A short example of how to us setenv in a c program to set an enviroment variable which is then made avilable for any child processes to use.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) {

	if (setenv("VARNAME", "NEWVALUE", 1) < 0) {
		perror("setenv");
		exit(EXIT_FAILURE);
	}

	system("env | grep VARNAME");
	return 0;
}