C multiple pipes - c

Question

I am trying to implement multiple pipes in C like
ls - al | less | wc
I have trouble with creating the pipeline. I have a loop that is supposed to create the processes and connect them with pipes:
for(i=0;i<num_cmds;i++){
create_commands(cmds[i]);
}
My create_commands() function looks like this
void create_commands (char cmd[MAX_CMD_LENGTH]) // Command be processed
{
int pipeid[2];
pipe(pipeid);
if (childpid = fork())
{
/* this is the parent process */
dup2(pipeid[1], 1); // dup2() the write end of the pipe to standard output.
close(pipeid[1]); // close() the write end of the pipe
//parse the command
parse_command(cmd, argvector);
// execute the command
execvp(argvector[0], argvector);
close(1); // close standard output
}
else
{
/* child process */
dup2( pipeid[0], 0); // the read end of the pipe to standard input
close( pipeid[0] ); // close() the read end of the pipe
}
}
But this doesn't work, I'm getting my stdin and stdout messed up.
Could anyone please point me to what I am doing wrong?
Thank you in advance!

Answer 1

The popen() function executes the command specified by the string command. It creates a pipe between the calling program and the executed command, and returns a pointer to a stream that can be used to either read from or write to the pipe.
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *fp;
int status;
int PATH_MAX = 1024;
char path[PATH_MAX];
fp = popen("ls -al | less | wc", "r");
if (fp == NULL)
/* Handle error */;
while (fgets(path, PATH_MAX, fp) != NULL)
printf("%s", path);
status = pclose(fp);
if (status == -1) {
/* Error reported by pclose() */
} else {
/* Use macros described under wait() to inspect `status' in order
to determine success/failure of command executed by popen() */
}
}
You can use a preset string to be called within popen(), you can also use your argv[] arguments to be piped in you'ld like.
popen() gives you a pipe, a FIFO First In First Out stream, and popen also feeds the STDOUT back to your program.
Here's the man page for popen():
http://linux.die.net/man/3/popen