I've got a CGI script (put together C program) that results its command line arguments (argv[], argv[1] etc.).

Basically try http://ajf.me/c/?abc I recieve "abc" because the second parameter.

However, basically try http://ajf.me/c/?a=bc I don't have any second parameter.

How come using = stop the parameters from being passed towards the program?

Whether it matters this is actually the C code:

#include <stdio.h>

int main (int argc, char *argv[]) {
    int i;
    printf("Content-Type: text/html;charset=utf-8\n\n");
    printf("<!DOCTYPE html>\n");
    printf("<html>\n");
    printf("<head>\n");
    printf("<title>ajf.me powered by ANSI C!</title>\n");
    printf("</head>\n");
    printf("<body>\n");
    printf("<h2>Supplied Arguments</h2>\n");
    printf("argc: %d\n", argc);
    printf("<ol>\n");
    for (i = 0; i < argc; ++i) {
        printf("<li>%s</li>\n", argv[i]);
    }
    printf("</ol>\n");
    printf("<em>Yes, this is vulnerable to null-byte injection. For instance, <a href=\"?Injected%%00Null\" style=\"font-family: monospace; color: green;\">?Injected\\0Null</a>.</em>\n");
    printf("</body>\n");
    printf("</html>\n");
}

Passing parameters to some CGI program on its command line should be an anomaly of the web server. Normally the part following the ? will come in an atmosphere variable known as QUERY_STRING.

It might most likely be useful that you should evaluate the CGI specs.