How I can print to stderr in C

Question

In C  Printing to stdout is easy  with printf from stdio h   However  how can print to stderr  We can use fprintf to achieve it apparently  but its syntax seems strange  Maybe we can use printf to print to stderr

User · Answer

Do you know sprintf  It s basically the same thing with fprintf  The first argument is the destination  the file in the case of fprintf i e  stderr   the second argument is the format string  and the rest are the arguments as usual   I also recommend this printf  and family  reference

User · Answer

The syntax is almost the same as printf  With printf you give the string format and its contents ie   printf  my  s has  d chars n    string format   30     With fprintf it is the same  except now you are also specifying the place to print to   File  myFile      fprintf  myFile   my  s has  d chars n    string format   30     Or in your case   fprintf  stderr   my  s has  d chars n    string format   30

User · Answer

If you don t want to modify current codes and just for debug usage   Add this macro    define printf args     fprintf stderr    args    under GCC  define printf args     fprintf stderr    VA ARGS      under MSVC   Change stderr to stdout if you want to roll back   It s helpful for debug  but it s not a good practice

User · Answer

Some examples of formatted output to stdout and stderr  printf  quot  s quot    quot Hello world n quot                     quot Hello world quot  on stdout  using printf  fprintf stdout   quot  s quot    quot Hello world n quot            quot Hello world quot  on stdout  using fprintf  fprintf stderr   quot  s quot    quot Stack overflow  n quot       Error message on stderr  using fprintf

User · Answer

include lt stdio h gt   int main           printf   hello          fprintf  stderr   HELP          printf    world n         return 0         a exe HELP hello  world     a exe 2 gt  tmp1 hello  world     a exe 1 gt  tmp1 HELP      stderr is usually unbuffered and stdout usually is  This can lead to odd looking output like this  which suggests code is executing in the wrong order  It isn t  it s just that the stdout buffer has yet to be flushed  Redirected or piped streams would of course not see this interleave as they would normally only see the output of stdout only or stderr only  Although initially both stdout and stderr come to the console  both are separate and can be individually redirected

User · Answer

To print your context  you can write code like this    FILE  fp  char  of  sprintf of   s s  text1 text2   fp fopen of  w    fprintf fp  your print line

[c] How I can print to stderr in C?

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