Pipe stdout of system() to stdin of other system()

Question

I want to learn how to use pipes in C, and tried to do basic things like for example cloning the behaviour of | in shell.

This is my first try:

#include <stdio.h>
#include <stdlib.h>

#include <unistd.h>


int main(void)
{
    FILE    *stdin_tmp;

    stdin_tmp   = stdin;
    stdin       = stdout;
    system("cat /tmp/test.txt");
    system("less");
    stdin       = stdin_tmp;

    return  0;
}

This is what I want to do (written in shell):

cat /tmp/test.txt |less

The behaviour is obviously not what I expected. less isn't receiving the output of cat.

How is it done correctly?

Answer 1

Try the popen() function.

Here's the prototype for it:

FILE *popen(const char *command, const char *type);

And here's a correct way to use it:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int ch;
    FILE *input;
    FILE *output;

    input = popen("cat /tmp/test.txt", "r");
    output = popen("less", "w");
    if (!input || !output)
        return EXIT_FAILURE;
    while( (ch = fgetc(input)) != EOF )
        fputc(ch, output);
    pclose(input);
    pclose(output);

    return 0;
}