How to change environment variable in shell executing a C program from that C program?

Question

I want to change the value of PATH variable inside the C program and then see the changed value in the shell using which I run this program.

Doing something like this,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main () {

    char *path = getenv ("PATH");
    printf ("%s\n\n", path);
    setenv ("PATH", strcat (path, ":~/myNewPath/"), 1);
    printf ("%s\n\n", path);

    int pid = fork ();
    if (pid == -1)
        abort ();
    if (pid == 0) {

    } else {
        // use execlp? how? source? any hints?
    }

    return 0;
}

If I use source command in the exec* system call. What will be the syntax to update this PATH variable in the shell backwards?

Answer 1

This is impossible. There is no way for a subprocess to change its parent's environment variables.

To understand why it is impossible, look at the signature of execve

int execve(const char *program, char *const *argv, char *const *envp);

which is paired with the true signature of main on Unix systems

int main(int argc, char **argv, char **envp);

and perhaps you begin to understand that as far as the kernel is concerned, the environment variables are a second set of command line arguments. That they appear to be independently accessible via getenv and setenv etc, and appear to inherit from parent to child, is an illusion maintained by the C library.

For more detail on how this works, study the x86-64 ELF ABI specification, section 3.4.1 "Initial Stack and Register State" paying particular attention to figure 3.9 which shows the layout of the data copied by execve onto the newly created stack. (The document linked is specific to one CPU architecture, but the way this works is generally consistent across modern Unixes; fine details will of course vary from CPU to CPU and OS to OS.)