problems with fork() and execve()

I'm writing a small utility that uses the nidump command (I'm on OS X, and a lot of info on the system is located in the NetInfo database - that can be reached through the command line via the nidump command), fork() an execve(). The problem is that no file is created and no output is collected from NetInfo; The application failes completely.

The filename is given by the user (argv[1]), and the exec funtion I chose to execute the nidump command, with arguments, was execve. I'm not sure it's the right one to use, though, as it doesn't seem to work right (maybe it's just me that's a bit rusty). Anyway, here's some code:

Code:

---

int main(int argc, char *argv[]) {
 
    int PID, status, waitsignal;
    char *args[] = {">> . ", argv[1], NULL}; // this isn't working correct
   
    printf("OS X pssword file dumping\n");
   
    if(argc < 2) {
        printf("usage: %s <filename>\n", argv[0]);
        exit(0);
    }
   
    PID = fork();
   
    if(PID == -1) {
       
        perror("fork error");
        exit(EXIT_FAILURE);
       
    } else if(PID == 0) {
       
        printf("child pid: %d\n", getpid());
        execve("/usr/bin/nidump passwd", args, NULL); // this isn't working correct either
       
        if(errno == -1) {
            perror("error");
        } else {
            //printf("nidump process completed!\n");
        }

[...]

---

I know there's something wrong with the args variable and the execve function, but I can't figure it out. Where should the argument for the command (nidump) be placed? Should I use another exec function instead?

The problem is, you're treating exec like system()

None of that friendly stuff like command line parsing into parameters, and I/O redirection is available (Wait a minute, I've just said that in another post).

Yeah, I guess I'm treating exec like system(), but I know it can be used that way. Here's some of my other code:

Code:

---

[...]
char *argv[] = {"ping", "-c 5", ping_host, NULL};

    ping_pid = (int)fork();

    if(ping_pid == -1) {

        perror("cannot execute command, an error occured\n");
        exit(EXIT_FAILURE);

    } else if(ping_pid == 0) {

        // child prosess
        execve("/sbin/ping", argv, NULL);
[...]

---

- Where execve is used like system(). Since this works, I figured the nidump command with parameters would work too. Anyone got any ideas on what I'm doing wrong?

Edit: After re-reading your post Salem, I realized you said it wasn't possible to re-direct I/O. Is there another way to do what I want to do (without using system())?

> I figured the nidump command with parameters would work too
But only if you put "passwd" into args

In fact, it should be more like this

Code:

---

char *args[] = {
"/usr/bin/nidump",
"passwd"
NULL};

...

execve(args[0], args, NULL);

---

> I realized you said it wasn't possible to re-direct I/O
No, I said it was tricky in DOS
In unix/linux, its much easier

Code:

---

    } else if(PID == 0) {
        int fd = open( argv[1], O_WRONLY ); // This is >, if you want >> bitwise the flag with O_APPEND
        close( 1 );  // existing stdout is no more
        dup( fd );  // stdout is now fd
        printf("child pid: %d\n", getpid());
        execve(args[0], args, NULL);
        if(errno == -1) {
            perror("error");
        }
    }

---

> char *argv[] = {"ping", "-c 5", ping_host, NULL};
It might work, though I would have said
char *argv[] = {"ping", "-c", "5", ping_host, NULL};
Remember, every white space separates the args on the command line (as you type it in), and this must be reflected in any argv[] you construct.

Code:

---

    } else if(PID == 0) {
        int fd = open( argv[1], O_WRONLY ); // This is >, if you want >> bitwise the flag with O_APPEND
        close( 1 );  // existing stdout is no more
        dup( fd );  // stdout is now fd
        printf("child pid: %d\n", getpid());
        execve(args[0], args, NULL);
        if(errno == -1) {
            perror("error");
        }
    }

---

Thanks a million for this! I never knew you could do this that easy. :)