FILE LINE and FUNCTION usage in C

Question

Presuming that your C   compiler supports them  is there any particular reason not to use   FILE      LINE   and   FUNCTION   for logging and debugging purposes   I m primarily concerned with giving the user misleading data   for example  reporting the incorrect line number or function as a result of optimization   or taking a performance hit as a result   Basically  can I trust   FILE      LINE   and   FUNCTION   to always do the right thing

User · Answer

In rare cases  it can be useful to change the line that is given by   LINE   to something else  I ve seen GNU configure does that for some tests to report appropriate line numbers after it inserted some voodoo between lines that do not appear in original source files  For example    line 100   Will make the following lines start with   LINE   100  You can optionally add a new file-name   line 100  file c    It s only rarely useful  But if it is needed  there are no alternatives I know of  Actually  instead of the line  a macro can be used too which must result in any of the above two forms  Using the boost preprocessor library  you can increment the current line by 50    line BOOST PP ADD   LINE    50    I thought it s useful to mention it since you asked about the usage of   LINE   and   FILE    One never gets enough surprises out of C       Edit   Jonathan Leffler provides some more good use-cases in the comments      Messing with  line is very useful for pre-processors that want to keep errors reported in the user s C code in line with the user s source file  Yacc  Lex  and  more at home to me  ESQL C preprocessors do that

User · Answer

FUNCTION   is non standard    func   exists in C99   C  11  The others    LINE   and   FILE    are just fine  It will always report the right file and line  and function if you choose to use   FUNCTION     func     Optimization is a non-factor since it is a compile time macro expansion  it will never affect performance in any way

User · Answer

Personally  I m reluctant to use these for anything but debugging messages  I have done it  but I try not to show that kind of information to customers or end users  My customers are not engineers and are sometimes not computer savvy  I might log this info to the console  but  as I said  reluctantly except for debug builds or for internal tools  I suppose it does depend on the customer base you have  though

User · Answer

I use them all the time  The only thing I worry about is giving away IP in log files  If your function names are really good you might be making a trade secret easier to uncover  It s sort of like shipping with debug symbols  only more difficult to find things  In 99 999  of the cases nothing bad will come of it

User · Answer

C  20 std  source location  C   has finally added a non-macro option  and it will likely dominate at some point in the future when C  20 becomes widespread    https   en cppreference com w cpp utility source location http   www open-std org jtc1 sc22 wg21 docs papers 2019 p1208r5 pdf   The documentation says      constexpr const char  function name   const noexcept       6 Returns  If this object represents a position in the body of a function    returns an implementation-defined NTBS that should correspond to the   function name  Otherwise  returns an empty string    where NTBS means  Null Terminated Byte String    I ll give it a try when support arrives to GCC  GCC 9 1 0 with g  -9 -std c  2a still doesn t support it   https   en cppreference com w cpp utility source location claims usage will be like    include  lt iostream gt   include  lt string view gt   include  lt source location gt   void log std  string view message           const std  source location amp  location std  source location  current           std  cout  lt  lt   info                  lt  lt  location file name    lt  lt                     lt  lt  location line    lt  lt                     lt  lt  location function name    lt  lt                     lt  lt  message  lt  lt    n      int main         log  Hello world         Possible output   info main cpp 16 main Hello world      PRETTY FUNCTION   vs   FUNCTION   vs   func   vs std  source location  function name  Answered at  What  39 s the difference between   PRETTY FUNCTION      FUNCTION      func

User · Answer

FYI   g   offers the non-standard   PRETTY FUNCTION   macro   Until just now I did not know about C99   func    thanks Evan     I think I still prefer   PRETTY FUNCTION   when it s available for the extra class scoping   PS   static string  getScopedClassMethod  string thePrettyFunction       size t index   thePrettyFunction   find           if   index    string  npos       return thePrettyFunction      Degenerate case       thePrettyFunction   erase  index       index   thePrettyFunction   rfind           if   index    string  npos       return thePrettyFunction      Degenerate case       thePrettyFunction   erase  0  index   1       return thePrettyFunction       The scoped class name

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