Passing variable number of arguments around

Question

Say I have a C function which takes a variable number of arguments  How can I call another function which expects a variable number of arguments from inside of it  passing all the arguments that got into the first function   Example   void format string char  fmt         void debug print int dbg lvl  char  fmt             format string fmt     how do I pass all the arguments from                 fprintf stdout  fmt

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Ross  solution cleaned-up a bit  Only works if all args are pointers  Also language implementation must support eliding of previous comma if   VA ARGS   is empty  both Visual Studio C   and GCC do       pass number of arguments version   define callVardicMethodSafely       value t  args      NULL    VA ARGS      actualFunction args 1 sizeof args    sizeof  args  - 1         NULL terminated array version   define callVardicMethodSafely       value t  args      NULL    VA ARGS    NULL    actualFunction args 1

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There s no way of calling  eg  printf without knowing how many arguments you re passing to it  unless you want to get into naughty and non-portable tricks   The generally used solution is to always provide an alternate form of vararg functions  so printf has vprintf which takes a va list in place of the      The     versions are just wrappers around the va list versions

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Variadic Functions can be dangerous  Here s a safer trick      void func type  values            while  values                x    values                   do whatever with x                     func  type    val1 val2 val3 val4 0

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You can use inline assembly for the function call   in this code I assume the arguments are characters    void format string char  fmt        void debug print int dbg level  int numOfArgs  char  fmt                     va list argumentsToPass          va start argumentsToPass  fmt           char  list   new char numOfArgs           for int n   0  n  lt  numOfArgs  n                list n    va arg argumentsToPass  char           va end argumentsToPass           for int n   numOfArgs - 1  n  gt   0  n--                        char next              next   list n                 asm push next                      asm push fmt            asm call format string          fprintf stdout  fmt

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To pass the ellipses on  you have to convert them to a va list and use that va list in your second function  Specifically   void format string char  fmt va list argptr  char  formatted string     void debug print int dbg lvl  char  fmt               char formatted string MAX FMT SIZE     va list argptr   va start argptr fmt    format string fmt  argptr  formatted string    va end argptr    fprintf stdout    s  formatted string

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Short answer     logs all messages below this level  level 0 turns off LOG   ifndef LOG LEVEL  define LOG LEVEL 5     0 off  1 error  2 warning  3  info  4  debug  5 verbose  endif  define  LOG FORMAT SHORT letter  format   quot   quot   letter  quot     quot  format  quot  n quot       short log  define log s level  format                                                                                                   if  level  lt   LOG LEVEL                                                                                                        printf  LOG FORMAT SHORT level  format       VA ARGS      usage log s 1   quot fatal error occurred quot    log s 3   quot x  d and name  s quot  2   quot ali quot     output  1   fatal error occurred  3   x 2 and name ali  log with file and line number const char   getFileName const char  path        size t i   0      size t pos   0      char  p    char  path      while   p            i            if   p            p                        pos   i                    p              return path   pos      define  LOG FORMAT letter  format                                                                                      quot   quot   letter  quot    s  u   s     quot  format  quot  n quot    getFileName   FILE       LINE      FUNCTION     ifndef LOG LEVEL  define LOG LEVEL 5    0 off  1 error  2 warning  3  info  4  debug  5 verbose  endif      long log  define log l level  format                                                                                                if  level  lt   LOG LEVEL                                                                                                            printf  LOG FORMAT level  format       VA ARGS     usage log s 1   quot fatal error occurred quot    log s 3   quot x  d and name  s quot  2   quot ali quot     output  1  test cpp 97  main    fatal error occurred  3  test cpp 98  main    x 2 and name ali  custom print function you can write custom print function and pass     args to it and it is also possible to combine this with methods above  source from here int print custom const char  format             static char loc buf 64       char  temp   loc buf      int len      va list arg      va list copy      va start arg  format       va copy copy  arg       len   vsnprintf NULL  0  format  arg       va end copy       if  len  gt   sizeof loc buf             temp    char  malloc len   1           if  temp    NULL                return 0                      vsnprintf temp  len   1  format  arg       printf temp      replace with any print function you want     va end arg       if  len  gt   sizeof loc buf             free temp             return len

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You can try macro also    define NONE    0x00  define DBG     0x1F  define INFO    0x0F  define ERR     0x07  define EMR     0x03  define CRIT    0x01   define DEBUG LEVEL ERR   define WHERESTR   FILE    s  FUNC    s  LINE    d      define WHEREARG   FILE     func     LINE    define DEBUG       fprintf stderr    VA ARGS     define DEBUG PRINT X   fmt        if  DEBUG LEVEL  amp  X     X                                          DEBUG WHERESTR  fmt  WHEREARG   VA ARGS     int main         int x 10      DEBUG PRINT DBG   i am x  d n   x       return 0

