt str     0                               q   q- gt next                                            if  good                                p- gt next   q- gt next                              q- gt next   p                                                   not-good  cleanup      if  good                if cleanup alloc str    free p- gt str               if cleanup alloc node   free p                good  return list or else return NULL     return  good   list   NULL        Source  http   blog staila com  p 114

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I definitely prefer the first solution    int size  if getObjectSize h   amp size     MYAPI SUCCESS         Error handling     i would slightly modify it  to    int size  MYAPIError rc   rc   getObjectSize h   amp size  if   rc    MYAPI SUCCESS         Error handling     In additional i will never mix legitimate return value with error even if currently the scope of function allowing you to do so  you never know which way function implementation will go in the future    And if we already talking about error handling i would suggest goto Error  as error handling code  unless some undo function can be called to handle error handling correctly

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EDIT If you need access only to the last error  and you don t work in multithreaded environment   You can return only true false  or some kind of  define if you work in C and don t support bool variables   and have a global Error buffer that will hold the last error   int getObjectSize MYAPIHandle h  int  returnedSize   MYAPI ERROR LastError  MYAPI ERROR  getLastError    return LastError     define FUNC SUCCESS 1  define FUNC FAIL 0  if getObjectSize h   amp size     FUNC SUCCESS         MYAPI ERROR  error   getLastError           error handling

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I like the error as return-value way  If you re designing the api and you want to make use of your library as painless as possible think about these additions    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out  I know this idea makes multithreaded use a bit difficult  but it would be nice if application programmer can set an global error-callback  That way they will be able to put a breakpoint into the callback during bug-hunt sessions    Hope it helps

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I like the error as return-value way  If you re designing the api and you want to make use of your library as painless as possible think about these additions    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out  I know this idea makes multithreaded use a bit difficult  but it would be nice if application programmer can set an global error-callback  That way they will be able to put a breakpoint into the callback during bug-hunt sessions    Hope it helps

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Second approach lets the compiler produce more optimized code  because when address of a variable is passed to a function  the compiler cannot keep its value in register s  during subsequent calls to other functions  The completion code usually is used only once  just after the call  whereas  real  data returned from the call may be used more often

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I have done a lot of C programming in the past  And I really apreciated the error code return value  But is has several possible pitfalls    Duplicate error numbers  this can be solved with a global errors h file  Forgetting to check the error code  this should be solved with a cluebat and long debugging hours  But in the end you will learn  or you will know that someone else will do the debugging

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Use setjmp   http   en wikipedia org wiki Setjmp h  http   aszt inf elte hu  gsd halado cpp ch02s03 html  http   www di unipi it  nids docs longjump try trow catch html   include  lt setjmp h gt   include  lt stdio h gt   jmp buf x   void f         longjmp x 5      throw 5     int main            output of this program is 5       int i   0       if    i   setjmp x      0     try                f               -- gt  end of try      else    catch i                 switch  i                     case  1          case  2          default  fprintf  stdout   error code    d n   i   break                       -- gt  end of catch i       return 0         include  lt stdio h gt   include  lt setjmp h gt    define TRY do  jmp buf ex buf    if   setjmp ex buf        define CATCH   else    define ETRY    while 0   define THROW longjmp ex buf    1   int main int argc  char   argv       TRY            printf  In Try Statement n          THROW        printf  I do not appear n            CATCH            printf  Got Exception  n            ETRY      return 0

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I have done a lot of C programming in the past  And I really apreciated the error code return value  But is has several possible pitfalls    Duplicate error numbers  this can be solved with a global errors h file  Forgetting to check the error code  this should be solved with a cluebat and long debugging hours  But in the end you will learn  or you will know that someone else will do the debugging

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I like the error as return-value way  If you re designing the api and you want to make use of your library as painless as possible think about these additions    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out  I know this idea makes multithreaded use a bit difficult  but it would be nice if application programmer can set an global error-callback  That way they will be able to put a breakpoint into the callback during bug-hunt sessions    Hope it helps

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If you want your program to crash and not know the reason  then go ahead and trust the programmers and c basic error handling  I think it s best to build in some kind of error reporting  call it debug mode  turn it off when your want best performance and turn it on when you want to debug a issue  Hopefully you can hit it again  There will be bugs  the question is how do you want to spend your days and nights looking for them

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The UNIX approach is most similar to your second suggestion  Return either the result or a single  it went wrong  value  For instance  open will return the file descriptor on success or -1 on failure  On failure it also sets errno  an external global integer to indicate which failure occurred   For what it s worth  Cocoa has also been adopting a similar approach  A number of methods return BOOL  and take an NSError    parameter  so that on failure they set the error and return NO  Then the error handling looks like   NSError  error   nil  if   myThing doThingError   amp error     NO         error handling     which is somewhere between your two options  -

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In addition to what has been said  prior to returning your error code  fire off an assert or similar diagnostic when an error is returned  as it will make tracing a lot easier   The way I do this is to have a customised assert that still gets compiled in at release but only gets fired when the software is in diagnostics mode  with an option to silently report to a log file or pause on screen   I personally return error codes as negative integers with no error as zero   but it does leave you with the possible following bug  if  MyFunc     DoSomething      An alternative is have a failure always returned as zero  and use a LastError   function to provide details of the actual error

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The UNIX approach is most similar to your second suggestion  Return either the result or a single  it went wrong  value  For instance  open will return the file descriptor on success or -1 on failure  On failure it also sets errno  an external global integer to indicate which failure occurred   For what it s worth  Cocoa has also been adopting a similar approach  A number of methods return BOOL  and take an NSError    parameter  so that on failure they set the error and return NO  Then the error handling looks like   NSError  error   nil  if   myThing doThingError   amp error     NO         error handling     which is somewhere between your two options  -

