C - pipe without using popen - c

Question

how can I transform this:
FILE *f;
char in_buffer[80];
f=popen("command","r");
fgets(in_buffer,sizeof(in_buffer),f)
without using popen(), but only pipe() or other instruction?

Answer 1

Here's my simple implementation, with comments explaining what's being done.
#include <unistd.h>
#include <stdio.h>
FILE *
my_popen (const char *cmd)
{
int fd[2];
int read_fd, write_fd;
int pid;
/* First, create a pipe and a pair of file descriptors for its both ends */
pipe(fd);
read_fd = fd[0];
write_fd = fd[1];
/* Now fork in order to create process from we'll read from */
pid = fork();
if (pid == 0) {
/* Child process */
/* Close "read" endpoint - child will only use write end */
close(read_fd);
/* Now "bind" fd 1 (standard output) to our "write" end of pipe */
dup2(write_fd,1);
/* Close original descriptor we got from pipe() */
close(write_fd);
/* Execute command via shell - this will replace current process */
execl("/bin/sh", "sh", "-c", cmd, NULL);
/* Don't let compiler be angry with us */
return NULL;
} else {
/* Parent */
/* Close "write" end, not needed in this process */
close(write_fd);
/* Parent process is simpler - just create FILE* from file descriptor,
for compatibility with popen() */
return fdopen(read_fd, "r");
}
}
int main ()
{
FILE *p = my_popen ("ls -l");
char buffer[1024];
while (fgets(buffer, 1024, p)) {
printf (" => %s", buffer);
}
fclose(p);
}
Notes:
Thir code supports only "r" mode of popen. Implementing other modes, namely "w" mode is left as an exercise for the reader.
System functions used in this example may fail - error handling is left as an exercise for the reader.
Implementation of pclose is left as an exercise for the reader - see close, waiptid, and fclose.
If you want to look at real impementations, you can look into sources of OSX, GNU glibc and OpenSolaris, among others.
Hope this helps!