Examples of good gotos in C or C

Question

In this thread  we look at examples of good uses of goto in C or C     It s inspired by an answer which people voted up because they thought I was joking   Summary  label changed from original to make intent even clearer    infinite loop          code goes here  goto infinite loop    Why it s better than the alternatives    It s specific   goto is the language construct which causes an unconditional branch   Alternatives depend on using structures supporting conditional branches  with a degenerate always-true condition  The label documents the intent without extra comments  The reader doesn t have to scan the intervening code for early breaks  although it s still possible for an unprincipled hacker to simulate continue with an early goto     Rules    Pretend that the gotophobes didn t win   It s understood that the above can t be used in real code because it goes against established idiom  Assume that we have all heard of  Goto considered harmful  and know that goto can be used to write spaghetti code  If you disagree with an example  criticize it on technical merit alone   Because people don t like goto  is not a technical reason     Let s see if we can talk about this like grown ups   Edit  This question seems finished now   It generated some high quality answers   Thanks to everyone   especially those who took my little loop example seriously   Most skeptics were concerned  by the lack of block scope   As  quinmars pointed out in a comment  you can always put braces around the  loop body   I note in passing that for     and while true  don t give you the braces  for free either  and omitting them can cause vexing bugs    Anyway  I won t waste any more of your brain power on this trifle - I can live with the harmless and idiomatic for     and while true   just as well if I want to keep my job      Considering the other responses  I see that many people view goto as something you always  have to rewrite in another way   Of course you can avoid a goto by introducing a loop   an extra flag  a stack of nested ifs  or whatever  but why not consider whether goto is  perhaps the best tool for the job  Put another way  how much ugliness are people prepared to endure to avoid using a built-in language feature for its intended purpose  My take is that even adding a flag is too high a price to pay   I like my variables to represent things in  the problem or solution domains   Solely to avoid a goto  doesn t cut it   I ll accept the first answer which gave the C pattern for branching to a cleanup block  IMO  this makes the strongest case for a goto of all the posted answers  certainly  if you measure it by the contortions a hater has to go through to avoid it

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The classic need for GOTO in C is as follows  for       for         if breakout condition         goto final   final    There is no straightforward way to break out of nested loops without a goto

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fizzer myopenid com  your posted code snippet is equivalent to the following       while  system call      -1                if  errno    EINTR                           handle real errors              break                    I definitely prefer this form

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Knuth has written a paper  Structured programming with GOTO statements   you can get it e g  from here   You ll find many examples there

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My gripe about this is that you lose block scoping  any local variables declared between the gotos remains in force if the loop is ever broken out of    Maybe you re assuming the loop runs forever  I don t think that s what the original question writer was asking  though    The problem of scoping is more of an issue with C    as some objects may be depending on their dtor being called at appropriate times   For me  the best reason to use goto is during a multi-step initialization process where the it s vital that all inits are backed out of if one fails  a la   if  foo init      goto bye   if  bar init      goto foo bye   if  xyzzy init      goto bar bye   return TRUE   bar bye   bar terminate     foo bye    foo terminate     bye    return FALSE

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Greg   Why not do your example like this   void foo         if  doA                      if  doB                        if   doC                             UndoA                 UndoB                                  else                     UndoA                        return

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Very common   do stuff thingy        lock thingy        foo      if  foo failed            status   -EFOO          goto OUT             bar      if  bar failed            status   -EBAR          goto OUT             do stuff to thingy    OUT      unlock thingy       return status      The only case I ever use goto is for jumping forwards  usually out of blocks  and never into blocks   This avoids abuse of do  while 0  and other constructs which increase nesting  while still maintaining readable  structured code

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include  lt stdio h gt   include  lt string h gt   int main         char name 64       char url 80     The final url name with http   www  com       char  pName      int x       pName   name       INPUT      printf   nWrite the name of a web page  Without www  http   com          gets name        for x 0 x lt   strlen name   x            if   pName 0       0       pName x                                printf  Name blank or with spaces                 getch                system  cls                goto INPUT                 strcpy url  http   www         strcat url name       strcat url   com         printf   s  url       return 0

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Even though I ve grown to hate this pattern over time  it s in-grained into COM programming      define IfFailGo x   hr    x   if  FAILED hr   goto Error      HRESULT SomeMethod IFoo  pFoo      HRESULT hr   S OK    IfFailGo  pFoo- gt PerformAction        IfFailGo  pFoo- gt SomeOtherAction      Error    return hr