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I use the first approach whenever I create a library   There are several advantages of using a typedef ed enum as a return code    If the function returns a more complicated output such as an array and it s length you do not need to create arbitrary structures to return   rc   func      int   return array  size t  array length    It allows for simple  standardized error handling   if   rc   func          API SUCCESS          Error Handling       It allows for simple error handling in the library function      Check for valid arguments    if  NULL    return array    NULL    array length      return API INVALID ARGS   Using a typedef ed enum also allows for the enum name to be visible in the debugger   This allows for easier debugging without the need to constantly consult a header file   Having a function to translate this enum into a string is helpful as well    The most important issue regardless of approach used is to be consistent   This applies to function and argument naming  argument ordering and error handling

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EDIT If you need access only to the last error  and you don t work in multithreaded environment   You can return only true false  or some kind of  define if you work in C and don t support bool variables   and have a global Error buffer that will hold the last error   int getObjectSize MYAPIHandle h  int  returnedSize   MYAPI ERROR LastError  MYAPI ERROR  getLastError    return LastError     define FUNC SUCCESS 1  define FUNC FAIL 0  if getObjectSize h   amp size     FUNC SUCCESS         MYAPI ERROR  error   getLastError           error handling

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There s a nice set of slides from CMU s CERT with recommendations for when to use each of the common C  and C    error handling techniques  One of the best slides is this decision tree     I would personally change two things about this flowcart   First  I would clarify that sometimes objects should use return values to indicate errors  If a function only extracts data from an object but doesn t mutate the object  then the integrity of the object itself is not at risk and indicating errors using a return value is more appropriate   Second  it s not always appropriate to use exceptions in C    Exceptions are good because they can reduce the amount of source code devoted to error handling  they mostly don t affect function signatures  and they re very flexible in what data they can pass up the callstack  On the other hand  exceptions might not be the right choice for a few reasons    C   exceptions have very particular semantics  If you don t want those semantics  then C   exceptions are a bad choice  An exception must be dealt with immediately after being thrown and the design favors the case where an error will need to unwind the callstack a few levels  C   functions that throw exceptions can t later be wrapped to not throw exceptions  at least not without paying the full cost of exceptions anyway  Functions that return error codes can be wrapped to throw C   exceptions  making them more flexible  C   s new gets this right by providing a non-throwing variant  C   exceptions are relatively expensive but this downside is mostly overblown for programs making sensible use of exceptions  A program simply shouldn t throw exceptions on a codepath where performance is a concern  It doesn t really matter how fast your program can report an error and exit  Sometimes C   exceptions are not available  Either they re literally not available in one s C   implementation  or one s code guidelines ban them      Since the original question was about a multithreaded context  I think the local error indicator technique  what s described in SirDarius s answer  was underappreciated in the original answers  It s threadsafe  doesn t force the error to be immediately dealt with by the caller  and can bundle arbitrary data describing the error  The downside is that it must be held by an object  or I suppose somehow associated externally  and is arguably easier to ignore than a return code

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I was pondering this issue recently as well  and wrote up some macros for C that simulate try-catch-finally semantics using purely local return values  Hope you find it useful

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I personally prefer the former approach  returning an error indicator    Where necessary the return result should just indicate that an error occurred  with another function being used to find out the exact error   In your getSize   example I d consider that sizes must always be zero or positive  so returning a negative result can indicate an error  much like UNIX system calls do   I can t think of any library that I ve used that goes for the latter approach with an error object passed in as a pointer   stdio  etc all go with a return value

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In addition to what has been said  prior to returning your error code  fire off an assert or similar diagnostic when an error is returned  as it will make tracing a lot easier   The way I do this is to have a customised assert that still gets compiled in at release but only gets fired when the software is in diagnostics mode  with an option to silently report to a log file or pause on screen   I personally return error codes as negative integers with no error as zero   but it does leave you with the possible following bug  if  MyFunc     DoSomething      An alternative is have a failure always returned as zero  and use a LastError   function to provide details of the actual error

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Second approach lets the compiler produce more optimized code  because when address of a variable is passed to a function  the compiler cannot keep its value in register s  during subsequent calls to other functions  The completion code usually is used only once  just after the call  whereas  real  data returned from the call may be used more often

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In addition the other great answers  I suggest that you try to separate the error flag and the error code in order to save one line on each call  i e    if   doit a  b  c   amp errcode           handle           thine            error         When you have lots of error-checking  this little simplification really helps

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I definitely prefer the first solution    int size  if getObjectSize h   amp size     MYAPI SUCCESS         Error handling     i would slightly modify it  to    int size  MYAPIError rc   rc   getObjectSize h   amp size  if   rc    MYAPI SUCCESS         Error handling     In additional i will never mix legitimate return value with error even if currently the scope of function allowing you to do so  you never know which way function implementation will go in the future    And if we already talking about error handling i would suggest goto Error  as error handling code  unless some undo function can be called to handle error handling correctly

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EDIT If you need access only to the last error  and you don t work in multithreaded environment   You can return only true false  or some kind of  define if you work in C and don t support bool variables   and have a global Error buffer that will hold the last error   int getObjectSize MYAPIHandle h  int  returnedSize   MYAPI ERROR LastError  MYAPI ERROR  getLastError    return LastError     define FUNC SUCCESS 1  define FUNC FAIL 0  if getObjectSize h   amp size     FUNC SUCCESS         MYAPI ERROR  error   getLastError           error handling