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I m unsure if this works for all compilers  but it has worked so far for me   void inner func int  amp i      va list vars    va start vars  i     int j   va arg vars     va end vars      Generally useless  but should be included     void func int i           inner func i       You can add the     to inner func   if you want  but you don t need it   It works because va start uses the address of the given variable as the start point   In this case  we are giving it a reference to a variable in func     So it uses that address and reads the variables after that on the stack   The inner func   function is reading from the stack address of func     So it only works if both functions use the same stack segment   The va start and va arg macros will generally work if you give them any var as a starting point   So if you want you can pass pointers to other functions and use those too   You can make your own macros easily enough   All the macros do is typecast memory addresses   However making them work for all the compilers and calling conventions is annoying   So it s generally easier to use the ones that come with the compiler

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You can use inline assembly for the function call   in this code I assume the arguments are characters    void format string char  fmt        void debug print int dbg level  int numOfArgs  char  fmt                     va list argumentsToPass          va start argumentsToPass  fmt           char  list   new char numOfArgs           for int n   0  n  lt  numOfArgs  n                list n    va arg argumentsToPass  char           va end argumentsToPass           for int n   numOfArgs - 1  n  gt   0  n--                        char next              next   list n                 asm push next                      asm push fmt            asm call format string          fprintf stdout  fmt

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Short answer     logs all messages below this level  level 0 turns off LOG   ifndef LOG LEVEL  define LOG LEVEL 5     0 off  1 error  2 warning  3  info  4  debug  5 verbose  endif  define  LOG FORMAT SHORT letter  format   quot   quot   letter  quot     quot  format  quot  n quot       short log  define log s level  format                                                                                                   if  level  lt   LOG LEVEL                                                                                                        printf  LOG FORMAT SHORT level  format       VA ARGS      usage log s 1   quot fatal error occurred quot    log s 3   quot x  d and name  s quot  2   quot ali quot     output  1   fatal error occurred  3   x 2 and name ali  log with file and line number const char   getFileName const char  path        size t i   0      size t pos   0      char  p    char  path      while   p            i            if   p            p                        pos   i                    p              return path   pos      define  LOG FORMAT letter  format                                                                                      quot   quot   letter  quot    s  u   s     quot  format  quot  n quot    getFileName   FILE       LINE      FUNCTION     ifndef LOG LEVEL  define LOG LEVEL 5    0 off  1 error  2 warning  3  info  4  debug  5 verbose  endif      long log  define log l level  format                                                                                                if  level  lt   LOG LEVEL                                                                                                            printf  LOG FORMAT level  format       VA ARGS     usage log s 1   quot fatal error occurred quot    log s 3   quot x  d and name  s quot  2   quot ali quot     output  1  test cpp 97  main    fatal error occurred  3  test cpp 98  main    x 2 and name ali  custom print function you can write custom print function and pass     args to it and it is also possible to combine this with methods above  source from here int print custom const char  format             static char loc buf 64       char  temp   loc buf      int len      va list arg      va list copy      va start arg  format       va copy copy  arg       len   vsnprintf NULL  0  format  arg       va end copy       if  len  gt   sizeof loc buf             temp    char  malloc len   1           if  temp    NULL                return 0                      vsnprintf temp  len   1  format  arg       printf temp      replace with any print function you want     va end arg       if  len  gt   sizeof loc buf             free temp             return len

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Let s say you have a typical variadic function you ve written  Because at least one argument is required before the variadic one      you have to always write an extra argument in usage   Or do you   If you wrap your variadic function in a macro  you need no preceding arg  Consider this example     define LOGI            void   android log print ANDROID LOG INFO  LOG TAG    VA ARGS       This is obviously far more convenient  since you needn t specify the initial argument every time

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You can try macro also    define NONE    0x00  define DBG     0x1F  define INFO    0x0F  define ERR     0x07  define EMR     0x03  define CRIT    0x01   define DEBUG LEVEL ERR   define WHERESTR   FILE    s  FUNC    s  LINE    d      define WHEREARG   FILE     func     LINE    define DEBUG       fprintf stderr    VA ARGS     define DEBUG PRINT X   fmt        if  DEBUG LEVEL  amp  X     X                                          DEBUG WHERESTR  fmt  WHEREARG   VA ARGS     int main         int x 10      DEBUG PRINT DBG   i am x  d n   x       return 0

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To pass the ellipses on  you have to convert them to a va list and use that va list in your second function  Specifically   void format string char  fmt va list argptr  char  formatted string     void debug print int dbg lvl  char  fmt               char formatted string MAX FMT SIZE     va list argptr   va start argptr fmt    format string fmt  argptr  formatted string    va end argptr    fprintf stdout    s  formatted string

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In magnificent C  0x you could use variadic templates   template  lt typename     Ts gt  void format string char  fmt  Ts     ts      template  lt typename     Ts gt  void debug print int dbg lvl  char  fmt  Ts     ts      format string fmt  ts

