Execute program from within a C program

You want to use popen. It gives you a unidirectional pipe with which you can access stdin and stdout of the program.

popen is standard on modern unix and unix-like OS, of which Linux is one :-)

Type

man popen

in a terminal to read more about it.

EDIT

Whether popen produces unidirectional or bidirectional pipes depends on the implementation. In Linux and OpenBSD, popen produces unidirectional pipes, which are read-only or write-only. On OS X, FreeBSD and NetBSD popen produces bidirectional pipes.

I wrote some example C code for someone else a while back that shows how to do this. Here it is for you:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

void error(char *s);
char *data = "Some input data\n";

main()
{
  int in[2], out[2], n, pid;
  char buf[255];

  /* In a pipe, xx[0] is for reading, xx[1] is for writing */
  if (pipe(in) < 0) error("pipe in");
  if (pipe(out) < 0) error("pipe out");

  if ((pid=fork()) == 0) {
    /* This is the child process */

    /* Close stdin, stdout, stderr */
    close(0);
    close(1);
    close(2);
    /* make our pipes, our new stdin,stdout and stderr */
    dup2(in[0],0);
    dup2(out[1],1);
    dup2(out[1],2);

    /* Close the other ends of the pipes that the parent will use, because if
     * we leave these open in the child, the child/parent will not get an EOF
     * when the parent/child closes their end of the pipe.
     */
    close(in[1]);
    close(out[0]);

    /* Over-write the child process with the hexdump binary */
    execl("/usr/bin/hexdump", "hexdump", "-C", (char *)NULL);
    error("Could not exec hexdump");
  }

  printf("Spawned 'hexdump -C' as a child process at pid %d\n", pid);

  /* This is the parent process */
  /* Close the pipe ends that the child uses to read from / write to so
   * the when we close the others, an EOF will be transmitted properly.
   */
  close(in[0]);
  close(out[1]);

  printf("<- %s", data);
  /* Write some data to the childs input */
  write(in[1], data, strlen(data));

  /* Because of the small amount of data, the child may block unless we
   * close it's input stream. This sends an EOF to the child on it's
   * stdin.
   */
  close(in[1]);

  /* Read back any output */
  n = read(out[0], buf, 250);
  buf[n] = 0;
  printf("-> %s",buf);
  exit(0);
}

void error(char *s)
{
  perror(s);
  exit(1);
}

1. Create two pipes with pipe(...), one for stdin, one for stdout.
2. fork(...) the process.
3. In the child process (the one where fork(...) returns 0) dup (...) the pipes to stdin/stdout.
4. exec[v][e] the to be started programm file in the child process.
5. In the parent process (the one where fork) returns the PID of the child) do a loop that reads from the child's stdout (select(...) or poll(...), read(...) ) into a buffer, until the child terminates (waitpid(...)).
6. Eventually supply the child with input on stdin if it expects some.
7. When done close(...) the pipes.