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What you could do instead of returning your error  and thus forbidding you from returning data with your function  is using a wrapper for your return type   typedef struct       enum  SUCCESS  ERROR  status      union           int errCode          MyType value        ret    MyTypeWrapper    Then  in the called function   MyTypeWrapper MYAPIFunction MYAPIHandle h        MyTypeWrapper wrapper                      If there is an error somewhere      wrapper status   ERROR      wrapper ret errCode   MY ERROR CODE          Everything went well      wrapper status   SUCCESS      wrapper ret value   myProcessedData      return wrapper       Please note that with the following method  the wrapper will have the size of MyType plus one byte  on most compilers   which is quite profitable  and you won t have to push another argument on the stack when you call your function  returnedSize or returnedError in both of the methods you presented

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Use setjmp   http   en wikipedia org wiki Setjmp h  http   aszt inf elte hu  gsd halado cpp ch02s03 html  http   www di unipi it  nids docs longjump try trow catch html   include  lt setjmp h gt   include  lt stdio h gt   jmp buf x   void f         longjmp x 5      throw 5     int main            output of this program is 5       int i   0       if    i   setjmp x      0     try                f               -- gt  end of try      else    catch i                 switch  i                     case  1          case  2          default  fprintf  stdout   error code    d n   i   break                       -- gt  end of catch i       return 0         include  lt stdio h gt   include  lt setjmp h gt    define TRY do  jmp buf ex buf    if   setjmp ex buf        define CATCH   else    define ETRY    while 0   define THROW longjmp ex buf    1   int main int argc  char   argv       TRY            printf  In Try Statement n          THROW        printf  I do not appear n            CATCH            printf  Got Exception  n            ETRY      return 0

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I ve used both approaches  and they both worked fine for me   Whichever one I use  I always try to apply this principle   If the only possible errors are programmer errors  don t return an error code  use asserts inside the function   An assertion that validates the inputs clearly communicates what the function expects   while too much error checking can obscure the program logic   Deciding what to do for all the various error cases can really complicate the design   Why figure out how functionX should handle a null pointer if you can instead insist that the programmer never pass one

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I use the first approach whenever I create a library   There are several advantages of using a typedef ed enum as a return code    If the function returns a more complicated output such as an array and it s length you do not need to create arbitrary structures to return   rc   func      int   return array  size t  array length    It allows for simple  standardized error handling   if   rc   func          API SUCCESS          Error Handling       It allows for simple error handling in the library function      Check for valid arguments    if  NULL    return array    NULL    array length      return API INVALID ARGS   Using a typedef ed enum also allows for the enum name to be visible in the debugger   This allows for easier debugging without the need to constantly consult a header file   Having a function to translate this enum into a string is helpful as well    The most important issue regardless of approach used is to be consistent   This applies to function and argument naming  argument ordering and error handling

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When I write programs  during initialization  I usually spin off a thread for error handling  and initialize a special structure for errors  including a lock  Then  when I detect an error  through return values  I enter in the info from the exception into the structure and send a SIGIO to the exception handling thread  then see if I can t continue execution  If I can t  I send a SIGURG to the exception thread  which stops the program gracefully

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Here is an approach which I think is interesting  while requiring some discipline  This assumes a handle-type variable is the instance on which operate all API functions  The idea is that the struct behind the handle stores the previous error as a struct with necessary data  code  message      and the user is provided with a function that returns a pointer to this error object  Each operation will update the pointed object so the user can check its status without even calling functions  As opposed to the errno pattern  the error code is not global  which make the approach thread-safe  as long as each handle is properly used  Example  MyHandle   h   MyApiCreateHandle        first call checks for pointer nullity  since we cannot retrieve error code    on a NULL pointer    if  h    NULL       return 0       from here h is a valid handle        get a pointer to the error struct that will be updated with each call    MyApiError   err   MyApiGetError h    MyApiFileDescriptor   fd   MyApiOpenFile  quot  path to file ext quot        we want to know what can go wrong    if  err- gt code    MyApi ERROR OK        fprintf stderr   quot   d   s n quot   err- gt code  err- gt message       MyApiDestroy h       return 0     MyApiRecord record      here the API could refuse to execute the operation if the previous one    yielded an error  and eventually close the file descriptor itself if    the error is not recoverable    MyApiReadFileRecord h   amp record  sizeof record        we want to know what can go wrong  here using a macro checking for failure    if  MyApi FAILED err         fprintf stderr   quot   d   s n quot   err- gt code  err- gt message       MyApiDestroy h       return 0

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I like the error as return-value way  If you re designing the api and you want to make use of your library as painless as possible think about these additions    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out  I know this idea makes multithreaded use a bit difficult  but it would be nice if application programmer can set an global error-callback  That way they will be able to put a breakpoint into the callback during bug-hunt sessions    Hope it helps

