Multiple fork and printing something before exec - c

Question

// I have commands in commands[] array
pid_t pid[command_count];
for (int i = 0; i < command_count; i++) {
if ((pid[i]=fork()) == 0) {
printf("--%s\n", commands[i][0]);
execvp(commands[i][0],commands[i]);
_exit(1);
}
if (pid[i] < 0) {
}
}
for (i = 0; i < command_count; i++) {
if (pid[i] > 0) {
int status;
waitpid(pid[i], &status, 0);
}
}
I have the above code and want to run commands at once (paralel) that is in commands array and before each run, want to print the command. For example;
ls | pwd | ls -a
It should print each command name before that command run like
--ls
.. a b.txt
--pwd
/a/a/
--ls -a
*some output*
But it prints like following
--ls
--pwd
--ls -a
.. a b.txt
*some directory as a result of pwd*
*some output*
What could be the reason and how can I fix it?

Answer 1

You will have to use pipe() to create new standard output and optionally standard error file descriptors for each command. Then you can read the pipes in order until each command completes.
Otherwise, because each command is forked into its own process it will run at its own convenience. Text output from commands running all at once and producing output to the same terminal can be mixed up even more than you show here.
Maybe something like
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>
struct cmd_data {
const char *cmd;
int fd[2]; // stdout pipe for command
pid_t pid;
int status; // exit status
};
void cmd_launch(struct cmd_data *p, const char *cmd) {
int r;
p->cmd = cmd;
r = pipe(p->fd);
if(r<0) {
perror("pipe");
exit(EXIT_FAILURE);
}
r = fork();
if(r < 0) {
perror("fork");
close(p->fd[0]);
close(p->fd[1]);
} else if( r > 0 ) {
p->pid = r;
close(p->fd[1]);
} else {
close(p->fd[0]);
dup2(p->fd[1], STDOUT_FILENO);
close(p->fd[1]);
r = execlp(cmd, cmd, NULL);
perror("execlp");
exit(EXIT_FAILURE);
}
}
void cmd_join(struct cmd_data *p) {
char buf[4096];
const size_t buflen = sizeof buf;
ssize_t bytes;
printf("-- %s\n", p->cmd);
fflush(stdout);
while( 0 != (bytes = read(p->fd[0], buf, buflen)) ) {
write(STDOUT_FILENO, buf, bytes);
}
close(p->fd[0]);
pid_t r = waitpid(p->pid, &p->status, 0);
if(r<0){
perror("waitpid");
}
printf("-- completed with status %d\n", p->status);
fflush(stdout);
}
int main(int argc, char *argv[]) {
size_t cmd_c = argc - 1;
struct cmd_data *p = calloc(argc, sizeof *p);
size_t i;
for(i = 0; i < cmd_c; ++i) {
cmd_launch(p + i, argv[i + 1]);
}
for(i = 0; i < cmd_c; ++i) {
cmd_join(p + i);
}
free(p);
return EXIT_SUCCESS;
}