[SOLVED] Where stdout leads

ModMaker · 12-28-2010, 08:49 PM

I am writing a program in C and I need to determine if stdout has been redirected. I know that I can write to /dev/tty to write to the screen. I want to print to the screen always but if stdout is redirected, to print a log to that file.

So is there a way to determine if stdout has been redirected?

wje_lq · 12-28-2010, 10:47 PM

Quote:

Originally Posted by ModMaker

I want to print to the screen always but if stdout is redirected, to print a log to that file.

Perhaps a more useful way too look at it: you want to print to stdout always, but if stdout is redirected, print also to /dev/tty.

Quote:

Originally Posted by ModMaker

So is there a way to determine if stdout has been redirected?

Sure. A quick and dirty way is to use isatty(STDOUT_FILENO).

ModMaker · 12-29-2010, 02:14 AM

Thanks, it worked.

wje_lq · 12-29-2010, 03:57 AM

Quote:

Originally Posted by ModMaker

Thanks, it worked.

You're welcome. But I'll bet you're not welcoming the task of all those duplicate print statements. Try something like what's in this shell script. (Run it, you'll like it.)

Code:

rm -f 1 1.x;cat > 1.c <<EOD;gcc -Wall -Werror 1.c -o 1;./1 aaa;./1 bbb >> 1.x
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>      /* for exit() */
#include <unistd.h>      /* for STDOUT_FILENO */

FILE *dev_tty;

void printboth(char *the_format,
               ...
              )
{
  va_list argument_list;

  va_start(argument_list,the_format);
  vfprintf(stdout,the_format,argument_list);
  va_end(argument_list);

  if(isatty(STDOUT_FILENO)==0)
  {
    va_start(argument_list,the_format);
    vfprintf(dev_tty,the_format,argument_list);
    va_end(argument_list);
  }

} /* printboth() */

int main(int    argc,
         char **argv
        )
{
  dev_tty=fopen("/dev/tty","w");

  if(dev_tty==NULL)
  {
    fprintf(stderr,"could not open /dev/tty\n");

    exit(1);
  }

  printboth("running with argument %s\n",
            argv[1]
           );

  return 0;

} /* main() */
EOD