C C line number

Question

In the sake of debugging purposes  can I get the line number in C C   compilers   standard way or specific ways for certain compilers   e g  if  Logical      printf  Not logical value at line number  d  n  LineNumber          How to get LineNumber without writing it by my hand  dynamic compilation

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Use   LINE    that s double-underscore LINE double-underscore   the preprocessor will replace it with the line number on which it is encountered

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Checkout   FILE   and   LINE   macros

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For those who might need it  a  FILE LINE  macro to easily print file and line    define STRINGIZING x   x  define STR x  STRINGIZING x   define FILE LINE   FILE       STR   LINE

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C  20 offers a new way to achieve this by using std  source location  This is currently accessible in gcc an clang as std  experimental  source location with  include  lt experimental source location gt    The problem with macros like   LINE   is that if you want to create for example a logging function that outputs the current line number along with a message  you always have to pass   LINE   as a function argument  because it is expanded at the call site  Something like this   void log const std  string msg        std  cout  lt  lt    LINE    lt  lt       lt  lt  msg  lt  lt  std  endl      Will always output the line of the function declaration and not the line where log was actually called from  On the other hand  with std  source location you can write something like this    include  lt experimental source location gt  using std  experimental  source location   void log const std  string msg  const source location loc   source location  current          std  cout  lt  lt  loc line    lt  lt       lt  lt  msg  lt  lt  std  endl      Here  loc is initialized with the line number pointing to the location where log was called  You can try it online here

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Use   LINE    but what is its type      LINE The presumed line number  within the current source file  of the current source line  an integer constant     As an integer constant  code can often assume the value is   LINE    lt   INT MAX and so the type is  int   To print in C  printf   needs the matching specifier    d    This is a far lesser  concern in C   with cout   Pedantic concern  If the line number exceeds INT MAX1  somewhat conceivable with 16-bit int    hopefully the compiler will produce a warning   Example   format   d  expects argument of type  int   but argument 2 has type  long int   -Wformat     Alternatively  code could force wider types to forestall such warnings   printf  Not logical value at line number  ld n    long    LINE       or  include  lt stdint h gt  printf  Not logical value at line number  jd n   INTMAX C   LINE          Avoid printf    To avoid all integer limitations  stringify   Code could directly print without a printf   call  a nice thing to avoid in error handling2     define xstr a  str a   define str a   a  fprintf stderr   Not logical value at line number  s n   xstr   LINE      fputs  Not logical value at line number   xstr   LINE      n   stderr       1 Certainly poor programming practice to have such a large file  yet perhaps machine generated code may go high   2 In debugging  sometimes code simply is not working as hoped   Calling complex functions like  printf   can itself incur issues vs  a simple fputs

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You should use the preprocessor macro   LINE   and   FILE    They are predefined macros and part of the C C   standard  During preprocessing  they are replaced respectively by a constant string holding an integer representing the current line number and by the current file name   Others preprocessor variables       func     function name  this is part of C99  not all C   compilers support it    DATE     a string of form  Mmm dd yyyy    TIME     a string of form  hh mm ss      Your code will be    if  Logical    printf  Not logical value at line number  d in file  s n     LINE      FILE

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You could use a macro with the same behavior as printf     except that it also includes debug information such as  function name  class  and line number    include  lt cstdio gt     needed for printf  define print a  args     printf   s  s  d    a     func     FILE      LINE      args   define println a  args     print a   n     args    These macros should behave identically to printf    while including java stacktrace-like information  Here s an example main   void exampleMethod         println  printf   syntax  string    s  int    d    foobar   42      int main int argc  char   argv        print  Before exampleMethod      n        exampleMethod        println  Success         Which results in the following output      main main cpp 11  Before exampleMethod        exampleMethod main cpp 7  printf   syntax  string   foobar  int   42   main main cpp 13  Success

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Try   FILE   and   LINE    You might also find   DATE   and   TIME   useful  Though unless you have to debug a program on the clientside and thus need to log these informations you should use normal debugging

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As part of the C   standard there exists some pre-defined macros that you can use   Section 16 8 of the C   standard defines amongst other things  the   LINE   macro        LINE    The line number of the current source line  a decimal   constant       FILE    The presumed name of the source file  a character string   literal       DATE    The date of translation of the source file  a character string   literal         TIME    The time of translation of the source file  a character string   literal         STDC    Whether  STDC   is predefined     cplusplus  The name   cplusplus is defined to the value 199711L when   compiling a C    translation unit   So your code would be   if  Logical    printf  Not logical value at line number  d  n    LINE

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Since i m also facing this problem now and i cannot add an answer to a different but also valid question asked here  i ll provide an example solution for the problem of  getting only the line number of where the function has been called in C   using templates  Background  in C   one can use non-type integer values as a template argument  This is different than the typical usage of data types as template arguments  So the idea is to use such integer values for a function call   include  lt iostream gt   class Test      public          template lt unsigned int L gt          int test                std  cout  lt  lt   quot the function has been called at line number   quot   lt  lt  L  lt  lt  std  endl              return 0                    int test    return this- gt test lt 0 gt           int main int argc  char   argv       Test t      t test        t test lt   LINE   gt         return 0     Output   the function has been called at line number  0 the function has been called at line number  16  One thing to mention here is that in C  11 Standard it s possible to give default template values for functions using template  In pre C  11 default values for non-type arguments seem to only work for class template arguments  Thus  in C  11  there would be no need to have duplicate function definitions as above  In C  11 its also valid to have const char  template arguments but its not possible to use them with literals like   FILE   or   func   as mentioned here  So in the end if you re using C   or C  11 this might be a very interesting alternative than using macro s to get the calling line

[c++] C/C++ line number

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