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Very common   do stuff thingy        lock thingy        foo      if  foo failed            status   -EFOO          goto OUT             bar      if  bar failed            status   -EBAR          goto OUT             do stuff to thingy    OUT      unlock thingy       return status      The only case I ever use goto is for jumping forwards  usually out of blocks  and never into blocks   This avoids abuse of do  while 0  and other constructs which increase nesting  while still maintaining readable  structured code

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The classic need for GOTO in C is as follows  for       for         if breakout condition         goto final   final    There is no straightforward way to break out of nested loops without a goto

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Heres one trick I ve heard of people using  I ve never seen it in the wild though  And it only applies to C because C   has RAII to do this more idiomatically    void foo         if   doA            goto exit      if   doB            goto cleanupA      if   doC            goto cleanupB          everything has succeeded        return   cleanupB      undoB    cleanupA      undoA    exit      return

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I don t use goto s myself  however I did work with a person once that would use them in specific cases   If I remember correctly  his rationale was around performance issues - he also had specific rules for how   Always in the same function  and the label was always BELOW the goto statement

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One good place to use a goto is in a procedure that can abort at several points  each of which requires various levels of cleanup   Gotophobes can always replace the gotos with structured code and a series of tests  but I think this is more straightforward because it eliminates excessive indentation    if   openDataFile      goto quit   if   getDataFromFile      goto closeFileAndQuit   if   allocateSomeResources    goto freeResourcesAndQuit      Do more work here      freeResourcesAndQuit        free resources closeFileAndQuit        close file quit        quit

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Even though I ve grown to hate this pattern over time  it s in-grained into COM programming      define IfFailGo x   hr    x   if  FAILED hr   goto Error      HRESULT SomeMethod IFoo  pFoo      HRESULT hr   S OK    IfFailGo  pFoo- gt PerformAction        IfFailGo  pFoo- gt SomeOtherAction      Error    return hr

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One good place to use a goto is in a procedure that can abort at several points  each of which requires various levels of cleanup   Gotophobes can always replace the gotos with structured code and a series of tests  but I think this is more straightforward because it eliminates excessive indentation    if   openDataFile      goto quit   if   getDataFromFile      goto closeFileAndQuit   if   allocateSomeResources    goto freeResourcesAndQuit      Do more work here      freeResourcesAndQuit        free resources closeFileAndQuit        close file quit        quit

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If Duff s device doesn t need a goto  then neither should you      void dsend int count        int n      if   count  return      n    count   7    8      switch  count   8          case 0  do   puts  case 0          case 7       puts  case 7          case 6       puts  case 6          case 5       puts  case 5          case 4       puts  case 4          case 3       puts  case 3          case 2       puts  case 2          case 1       puts  case 1                       while  --n  gt  0             code above from Wikipedia entry

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Greg   Why not do your example like this   void foo         if  doA                      if  doB                        if   doC                             UndoA                 UndoB                                  else                     UndoA                        return

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I ve seen goto used correctly but the situations are normaly ugly  It is only when the use of goto itself is so much less worse than the original   Johnathon Holland the poblem is you re version is less clear  people seem to be scared of local variables   void foo         bool doAsuccess   doA        bool doBsuccess   doAsuccess  amp  amp  doB        bool doCsuccess   doBsuccess  amp  amp  doC         if   doCsuccess                if  doBsuccess              undoB            if  doAsuccess              undoA              And I prefer loops like this but some people prefer while true    for              code goes here

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Here is an example of a good goto      No Code

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If Duff s device doesn t need a goto  then neither should you      void dsend int count        int n      if   count  return      n    count   7    8      switch  count   8          case 0  do   puts  case 0          case 7       puts  case 7          case 6       puts  case 6          case 5       puts  case 5          case 4       puts  case 4          case 3       puts  case 3          case 2       puts  case 2          case 1       puts  case 1                       while  --n  gt  0             code above from Wikipedia entry

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Heres one trick I ve heard of people using  I ve never seen it in the wild though  And it only applies to C because C   has RAII to do this more idiomatically    void foo         if   doA            goto exit      if   doB            goto cleanupA      if   doC            goto cleanupB          everything has succeeded        return   cleanupB      undoB    cleanupA      undoA    exit      return