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There s a nice set of slides from CMU s CERT with recommendations for when to use each of the common C  and C    error handling techniques  One of the best slides is this decision tree     I would personally change two things about this flowcart   First  I would clarify that sometimes objects should use return values to indicate errors  If a function only extracts data from an object but doesn t mutate the object  then the integrity of the object itself is not at risk and indicating errors using a return value is more appropriate   Second  it s not always appropriate to use exceptions in C    Exceptions are good because they can reduce the amount of source code devoted to error handling  they mostly don t affect function signatures  and they re very flexible in what data they can pass up the callstack  On the other hand  exceptions might not be the right choice for a few reasons    C   exceptions have very particular semantics  If you don t want those semantics  then C   exceptions are a bad choice  An exception must be dealt with immediately after being thrown and the design favors the case where an error will need to unwind the callstack a few levels  C   functions that throw exceptions can t later be wrapped to not throw exceptions  at least not without paying the full cost of exceptions anyway  Functions that return error codes can be wrapped to throw C   exceptions  making them more flexible  C   s new gets this right by providing a non-throwing variant  C   exceptions are relatively expensive but this downside is mostly overblown for programs making sensible use of exceptions  A program simply shouldn t throw exceptions on a codepath where performance is a concern  It doesn t really matter how fast your program can report an error and exit  Sometimes C   exceptions are not available  Either they re literally not available in one s C   implementation  or one s code guidelines ban them      Since the original question was about a multithreaded context  I think the local error indicator technique  what s described in SirDarius s answer  was underappreciated in the original answers  It s threadsafe  doesn t force the error to be immediately dealt with by the caller  and can bundle arbitrary data describing the error  The downside is that it must be held by an object  or I suppose somehow associated externally  and is arguably easier to ignore than a return code

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Here is an approach which I think is interesting  while requiring some discipline  This assumes a handle-type variable is the instance on which operate all API functions  The idea is that the struct behind the handle stores the previous error as a struct with necessary data  code  message      and the user is provided with a function that returns a pointer to this error object  Each operation will update the pointed object so the user can check its status without even calling functions  As opposed to the errno pattern  the error code is not global  which make the approach thread-safe  as long as each handle is properly used  Example  MyHandle   h   MyApiCreateHandle        first call checks for pointer nullity  since we cannot retrieve error code    on a NULL pointer    if  h    NULL       return 0       from here h is a valid handle        get a pointer to the error struct that will be updated with each call    MyApiError   err   MyApiGetError h    MyApiFileDescriptor   fd   MyApiOpenFile  quot  path to file ext quot        we want to know what can go wrong    if  err- gt code    MyApi ERROR OK        fprintf stderr   quot   d   s n quot   err- gt code  err- gt message       MyApiDestroy h       return 0     MyApiRecord record      here the API could refuse to execute the operation if the previous one    yielded an error  and eventually close the file descriptor itself if    the error is not recoverable    MyApiReadFileRecord h   amp record  sizeof record        we want to know what can go wrong  here using a macro checking for failure    if  MyApi FAILED err         fprintf stderr   quot   d   s n quot   err- gt code  err- gt message       MyApiDestroy h       return 0

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Here s a simple program to demonstrate the first 2 bullets of Nils Pipenbrinck s answer here  His first 2 bullets are    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out     Assume you have written a module named mymodule  First  in mymodule h  you define your enum-based error codes  and you write some error strings which correspond to these codes  Here I am using an array of C strings  char     which only works well if your first enum-based error code has value 0  and you don t manipulate the numbers thereafter  If you do use error code numbers with gaps or other starting values  you ll simply have to change from using a mapped C-string array  as I do below  to using a function which uses a switch statement or if   else if statements to map from enum error codes to printable C strings  which I don t demonstrate   The choice is yours  mymodule h      brief Error codes for library  quot mymodule quot  typedef enum mymodule error e           No error     MYMODULE ERROR OK   0               Invalid arguments  ex  NULL pointer where a valid pointer is required      MYMODULE ERROR INVARG           Out of memory  RAM      MYMODULE ERROR NOMEM           Make up your error codes as you see fit     MYMODULE ERROR MYERROR           etc etc              Total   of errors in this list  NOT AN ACTUAL ERROR CODE           NOTE  that for this to work  it assumes your first error code is value 0 and you let it naturally          increment from there  as is done above  without explicitly altering any error values above     MYMODULE ERROR COUNT    mymodule error t      Array of strings to map enum error types to printable strings    - see important NOTE above  const char  const MYMODULE ERROR STRS             quot MYMODULE ERROR OK quot        quot MYMODULE ERROR INVARG quot        quot MYMODULE ERROR NOMEM quot        quot MYMODULE ERROR MYERROR quot          To get a printable error string const char  mymodule error str mymodule error t err       Other functions in mymodule mymodule error t mymodule func1 void   mymodule error t mymodule func2 void   mymodule error t mymodule func3 void    mymodule c contains my mapping function to map from enum error codes to printable C strings  mymodule c  include  lt stdio h gt        brief      Function to get a printable string from an enum error type      param in   err     a valid error code for this module      return     A printable C string corresponding to the error code input above  or NULL if an invalid error code                 was passed in const char  mymodule error str mymodule error t err        const char  err str   NULL          Ensure error codes are within the valid array index range     if  err  gt   MYMODULE ERROR COUNT                goto done             err str   MYMODULE ERROR STRS err    done      return err str        Let s just make some empty dummy functions to return some errors  fill these in as appropriate for your     library module  mymodule error t mymodule func1 void        return MYMODULE ERROR OK     mymodule error t mymodule func2 void        return MYMODULE ERROR INVARG     mymodule error t mymodule func3 void        return MYMODULE ERROR MYERROR     main c contains a test program to demonstrate calling some functions and printing some error codes from them  main c  include  lt stdio h gt   int main         printf  quot Demonstration of enum-based error codes in C  or C    n quot         printf  quot err code from mymodule func1      s n quot   mymodule error str mymodule func1          printf  quot err code from mymodule func2      s n quot   mymodule error str mymodule func2          printf  quot err code from mymodule func3      s n quot   mymodule error str mymodule func3           return 0     Output   Demonstration of enum-based error codes in C  or C    err code from mymodule func1     MYMODULE ERROR OK err code from mymodule func2     MYMODULE ERROR INVARG err code from mymodule func3     MYMODULE ERROR MYERROR  References  You can run this code yourself here  https   onlinegdb com ByEbKLupS

