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cstdincat

C program does not work as expected when stdin is supplied using cat

Here is a short C program:

#include<stdio.h>

int main (void){

    char cmd;

    while(scanf("%c", &cmd) != EOF){

        if(cmd == 'q'){
            printf("Thanks\n");
            return 0;
        }
    }

return 0;
}

When the following program is executed and value of cmd is typed using keyboard, everything works as expected.

$ ./catproblem
q
Thanks
$

However, when I am trying to pipe the input using cat, the program doesn't terminate right away when q is entered. For some reason it waits for any other input and only then terminates.

$ cat | ./catproblem
q
Thanks
anything
$

What causes this behavior? And can this be fixed, so that the program works as expected if cat is used for input?

Solution

  • The problem is that cat has not terminated, and your shell (bash) waits for all processes in the pipeline to terminate before continuing. When you type the line with anything, cat attempts to write to the pipe, gets a SIGPIPE signal, and terminates.