How does one properly use the Unix exec C(++)-command?

Question

Specifically, I need to call a version of exec that maintains the current working directory and sends standard out to the same terminal as the program calling exec. I also have a vector of string arguments I need to pass somehow, and I'm wondering how I would go about doing all of this. I've been told that all of this is possible exclusively with fork and exec, and given the terrible lack of documentation on the google, I've been unable to get the exec part working.

What exec method am I looking for that can accomplish this, and how do I call it?

Answer 1

You may be looking for execv() or execvp().

Answer 2

If you have a vector of strings then you need to convert it to an array of char* and call execvp

#include <cstdio>
#include <string>
#include <vector>

#include <sys/wait.h>
#include <unistd.h>

int main() {
    using namespace std;

    vector<string> args;
    args.push_back("Hello");
    args.push_back("World");

    char **argv = new char*[args.size() + 2];
    argv[0] = "echo";
    argv[args.size() + 1] = NULL;
    for(unsigned int c=0; c<args.size(); c++)
        argv[c+1] = (char*)args[c].c_str();

    switch (fork()) {
    case -1:
        perror("fork");
        return 1;

    case 0:
        execvp(argv[0], argv);
        // execvp only returns on error
        perror("execvp");
        return 1;

    default:
        wait(0);
    }
    return 0;
}

Answer 3

You don't necessarily need google to find this out, you should have the man command available so you can man fork and man exec (or maybe man 2 fork and man 3 exec) to find out about how the parameters to these system and library functions should be formed.

In Debian and Ubuntu, these man pages are in the manpages-dev package which can be installed using synaptic or with:

sudo apt-get install manpages-dev