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I have done a lot of C programming in the past  And I really apreciated the error code return value  But is has several possible pitfalls    Duplicate error numbers  this can be solved with a global errors h file  Forgetting to check the error code  this should be solved with a cluebat and long debugging hours  But in the end you will learn  or you will know that someone else will do the debugging

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When I write programs  during initialization  I usually spin off a thread for error handling  and initialize a special structure for errors  including a lock  Then  when I detect an error  through return values  I enter in the info from the exception into the structure and send a SIGIO to the exception handling thread  then see if I can t continue execution  If I can t  I send a SIGURG to the exception thread  which stops the program gracefully

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The UNIX approach is most similar to your second suggestion  Return either the result or a single  it went wrong  value  For instance  open will return the file descriptor on success or -1 on failure  On failure it also sets errno  an external global integer to indicate which failure occurred   For what it s worth  Cocoa has also been adopting a similar approach  A number of methods return BOOL  and take an NSError    parameter  so that on failure they set the error and return NO  Then the error handling looks like   NSError  error   nil  if   myThing doThingError   amp error     NO         error handling     which is somewhere between your two options  -

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I personally prefer the former approach  returning an error indicator    Where necessary the return result should just indicate that an error occurred  with another function being used to find out the exact error   In your getSize   example I d consider that sizes must always be zero or positive  so returning a negative result can indicate an error  much like UNIX system calls do   I can t think of any library that I ve used that goes for the latter approach with an error object passed in as a pointer   stdio  etc all go with a return value

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Here s a simple program to demonstrate the first 2 bullets of Nils Pipenbrinck s answer here  His first 2 bullets are    store all possible error-states in one typedef ed enum and use it in your lib  Don t just return ints or even worse  mix ints or different enumerations with return-codes   provide a function that converts errors into something human readable  Can be simple  Just error-enum in  const char  out     Assume you have written a module named mymodule  First  in mymodule h  you define your enum-based error codes  and you write some error strings which correspond to these codes  Here I am using an array of C strings  char     which only works well if your first enum-based error code has value 0  and you don t manipulate the numbers thereafter  If you do use error code numbers with gaps or other starting values  you ll simply have to change from using a mapped C-string array  as I do below  to using a function which uses a switch statement or if   else if statements to map from enum error codes to printable C strings  which I don t demonstrate   The choice is yours  mymodule h      brief Error codes for library  quot mymodule quot  typedef enum mymodule error e           No error     MYMODULE ERROR OK   0               Invalid arguments  ex  NULL pointer where a valid pointer is required      MYMODULE ERROR INVARG           Out of memory  RAM      MYMODULE ERROR NOMEM           Make up your error codes as you see fit     MYMODULE ERROR MYERROR           etc etc              Total   of errors in this list  NOT AN ACTUAL ERROR CODE           NOTE  that for this to work  it assumes your first error code is value 0 and you let it naturally          increment from there  as is done above  without explicitly altering any error values above     MYMODULE ERROR COUNT    mymodule error t      Array of strings to map enum error types to printable strings    - see important NOTE above  const char  const MYMODULE ERROR STRS             quot MYMODULE ERROR OK quot        quot MYMODULE ERROR INVARG quot        quot MYMODULE ERROR NOMEM quot        quot MYMODULE ERROR MYERROR quot          To get a printable error string const char  mymodule error str mymodule error t err       Other functions in mymodule mymodule error t mymodule func1 void   mymodule error t mymodule func2 void   mymodule error t mymodule func3 void    mymodule c contains my mapping function to map from enum error codes to printable C strings  mymodule c  include  lt stdio h gt        brief      Function to get a printable string from an enum error type      param in   err     a valid error code for this module      return     A printable C string corresponding to the error code input above  or NULL if an invalid error code                 was passed in const char  mymodule error str mymodule error t err        const char  err str   NULL          Ensure error codes are within the valid array index range     if  err  gt   MYMODULE ERROR COUNT                goto done             err str   MYMODULE ERROR STRS err    done      return err str        Let s just make some empty dummy functions to return some errors  fill these in as appropriate for your     library module  mymodule error t mymodule func1 void        return MYMODULE ERROR OK     mymodule error t mymodule func2 void        return MYMODULE ERROR INVARG     mymodule error t mymodule func3 void        return MYMODULE ERROR MYERROR     main c contains a test program to demonstrate calling some functions and printing some error codes from them  main c  include  lt stdio h gt   int main         printf  quot Demonstration of enum-based error codes in C  or C    n quot         printf  quot err code from mymodule func1      s n quot   mymodule error str mymodule func1          printf  quot err code from mymodule func2      s n quot   mymodule error str mymodule func2          printf  quot err code from mymodule func3      s n quot   mymodule error str mymodule func3           return 0     Output   Demonstration of enum-based error codes in C  or C    err code from mymodule func1     MYMODULE ERROR OK err code from mymodule func2     MYMODULE ERROR INVARG err code from mymodule func3     MYMODULE ERROR MYERROR  References  You can run this code yourself here  https   onlinegdb com ByEbKLupS

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In addition to what has been said  prior to returning your error code  fire off an assert or similar diagnostic when an error is returned  as it will make tracing a lot easier   The way I do this is to have a customised assert that still gets compiled in at release but only gets fired when the software is in diagnostics mode  with an option to silently report to a log file or pause on screen   I personally return error codes as negative integers with no error as zero   but it does leave you with the possible following bug  if  MyFunc     DoSomething      An alternative is have a failure always returned as zero  and use a LastError   function to provide details of the actual error