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I have nothing against gotos in general  but I can think of several reasons why you wouldn t want to use them for a loop like you mentioned    It does not limit scope hence any temp variables you use inside won t be freed until later  It does not limit scope hence it could lead to bugs  It does not limit scope hence you cannot re-use the same variable names later in future code in the same scope   It does not limit scope hence you have the chance of skipping over a variable declaration  People are not accustomed to it and it will make your code harder to read  Nested loops of this type can lead to spaghetti code  normals loops will not lead to spaghetti code

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I have nothing against gotos in general  but I can think of several reasons why you wouldn t want to use them for a loop like you mentioned    It does not limit scope hence any temp variables you use inside won t be freed until later  It does not limit scope hence it could lead to bugs  It does not limit scope hence you cannot re-use the same variable names later in future code in the same scope   It does not limit scope hence you have the chance of skipping over a variable declaration  People are not accustomed to it and it will make your code harder to read  Nested loops of this type can lead to spaghetti code  normals loops will not lead to spaghetti code

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One good place to use a goto is in a procedure that can abort at several points  each of which requires various levels of cleanup   Gotophobes can always replace the gotos with structured code and a series of tests  but I think this is more straightforward because it eliminates excessive indentation    if   openDataFile      goto quit   if   getDataFromFile      goto closeFileAndQuit   if   allocateSomeResources    goto freeResourcesAndQuit      Do more work here      freeResourcesAndQuit        free resources closeFileAndQuit        close file quit        quit

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Even though I ve grown to hate this pattern over time  it s in-grained into COM programming      define IfFailGo x   hr    x   if  FAILED hr   goto Error      HRESULT SomeMethod IFoo  pFoo      HRESULT hr   S OK    IfFailGo  pFoo- gt PerformAction        IfFailGo  pFoo- gt SomeOtherAction      Error    return hr

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Here is an example of a good goto      No Code

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fizzer myopenid com  your posted code snippet is equivalent to the following       while  system call      -1                if  errno    EINTR                           handle real errors              break                    I definitely prefer this form

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Heres one trick I ve heard of people using  I ve never seen it in the wild though  And it only applies to C because C   has RAII to do this more idiomatically    void foo         if   doA            goto exit      if   doB            goto cleanupA      if   doC            goto cleanupB          everything has succeeded        return   cleanupB      undoB    cleanupA      undoA    exit      return

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Here s my non-silly example   from Stevens APITUE  for Unix system calls which may be interrupted by a signal   restart      if  system call      -1            if  errno    EINTR  goto restart              handle real errors         The alternative is a degenerate loop   This version reads like English  if the system call was interrupted by a signal  restart it

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Even though I ve grown to hate this pattern over time  it s in-grained into COM programming      define IfFailGo x   hr    x   if  FAILED hr   goto Error      HRESULT SomeMethod IFoo  pFoo      HRESULT hr   S OK    IfFailGo  pFoo- gt PerformAction        IfFailGo  pFoo- gt SomeOtherAction      Error    return hr

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I don t use goto s myself  however I did work with a person once that would use them in specific cases   If I remember correctly  his rationale was around performance issues - he also had specific rules for how   Always in the same function  and the label was always BELOW the goto statement

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Greg   Why not do your example like this   void foo         if  doA                      if  doB                        if   doC                             UndoA                 UndoB                                  else                     UndoA                        return

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My gripe about this is that you lose block scoping  any local variables declared between the gotos remains in force if the loop is ever broken out of    Maybe you re assuming the loop runs forever  I don t think that s what the original question writer was asking  though    The problem of scoping is more of an issue with C    as some objects may be depending on their dtor being called at appropriate times   For me  the best reason to use goto is during a multi-step initialization process where the it s vital that all inits are backed out of if one fails  a la   if  foo init      goto bye   if  bar init      goto foo bye   if  xyzzy init      goto bar bye   return TRUE   bar bye   bar terminate     foo bye    foo terminate     bye    return FALSE

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I ve seen goto used correctly but the situations are normaly ugly  It is only when the use of goto itself is so much less worse than the original   Johnathon Holland the poblem is you re version is less clear  people seem to be scared of local variables   void foo         bool doAsuccess   doA        bool doBsuccess   doAsuccess  amp  amp  doB        bool doCsuccess   doBsuccess  amp  amp  doC         if   doCsuccess                if  doBsuccess              undoB            if  doAsuccess              undoA              And I prefer loops like this but some people prefer while true    for              code goes here