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I m unsure if this works for all compilers  but it has worked so far for me   void inner func int  amp i      va list vars    va start vars  i     int j   va arg vars     va end vars      Generally useless  but should be included     void func int i           inner func i       You can add the     to inner func   if you want  but you don t need it   It works because va start uses the address of the given variable as the start point   In this case  we are giving it a reference to a variable in func     So it uses that address and reads the variables after that on the stack   The inner func   function is reading from the stack address of func     So it only works if both functions use the same stack segment   The va start and va arg macros will generally work if you give them any var as a starting point   So if you want you can pass pointers to other functions and use those too   You can make your own macros easily enough   All the macros do is typecast memory addresses   However making them work for all the compilers and calling conventions is annoying   So it s generally easier to use the ones that come with the compiler

User · Answer

To pass the ellipses on  you have to convert them to a va list and use that va list in your second function  Specifically   void format string char  fmt va list argptr  char  formatted string     void debug print int dbg lvl  char  fmt               char formatted string MAX FMT SIZE     va list argptr   va start argptr fmt    format string fmt  argptr  formatted string    va end argptr    fprintf stdout    s  formatted string

User · Answer

To pass the ellipses on  you have to convert them to a va list and use that va list in your second function  Specifically   void format string char  fmt va list argptr  char  formatted string     void debug print int dbg lvl  char  fmt               char formatted string MAX FMT SIZE     va list argptr   va start argptr fmt    format string fmt  argptr  formatted string    va end argptr    fprintf stdout    s  formatted string

User · Answer

There s no way of calling  eg  printf without knowing how many arguments you re passing to it  unless you want to get into naughty and non-portable tricks   The generally used solution is to always provide an alternate form of vararg functions  so printf has vprintf which takes a va list in place of the      The     versions are just wrappers around the va list versions

User · Answer

Let s say you have a typical variadic function you ve written  Because at least one argument is required before the variadic one      you have to always write an extra argument in usage   Or do you   If you wrap your variadic function in a macro  you need no preceding arg  Consider this example     define LOGI            void   android log print ANDROID LOG INFO  LOG TAG    VA ARGS       This is obviously far more convenient  since you needn t specify the initial argument every time

User · Answer

Ross  solution cleaned-up a bit  Only works if all args are pointers  Also language implementation must support eliding of previous comma if   VA ARGS   is empty  both Visual Studio C   and GCC do       pass number of arguments version   define callVardicMethodSafely       value t  args      NULL    VA ARGS      actualFunction args 1 sizeof args    sizeof  args  - 1         NULL terminated array version   define callVardicMethodSafely       value t  args      NULL    VA ARGS    NULL    actualFunction args 1

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Though you can solve passing the formatter by storing it in local buffer first  but that needs stack and can sometime be issue to deal with  I tried following and it seems to work fine    include  lt stdarg h gt   include  lt stdio h gt   void print char const  fmt             va list arg      va start arg  fmt       vprintf fmt  arg       va end arg      void printFormatted char const  fmt  va list arg        vprintf fmt  arg      void showLog int mdl  char const  type             print   nMDL   d  TYPE   s   mdl  type        va list arg      va start arg  type       char const  fmt   va arg arg  char const        printFormatted fmt  arg       va end arg      int main          int x   3  y   6      showLog 1   INF      Value    d   d Looks Good   s   x  y   Infact Awesome          showLog 1   ERR        Hope this helps

User · Answer

There s no way of calling  eg  printf without knowing how many arguments you re passing to it  unless you want to get into naughty and non-portable tricks   The generally used solution is to always provide an alternate form of vararg functions  so printf has vprintf which takes a va list in place of the      The     versions are just wrappers around the va list versions

User · Answer

Variadic Functions can be dangerous  Here s a safer trick      void func type  values            while  values                x    values                   do whatever with x                     func  type    val1 val2 val3 val4 0

User · Answer

In magnificent C  0x you could use variadic templates   template  lt typename     Ts gt  void format string char  fmt  Ts     ts      template  lt typename     Ts gt  void debug print int dbg lvl  char  fmt  Ts     ts      format string fmt  ts

User · Answer

Though you can solve passing the formatter by storing it in local buffer first  but that needs stack and can sometime be issue to deal with  I tried following and it seems to work fine    include  lt stdarg h gt   include  lt stdio h gt   void print char const  fmt             va list arg      va start arg  fmt       vprintf fmt  arg       va end arg      void printFormatted char const  fmt  va list arg        vprintf fmt  arg      void showLog int mdl  char const  type             print   nMDL   d  TYPE   s   mdl  type        va list arg      va start arg  type       char const  fmt   va arg arg  char const        printFormatted fmt  arg       va end arg      int main          int x   3  y   6      showLog 1   INF      Value    d   d Looks Good   s   x  y   Infact Awesome          showLog 1   ERR        Hope this helps

User · Answer

There s no way of calling  eg  printf without knowing how many arguments you re passing to it  unless you want to get into naughty and non-portable tricks   The generally used solution is to always provide an alternate form of vararg functions  so printf has vprintf which takes a va list in place of the      The     versions are just wrappers around the va list versions

[c] Passing variable number of arguments around

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