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I was pondering this issue recently as well  and wrote up some macros for C that simulate try-catch-finally semantics using purely local return values  Hope you find it useful

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First approach is better IMHO    It s easier to write function that way  When you notice an error in the middle of the function you just return an error value  In second approach you need to assign error value to one of the parameters and then return something     but what would you return - you don t have correct value and you don t return error value  it s more popular so it will be easier to understand  maintain

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First approach is better IMHO    It s easier to write function that way  When you notice an error in the middle of the function you just return an error value  In second approach you need to assign error value to one of the parameters and then return something     but what would you return - you don t have correct value and you don t return error value  it s more popular so it will be easier to understand  maintain

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I ran into this Q amp A a number of times  and wanted to contribute a more comprehensive answer   I think the best way to think about this is how to return errors to caller  and what you return   How  There are 3 ways to return information from a function    Return Value Out Argument s  Out of Band  that includes non-local goto  setjmp longjmp   file or global scoped variables  file system etc    Return Value  You can only return value is a single object  however  it can be an arbitrary complex   Here is an example of an error returning function     enum error hold my beer      One benefit of return values is that it allows chaining of calls for less intrusive error handling      hold my beer    amp  amp     hold my cigarette    amp  amp     hold my pants        abort      This not just about readability  but may also allow processing an array of such function pointers in a uniform way   Out Argument s   You can return more via more than one object via arguments  but best practice does suggest to keep the total number of arguments low  say   lt  4    void look ma enum error  e  char  what broke    enum error e  look ma e   if e    FURNITURE      reorder what broke     else if e    SELF      tell doctor what broke       Out of Band  With setjmp   you define a place and how you want to handle an int value  and you transfer control to that location via a longjmp     See Practical usage of setjmp and longjmp in C   What   Indicator Code Object Callback   Indicator  An error indicator only tells you that there is a problem but nothing about the nature of said problem   struct foo  f   foo init    if  f          handle the absence of foo     This is the least powerful way for a function to communicate error state  however  perfect if caller cannot respond to the error in a graduated manner anyways   Code  An error code tells the caller about the nature of the problem  and may allow for a suitable response  from the above    It can be return value  or like the look ma   example above an error argument   Object  With an error object  the caller can be informed about arbitrary complicated issues   For example  an error code and a suitable human readable message  It can also inform the caller that multiple things went wrong  or an error per item when processing a collection   struct collection friends  enum error  e   malloc c size   sizeof enum error        ask for favor friends  reason   for int i   0  i  lt  c size  i         if reason i     NOT FOUND  find friends i        Instead of pre-allocating the error array  you can also  re allocate it dynamically as needed of course   Callback  Callback is the most powerful way to handle errors  as you can tell the function what behavior you would like to see happen when something goes wrong   A callback argument can be added to each function  or if customization uis only required per instance of a struct like this    struct foo               void  error handler  char           void default error handler char  message         assert f       printf   s   message        void foo set error handler struct foo  f  void   eh  char           assert f       f- gt error handler   eh       struct foo  foo init         struct foo  f   malloc sizeof struct foo        foo set error handler f  default error handler       return f        struct foo  f   foo init     foo something      One interesting benefit of a callback is that it can be invoked multiple times  or none at all in the absence of errors in which there is no overhead on the happy path   There is  however  an inversion of control   The calling code does not know if the callback was invoked   As such  it may make sense to also use an indicator

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What you could do instead of returning your error  and thus forbidding you from returning data with your function  is using a wrapper for your return type   typedef struct       enum  SUCCESS  ERROR  status      union           int errCode          MyType value        ret    MyTypeWrapper    Then  in the called function   MyTypeWrapper MYAPIFunction MYAPIHandle h        MyTypeWrapper wrapper                      If there is an error somewhere      wrapper status   ERROR      wrapper ret errCode   MY ERROR CODE          Everything went well      wrapper status   SUCCESS      wrapper ret value   myProcessedData      return wrapper       Please note that with the following method  the wrapper will have the size of MyType plus one byte  on most compilers   which is quite profitable  and you won t have to push another argument on the stack when you call your function  returnedSize or returnedError in both of the methods you presented

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I prefer error handling in C using the following technique    struct lnode  insert char  data  int len  struct lnode  list        struct lnode  p   q      uint8 t good      struct               uint8 t alloc node   1              uint8 t alloc str   1        cleanup     0  0           allocate node      p    struct lnode   malloc sizeof struct lnode        good   cleanup alloc node    p    NULL          good  then allocate str     if  good                p- gt str    char   malloc sizeof char  len               good   cleanup alloc str    p- gt str    NULL                good  copy data     if good                memcpy   p- gt str  data  len                 still good  insert in list     if good                if NULL    list                        p- gt next   NULL                      list   p                else                       q   list                      while q- gt next    NULL  amp  amp  good                                   duplicate found--not good                             good    strcmp q- gt str p- gt str     0                               q   q- gt next                                            if  good                                p- gt next   q- gt next                              q- gt next   p                                                   not-good  cleanup      if  good                if cleanup alloc str    free p- gt str               if cleanup alloc node   free p                good  return list or else return NULL     return  good   list   NULL        Source  http   blog staila com  p 114

