setenv seems useless when encounter with execve

Question

I'm following the project TINY SERVER and there is a peice of code shown as below.

This code is used to passing the cgiargs to filename which is a cgi program.

void serve_dynamic(int fd, char *filename, char *cgiargs)
{
    char buf[MAXLINE], *emptylist[] = { NULL };

    /* Return first part of HTTP response */
    sprintf(buf, "HTTP/1.0 200 OK\r\n"); 
    rio_writen(fd, buf, strlen(buf));
    sprintf(buf, "Server: Tiny Web Server\r\n");
    rio_writen(fd, buf, strlen(buf));
  
    if (Fork() == 0) { /* child */ //line:netp:servedynamic:fork
    /* Real server would set all CGI vars here */
    setenv("QUERY_STRING", cgiargs, 1); //line:netp:servedynamic:setenv
    dup2(fd, STDOUT_FILENO);         /* Redirect stdout to client */ //line:netp:servedynamic:dup2
    execve(filename, emptylist, environ); /* Run CGI program */ //line:netp:servedynamic:execve
    }
    wait(NULL); /* Parent waits for and reaps child */ //line:netp:servedynamic:wait
}

In the adder.c which is the target cgi program, the code is shown as below.

int main(void) {
    char *buf, *p;
    char arg1[MAXLINE], arg2[MAXLINE], content[MAXLINE];
    int n1 = 0, n2 = 0;

    /* Extract the two arguments */
    if ((buf = getenv("QUERY_STRING")) != NULL) {
        p = strchr(buf, ’&’);
        *p = ’\0’;
        strcpy(arg1, buf);
        strcpy(arg2, p + 1);
        n1 = atoi(arg1);
        n2 = atoi(arg2);
    }
    
    /* Make the response body */
    sprintf(content, "QUERY_STRING=%s", buf);
    sprintf(content, "Welcome to add.com: ");
    sprintf(content, "%sTHE Internet addition portal.\r\n<p>", content);
    sprintf(content, "%sThe answer is: %d + %d = %d\r\n<p>",
            content, n1, n2, n1 + n2);
    sprintf(content, "%sThanks for visiting!\r\n", content);
    
    /* Generate the HTTP response */
    printf("Connection: close\r\n");
    printf("Content-length: %d\r\n", (int)strlen(content));
    printf("Content-type: text/html\r\n\r\n");
    printf("%s", content);
    fflush(stdout);

    exit(0);
}

In my opinion, the serve_dynamic function sets the "QUERY_STRING", an environment variable, and the adder.c gets the "QUERY_STRING" and use it. But actually in adder.c, the buf is NULL.

Then I try to set environ to pass the QUERY_STRING and it works. But it's very inconvenient.
I didn't think out why setenv function didn't work, and I hope someone can solve my problem.

Answer 1

I read the environ variable agian, and I found I lost to use extern char **environ. After adding this code, the setenv and getenv can work normally.

Answer 2

I'm doing the exact TINY SERVER project as you did, and I had the exact same problem as you did. I think the 'setenv' function didn't work is because --

We set the environment variable 'QUERY_STRING' in the child process we forked, and then we used execve() to start a new process, which is NOT a child process of the calling process, instead it will replace the child process we forked, so the 'QUERY_STRING' was not shared in the new process.

I think the right way to set environment variable (in the child process we forked) to be passed to the program refered by execve() is like this: newenviron[0] = "QUERY_STRING=VALUE";

and then we will be able to use getenv() to access it in the cgi adder program.

You can refer to: https://man7.org/linux/man-pages/man2/execve.2.html

execve(const char *pathname, --,--) executes the program referred to by pathname. This causes the program that is currently being run by the calling process to be replaced with a new program.with newly initialized stack, heap, and (initialized and uninitialized) data segments.