Question

I want to transfer some parameters from one program to another.

For example, here are two programs. a.c compiled as a

#include <stdio.h>

int main() {
    char a[10];
    scanf("%s", a);
    printf("%s\n", a);
    return 0;
}

and e.c compiled as e:

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

int main() {
    char *input = "here it is";
    execl("./a", "a", "a",  NULL);
    return 0;
}

Actually, I would like to transfer the parameter “a” from the e.c to a.c so that once I execute the program “e”, it will print out

 a
 a

However, I find out that excel cannot pass the parameters to the specified program.

If I cannot modify the program a.c, how can I execute this program using another program using standard input?

Answer 1

If you can’t modify a.c, then you’ll need to write the data to some file, then reset stdin to read from that file:

FILE *fp = fopen("somefile", "w");
if (fp == NULL)
    // handle error
fputs(input, fp);
fclose(fp);

if (freopen("somefile", "r", stdin) == NULL)
    // handle error
unlink("somefile");
execl("./a", "a", NULL);

The unlink system call makes sure the file disappears when a is done with it.

Note: you might be able to use a pipe instead of a file, because the message is so small that it will fit inside a pipe’s buffer. That’s not a reliable solution for larger messages, though — if you want to use a pipe, you need to fork.