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I use the first approach whenever I create a library   There are several advantages of using a typedef ed enum as a return code    If the function returns a more complicated output such as an array and it s length you do not need to create arbitrary structures to return   rc   func      int   return array  size t  array length    It allows for simple  standardized error handling   if   rc   func          API SUCCESS          Error Handling       It allows for simple error handling in the library function      Check for valid arguments    if  NULL    return array    NULL    array length      return API INVALID ARGS   Using a typedef ed enum also allows for the enum name to be visible in the debugger   This allows for easier debugging without the need to constantly consult a header file   Having a function to translate this enum into a string is helpful as well    The most important issue regardless of approach used is to be consistent   This applies to function and argument naming  argument ordering and error handling

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Returning error code is the usual approach for error handling in C   But recently we experimented with the outgoing error pointer approach as well   It has some advantages over the return value approach    You can use the return value for more meaningful purposes  Having to write out that error parameter reminds you to handle the error or propagate it   You never forget checking the return value of fclose  don t you   If you use an error pointer  you can pass it down as you call functions  If any of the functions set it  the value won t get lost  By setting a data breakpoint on the error variable  you can catch where does the error occurred first  By setting a conditional breakpoint you can catch specific errors too  It makes it easier to automatize the check whether you handle all errors  The code convention may force you to call your error pointer as err and it must be the last argument  So the script can match the string err   then check if it s followed by if   err  Actually in practice we made a macro called CER  check err return  and CEG  check err goto   So you don t need to type it out always when we just want to return on error  and can reduce the visual clutter    Not all functions in our code has this outgoing parameter though  This outgoing parameter thing are used for cases where you would normally throw an exception

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I personally prefer the former approach  returning an error indicator    Where necessary the return result should just indicate that an error occurred  with another function being used to find out the exact error   In your getSize   example I d consider that sizes must always be zero or positive  so returning a negative result can indicate an error  much like UNIX system calls do   I can t think of any library that I ve used that goes for the latter approach with an error object passed in as a pointer   stdio  etc all go with a return value

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I use the first approach whenever I create a library   There are several advantages of using a typedef ed enum as a return code    If the function returns a more complicated output such as an array and it s length you do not need to create arbitrary structures to return   rc   func      int   return array  size t  array length    It allows for simple  standardized error handling   if   rc   func          API SUCCESS          Error Handling       It allows for simple error handling in the library function      Check for valid arguments    if  NULL    return array    NULL    array length      return API INVALID ARGS   Using a typedef ed enum also allows for the enum name to be visible in the debugger   This allows for easier debugging without the need to constantly consult a header file   Having a function to translate this enum into a string is helpful as well    The most important issue regardless of approach used is to be consistent   This applies to function and argument naming  argument ordering and error handling

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In addition the other great answers  I suggest that you try to separate the error flag and the error code in order to save one line on each call  i e    if   doit a  b  c   amp errcode           handle           thine            error         When you have lots of error-checking  this little simplification really helps

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First approach is better IMHO    It s easier to write function that way  When you notice an error in the middle of the function you just return an error value  In second approach you need to assign error value to one of the parameters and then return something     but what would you return - you don t have correct value and you don t return error value  it s more popular so it will be easier to understand  maintain

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I personally prefer the former approach  returning an error indicator    Where necessary the return result should just indicate that an error occurred  with another function being used to find out the exact error   In your getSize   example I d consider that sizes must always be zero or positive  so returning a negative result can indicate an error  much like UNIX system calls do   I can t think of any library that I ve used that goes for the latter approach with an error object passed in as a pointer   stdio  etc all go with a return value

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Second approach lets the compiler produce more optimized code  because when address of a variable is passed to a function  the compiler cannot keep its value in register s  during subsequent calls to other functions  The completion code usually is used only once  just after the call  whereas  real  data returned from the call may be used more often

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Returning error code is the usual approach for error handling in C   But recently we experimented with the outgoing error pointer approach as well   It has some advantages over the return value approach    You can use the return value for more meaningful purposes  Having to write out that error parameter reminds you to handle the error or propagate it   You never forget checking the return value of fclose  don t you   If you use an error pointer  you can pass it down as you call functions  If any of the functions set it  the value won t get lost  By setting a data breakpoint on the error variable  you can catch where does the error occurred first  By setting a conditional breakpoint you can catch specific errors too  It makes it easier to automatize the check whether you handle all errors  The code convention may force you to call your error pointer as err and it must be the last argument  So the script can match the string err   then check if it s followed by if   err  Actually in practice we made a macro called CER  check err return  and CEG  check err goto   So you don t need to type it out always when we just want to return on error  and can reduce the visual clutter    Not all functions in our code has this outgoing parameter though  This outgoing parameter thing are used for cases where you would normally throw an exception

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I definitely prefer the first solution    int size  if getObjectSize h   amp size     MYAPI SUCCESS         Error handling     i would slightly modify it  to    int size  MYAPIError rc   rc   getObjectSize h   amp size  if   rc    MYAPI SUCCESS         Error handling     In additional i will never mix legitimate return value with error even if currently the scope of function allowing you to do so  you never know which way function implementation will go in the future    And if we already talking about error handling i would suggest goto Error  as error handling code  unless some undo function can be called to handle error handling correctly

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I ve used both approaches  and they both worked fine for me   Whichever one I use  I always try to apply this principle   If the only possible errors are programmer errors  don t return an error code  use asserts inside the function   An assertion that validates the inputs clearly communicates what the function expects   while too much error checking can obscure the program logic   Deciding what to do for all the various error cases can really complicate the design   Why figure out how functionX should handle a null pointer if you can instead insist that the programmer never pass one

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I ve used both approaches  and they both worked fine for me   Whichever one I use  I always try to apply this principle   If the only possible errors are programmer errors  don t return an error code  use asserts inside the function   An assertion that validates the inputs clearly communicates what the function expects   while too much error checking can obscure the program logic   Deciding what to do for all the various error cases can really complicate the design   Why figure out how functionX should handle a null pointer if you can instead insist that the programmer never pass one

