Question

Im writing a program that should read input via stdin, so I have the following contruct.

FILE *fp=stdin;

But this just hangs if the user hasn’t piped anything into the program, how can I check if the user is actually piping data into my program like

gunzip -c file.gz |./a.out #should work
./a.out  #should exit program with nice msg.

thanks

Answer 1

Since you’re using file pointers, you’ll need both isatty() and fileno() to do this:

#include <unistd.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
    FILE* fp = stdin;

    if(isatty(fileno(fp)))
    {
        fprintf(stderr, "A nice msg.\n");
        exit(1);
    }

    /* carry on... */
    return 0;
}

Actually, that’s the long way. The short way is to not use file pointers:

#include <unistd.h>

int main(int argc, char* argv[])
{
    if(isatty(STDIN_FILENO))
    {
        fprintf(stderr, "A nice msg.\n");
        exit(1);
    }

    /* carry on... */
    return 0;
}

Several standard Unix programs do this check to modify their behavior. For example, if you have ls set up to give you pretty colors, it will turn the colors off if you pipe its stdout to another program.