Having trouble with fork(), pipe(), dup2() and exec() (pipes exercise)
Here's my code:
Code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <readline/readline.h>
int main(int argc, char *argv[]) {
char *bBuffer, *sPtr, *aPtr = NULL, *pipeComms[2], *cmdArgs[10];
int fdPipe[2], pCount, aCount, i, status;
pid_t pid;
pipe(fdPipe);
while(1) {
bBuffer = readline("Shell> ");
if(!strcasecmp(bBuffer, "exit")) {
return 0;
}
sPtr = bBuffer;
pCount = -1;
do {
aPtr = strsep(&sPtr, "|");
pipeComms[++pCount] = aPtr;
} while(aPtr);
for(i = 0; i < pCount; i++) {
aCount = -1;
do {
aPtr = strsep(&pipeComms[i], " ");
cmdArgs[++aCount] = aPtr;
} while(aPtr);
if(strlen(cmdArgs[0]) > 0) {
pid = fork();
if(pid == 0) {
if(i == 0) {
close(fdPipe[0]);
dup2(fdPipe[1], STDOUT_FILENO);
close(fdPipe[1]);
} else if(i == 1) {
close(fdPipe[1]);
dup2(fdPipe[0], STDIN_FILENO);
close(fdPipe[0]);
}
execvp(cmdArgs[0], cmdArgs);
} else {
waitpid(pid, &status, 0);
if(WIFEXITED(status)) {
printf("[%d] TERMINATED (Status: %d)\n",
pid, WEXITSTATUS(status));
}
}
}
}
}
return 0;
}
Here's the test case where this fails:
Code:
nazgulled ~/Projects/SO/G08 $ ls -l
total 8
-rwxr-xr-x 1 nazgulled nazgulled 7181 2009-05-27 17:44 a.out
-rwxr-xr-x 1 nazgulled nazgulled 754 2009-05-27 01:42 data.h
-rwxr-xr-x 1 nazgulled nazgulled 1305 2009-05-27 17:50 main.c
-rwxr-xr-x 1 nazgulled nazgulled 320 2009-05-27 01:42 makefile
-rwxr-xr-x 1 nazgulled nazgulled 14408 2009-05-27 17:21 prog
-rwxr-xr-x 1 nazgulled nazgulled 9276 2009-05-27 17:21 prog.c
-rwxr-xr-x 1 nazgulled nazgulled 10496 2009-05-27 17:21 prog.o
-rwxr-xr-x 1 nazgulled nazgulled 16 2009-05-27 17:19 test
nazgulled ~/Projects/SO/G08 $ ./a.out
Shell> ls -l|grep prog
[4804] TERMINATED (Status: 0)
-rwxr-xr-x 1 nazgulled nazgulled 14408 2009-05-27 17:21 prog
-rwxr-xr-x 1 nazgulled nazgulled 9276 2009-05-27 17:21 prog.c
-rwxr-xr-x 1 nazgulled nazgulled 10496 2009-05-27 17:21 prog.o
The problem is that I should return to my shell after that, I should see "Shell> " waiting for more input. You can also notice that you don't see a message similar to "[4804] TERMINATED (Status: 0)" (but with a different pid), which means the second process didn't terminate.
I think it has something to do with grep, because this works:
Code:
nazgulled ~/Projects/SO/G08 $ ./a.out
Shell> echo q|sudo fdisk /dev/sda
[4838] TERMINATED (Status: 0)
The number of cylinders for this disk is set to 1305.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
(e.g., DOS FDISK, OS/2 FDISK)
Command (m for help):
[4839] TERMINATED (Status: 0)
You can easily see two "terminate" messages...
So, what's wrong with my code?
P.S: There's no error handling on purpose, this code is just like a proof of concept to show off my problem...
