Interpreter path relative to script location in shebang

delegate.c

/*
    Helper program for resolving the path to the interpreter executable
    relative to the script location in shebangs.

    Sample usage:
        #!/usr/bin/delegate ../my-virtualenv/bin/python

        import sys

        def main():
            print sys.executable

        if __name__ == '__main__':
            main()
*/

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

#define MAX_PATH_SIZE 4096

int main(int argc, char **argv) {
    if (argc < 3) {
        fprintf(stderr, "usage: %s <interpreter> <file>\n", argv[0]);
        return -1;
    }

    char *interp_path = argv[1];
    char *script_path = argv[2];

    int ret;
    if (strchr(interp_path, '/') == NULL) {
        ret = execvp(interp_path, argv + 1);
    } else if (interp_path[0] == '/') {
        ret = execv(interp_path, argv + 1);
    } else {
        char *base_high = strrchr(script_path, '/');
        size_t base_len = base_high != NULL ? base_high - script_path + 1 : 0;
        size_t interp_len = strlen(interp_path);

        if (base_len + interp_len < MAX_PATH_SIZE) {
            char buf[MAX_PATH_SIZE];
            memcpy(buf, script_path, base_len);
            memcpy(buf + base_len, interp_path, interp_len + 1); // with '\0'

            argv[1] = buf;
            ret = execv(buf, argv + 1);
        } else {
            errno = ENAMETOOLONG;
            ret = -1;
        }
    }
    perror(argv[0]);
    return ret;
}