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First approach is better IMHO    It s easier to write function that way  When you notice an error in the middle of the function you just return an error value  In second approach you need to assign error value to one of the parameters and then return something     but what would you return - you don t have correct value and you don t return error value  it s more popular so it will be easier to understand  maintain

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I ve used both approaches  and they both worked fine for me   Whichever one I use  I always try to apply this principle   If the only possible errors are programmer errors  don t return an error code  use asserts inside the function   An assertion that validates the inputs clearly communicates what the function expects   while too much error checking can obscure the program logic   Deciding what to do for all the various error cases can really complicate the design   Why figure out how functionX should handle a null pointer if you can instead insist that the programmer never pass one

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Second approach lets the compiler produce more optimized code  because when address of a variable is passed to a function  the compiler cannot keep its value in register s  during subsequent calls to other functions  The completion code usually is used only once  just after the call  whereas  real  data returned from the call may be used more often

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In addition to what has been said  prior to returning your error code  fire off an assert or similar diagnostic when an error is returned  as it will make tracing a lot easier   The way I do this is to have a customised assert that still gets compiled in at release but only gets fired when the software is in diagnostics mode  with an option to silently report to a log file or pause on screen   I personally return error codes as negative integers with no error as zero   but it does leave you with the possible following bug  if  MyFunc     DoSomething      An alternative is have a failure always returned as zero  and use a LastError   function to provide details of the actual error

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I ran into this Q amp A a number of times  and wanted to contribute a more comprehensive answer   I think the best way to think about this is how to return errors to caller  and what you return   How  There are 3 ways to return information from a function    Return Value Out Argument s  Out of Band  that includes non-local goto  setjmp longjmp   file or global scoped variables  file system etc    Return Value  You can only return value is a single object  however  it can be an arbitrary complex   Here is an example of an error returning function     enum error hold my beer      One benefit of return values is that it allows chaining of calls for less intrusive error handling      hold my beer    amp  amp     hold my cigarette    amp  amp     hold my pants        abort      This not just about readability  but may also allow processing an array of such function pointers in a uniform way   Out Argument s   You can return more via more than one object via arguments  but best practice does suggest to keep the total number of arguments low  say   lt  4    void look ma enum error  e  char  what broke    enum error e  look ma e   if e    FURNITURE      reorder what broke     else if e    SELF      tell doctor what broke       Out of Band  With setjmp   you define a place and how you want to handle an int value  and you transfer control to that location via a longjmp     See Practical usage of setjmp and longjmp in C   What   Indicator Code Object Callback   Indicator  An error indicator only tells you that there is a problem but nothing about the nature of said problem   struct foo  f   foo init    if  f          handle the absence of foo     This is the least powerful way for a function to communicate error state  however  perfect if caller cannot respond to the error in a graduated manner anyways   Code  An error code tells the caller about the nature of the problem  and may allow for a suitable response  from the above    It can be return value  or like the look ma   example above an error argument   Object  With an error object  the caller can be informed about arbitrary complicated issues   For example  an error code and a suitable human readable message  It can also inform the caller that multiple things went wrong  or an error per item when processing a collection   struct collection friends  enum error  e   malloc c size   sizeof enum error        ask for favor friends  reason   for int i   0  i  lt  c size  i         if reason i     NOT FOUND  find friends i        Instead of pre-allocating the error array  you can also  re allocate it dynamically as needed of course   Callback  Callback is the most powerful way to handle errors  as you can tell the function what behavior you would like to see happen when something goes wrong   A callback argument can be added to each function  or if customization uis only required per instance of a struct like this    struct foo               void  error handler  char           void default error handler char  message         assert f       printf   s   message        void foo set error handler struct foo  f  void   eh  char           assert f       f- gt error handler   eh       struct foo  foo init         struct foo  f   malloc sizeof struct foo        foo set error handler f  default error handler       return f        struct foo  f   foo init     foo something      One interesting benefit of a callback is that it can be invoked multiple times  or none at all in the absence of errors in which there is no overhead on the happy path   There is  however  an inversion of control   The calling code does not know if the callback was invoked   As such  it may make sense to also use an indicator

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If you want your program to crash and not know the reason  then go ahead and trust the programmers and c basic error handling  I think it s best to build in some kind of error reporting  call it debug mode  turn it off when your want best performance and turn it on when you want to debug a issue  Hopefully you can hit it again  There will be bugs  the question is how do you want to spend your days and nights looking for them

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The UNIX approach is most similar to your second suggestion  Return either the result or a single  it went wrong  value  For instance  open will return the file descriptor on success or -1 on failure  On failure it also sets errno  an external global integer to indicate which failure occurred   For what it s worth  Cocoa has also been adopting a similar approach  A number of methods return BOOL  and take an NSError    parameter  so that on failure they set the error and return NO  Then the error handling looks like   NSError  error   nil  if   myThing doThingError   amp error     NO         error handling     which is somewhere between your two options  -

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I definitely prefer the first solution    int size  if getObjectSize h   amp size     MYAPI SUCCESS         Error handling     i would slightly modify it  to    int size  MYAPIError rc   rc   getObjectSize h   amp size  if   rc    MYAPI SUCCESS         Error handling     In additional i will never mix legitimate return value with error even if currently the scope of function allowing you to do so  you never know which way function implementation will go in the future    And if we already talking about error handling i would suggest goto Error  as error handling code  unless some undo function can be called to handle error handling correctly

[c] Error handling in C code

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Examples related to error-handling