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I ve seen goto used correctly but the situations are normaly ugly  It is only when the use of goto itself is so much less worse than the original   Johnathon Holland the poblem is you re version is less clear  people seem to be scared of local variables   void foo         bool doAsuccess   doA        bool doBsuccess   doAsuccess  amp  amp  doB        bool doCsuccess   doBsuccess  amp  amp  doC         if   doCsuccess                if  doBsuccess              undoB            if  doAsuccess              undoA              And I prefer loops like this but some people prefer while true    for              code goes here

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include  lt stdio h gt   include  lt string h gt   int main         char name 64       char url 80     The final url name with http   www  com       char  pName      int x       pName   name       INPUT      printf   nWrite the name of a web page  Without www  http   com          gets name        for x 0 x lt   strlen name   x            if   pName 0       0       pName x                                printf  Name blank or with spaces                 getch                system  cls                goto INPUT                 strcpy url  http   www         strcat url name       strcat url   com         printf   s  url       return 0

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Knuth has written a paper  Structured programming with GOTO statements   you can get it e g  from here   You ll find many examples there

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Here s my non-silly example   from Stevens APITUE  for Unix system calls which may be interrupted by a signal   restart      if  system call      -1            if  errno    EINTR  goto restart              handle real errors         The alternative is a degenerate loop   This version reads like English  if the system call was interrupted by a signal  restart it

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Knuth has written a paper  Structured programming with GOTO statements   you can get it e g  from here   You ll find many examples there

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Greg   Why not do your example like this   void foo         if  doA                      if  doB                        if   doC                             UndoA                 UndoB                                  else                     UndoA                        return

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Knuth has written a paper  Structured programming with GOTO statements   you can get it e g  from here   You ll find many examples there

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If Duff s device doesn t need a goto  then neither should you      void dsend int count        int n      if   count  return      n    count   7    8      switch  count   8          case 0  do   puts  case 0          case 7       puts  case 7          case 6       puts  case 6          case 5       puts  case 5          case 4       puts  case 4          case 3       puts  case 3          case 2       puts  case 2          case 1       puts  case 1                       while  --n  gt  0             code above from Wikipedia entry

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The classic need for GOTO in C is as follows  for       for         if breakout condition         goto final   final    There is no straightforward way to break out of nested loops without a goto

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My gripe about this is that you lose block scoping  any local variables declared between the gotos remains in force if the loop is ever broken out of    Maybe you re assuming the loop runs forever  I don t think that s what the original question writer was asking  though    The problem of scoping is more of an issue with C    as some objects may be depending on their dtor being called at appropriate times   For me  the best reason to use goto is during a multi-step initialization process where the it s vital that all inits are backed out of if one fails  a la   if  foo init      goto bye   if  bar init      goto foo bye   if  xyzzy init      goto bar bye   return TRUE   bar bye   bar terminate     foo bye    foo terminate     bye    return FALSE

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fizzer myopenid com  your posted code snippet is equivalent to the following       while  system call      -1                if  errno    EINTR                           handle real errors              break                    I definitely prefer this form

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I have nothing against gotos in general  but I can think of several reasons why you wouldn t want to use them for a loop like you mentioned    It does not limit scope hence any temp variables you use inside won t be freed until later  It does not limit scope hence it could lead to bugs  It does not limit scope hence you cannot re-use the same variable names later in future code in the same scope   It does not limit scope hence you have the chance of skipping over a variable declaration  People are not accustomed to it and it will make your code harder to read  Nested loops of this type can lead to spaghetti code  normals loops will not lead to spaghetti code

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My gripe about this is that you lose block scoping  any local variables declared between the gotos remains in force if the loop is ever broken out of    Maybe you re assuming the loop runs forever  I don t think that s what the original question writer was asking  though    The problem of scoping is more of an issue with C    as some objects may be depending on their dtor being called at appropriate times   For me  the best reason to use goto is during a multi-step initialization process where the it s vital that all inits are backed out of if one fails  a la   if  foo init      goto bye   if  bar init      goto foo bye   if  xyzzy init      goto bar bye   return TRUE   bar bye   bar terminate     foo bye    foo terminate     bye    return FALSE

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Here is an example of a good goto      No Code

