1) exec fails on x86_64

$ gcc-4.0 -arch x86_64 -o varg varg.c && ./varg
ERROR: execlp failed with errno 14: Bad address
Child process started with pid 51425
Parent process complete.

2) exec succeeds on i386

$ gcc-4.0 -arch i386 -o varg varg.c && ./varg
Child process started with pid 51452
Parent process complete.
Child started with 5 arguments.

3) Of course, the problem was that the project was using gcc-4.0

$ gcc -arch x86_64 -o varg varg.c && echo "===="; ./varg
varg.c: In function ‘main’:
varg.c:39: warning: missing sentinel in function call
====
ERROR: execlp failed with errno 14: Bad address
Child process started with pid 51425
Parent process complete.

varg.c

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

int main (int argc, const char * argv[]) {
    char *foo = "foo";
    char *bar = "bar";
    
    if (argc > 1) {
        printf ("Child started with %d arguments.\n", argc);
        exit(0);
    }
    
    int pid = -1;
    switch (pid = vfork()) {
        case -1:
            printf("Something bad happened.\n");
            break;
        case 0:
            execlp(argv[0], argv[0], "--foo", foo, "--bar", bar, 0);
            printf("ERROR: execlp failed with errno %d: %s\n", errno, strerror(errno));
            _exit(1);
            break;
        default:
            printf("Child process started with pid %d\n", pid);
    }
    
    printf("Parent process complete.\n");
    return 0;
}