Function to create processes in C [duplicate] - c

Question

This question already has answers here:
implementing a shell in C
(2 answers)
Closed 9 years ago.
im trying to implement a shell using C.
I'm currently trying to deal with the pipes of the program with no luck yet. I was wondering how could i process a command such as
SHELL$: sort < txtFile | grep key
i parsed the command line so that i have sort < txtFile in one char* array, and grep key in another char* array
any help would be greatly appreciated.
this is the code i have so far:
PS: i didnt include the code that deals with the redirects
pid = fork();
if(pid == 0)
{
/* child process */
if( PIPE_FLAG )
{
close(c2p[0]);
if(dup2(c2p[1], STDOUT_FILENO) == -1){
perror("dup2() failed");
exit(2);
}
}
/* Execute command */
execvp(cmd_args[0], cmd_args);
perror("exec failed 1. "); /* return only when exec fails */
exit(-1);
}
else if(pid > 0)
{
/* parent process */
if(!async)
waitpid(pid, NULL, 0);
else
printf("this is an async call\n");
if(PIPE_FLAG)
{
close(c2p[1]);
if(dup2(c2p[0], STDIN_FILENO) == -1){
perror("dup2() failed");
exit(-1);
}
execvp(nxt_args[0], nxt_args);
perror("exec failed 2. ");
exit(-1);
}
}
else
{
/* error occurred */
perror("fork failed");
exit(1);
}
this is my main:
int main (int argc, char* argv [])
{
char commands[BUFSIZ];
for(;;)
{
printf("MYSHELL$ ");
if(fgets(commands, BUFSIZ, stdin) == NULL)
{
perror("fgets failed");
exit(1);
}
execute(commands) ;
}
return 0;
}
so after the call to the above command, my shell loop terminates.
How could i solve this issue?

Answer 1

It looks like your problem is you exec() in the parent process too when piping. exec() replaces the current process with the new one, so when you call it in the parent, your shell is replaced, so the loop doesn't even exist anymore.
For every exec() in your shell, you should fork() first, to ensure that the parent shell process remains alive.