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I ve seen goto used correctly but the situations are normaly ugly  It is only when the use of goto itself is so much less worse than the original   Johnathon Holland the poblem is you re version is less clear  people seem to be scared of local variables   void foo         bool doAsuccess   doA        bool doBsuccess   doAsuccess  amp  amp  doB        bool doCsuccess   doBsuccess  amp  amp  doC         if   doCsuccess                if  doBsuccess              undoB            if  doAsuccess              undoA              And I prefer loops like this but some people prefer while true    for              code goes here

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Here s my non-silly example   from Stevens APITUE  for Unix system calls which may be interrupted by a signal   restart      if  system call      -1            if  errno    EINTR  goto restart              handle real errors         The alternative is a degenerate loop   This version reads like English  if the system call was interrupted by a signal  restart it

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Very common   do stuff thingy        lock thingy        foo      if  foo failed            status   -EFOO          goto OUT             bar      if  bar failed            status   -EBAR          goto OUT             do stuff to thingy    OUT      unlock thingy       return status      The only case I ever use goto is for jumping forwards  usually out of blocks  and never into blocks   This avoids abuse of do  while 0  and other constructs which increase nesting  while still maintaining readable  structured code

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Very common   do stuff thingy        lock thingy        foo      if  foo failed            status   -EFOO          goto OUT             bar      if  bar failed            status   -EBAR          goto OUT             do stuff to thingy    OUT      unlock thingy       return status      The only case I ever use goto is for jumping forwards  usually out of blocks  and never into blocks   This avoids abuse of do  while 0  and other constructs which increase nesting  while still maintaining readable  structured code

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If Duff s device doesn t need a goto  then neither should you      void dsend int count        int n      if   count  return      n    count   7    8      switch  count   8          case 0  do   puts  case 0          case 7       puts  case 7          case 6       puts  case 6          case 5       puts  case 5          case 4       puts  case 4          case 3       puts  case 3          case 2       puts  case 2          case 1       puts  case 1                       while  --n  gt  0             code above from Wikipedia entry

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fizzer myopenid com  your posted code snippet is equivalent to the following       while  system call      -1                if  errno    EINTR                           handle real errors              break                    I definitely prefer this form

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I don t use goto s myself  however I did work with a person once that would use them in specific cases   If I remember correctly  his rationale was around performance issues - he also had specific rules for how   Always in the same function  and the label was always BELOW the goto statement

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The classic need for GOTO in C is as follows  for       for         if breakout condition         goto final   final    There is no straightforward way to break out of nested loops without a goto

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I have nothing against gotos in general  but I can think of several reasons why you wouldn t want to use them for a loop like you mentioned    It does not limit scope hence any temp variables you use inside won t be freed until later  It does not limit scope hence it could lead to bugs  It does not limit scope hence you cannot re-use the same variable names later in future code in the same scope   It does not limit scope hence you have the chance of skipping over a variable declaration  People are not accustomed to it and it will make your code harder to read  Nested loops of this type can lead to spaghetti code  normals loops will not lead to spaghetti code

User · Answer

I don t use goto s myself  however I did work with a person once that would use them in specific cases   If I remember correctly  his rationale was around performance issues - he also had specific rules for how   Always in the same function  and the label was always BELOW the goto statement

User · Answer

Here s my non-silly example   from Stevens APITUE  for Unix system calls which may be interrupted by a signal   restart      if  system call      -1            if  errno    EINTR  goto restart              handle real errors         The alternative is a degenerate loop   This version reads like English  if the system call was interrupted by a signal  restart it

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Here is an example of a good goto      No Code

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One good place to use a goto is in a procedure that can abort at several points  each of which requires various levels of cleanup   Gotophobes can always replace the gotos with structured code and a series of tests  but I think this is more straightforward because it eliminates excessive indentation    if   openDataFile      goto quit   if   getDataFromFile      goto closeFileAndQuit   if   allocateSomeResources    goto freeResourcesAndQuit      Do more work here      freeResourcesAndQuit        free resources closeFileAndQuit        close file quit        quit

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Heres one trick I ve heard of people using  I ve never seen it in the wild though  And it only applies to C because C   has RAII to do this more idiomatically    void foo         if   doA            goto exit      if   doB            goto cleanupA      if   doC            goto cleanupB          everything has succeeded        return   cleanupB      undoB    cleanupA      undoA    exit      return

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