Call C function with arguments on fastcgi web application

Question

I'm developing a web app for a reservation system and so far I achieved to send a mail with libcurl and display the form in HTML using fcgi_stdio.h, but both separately.

This is how it looks

#define __USE_XOPEN
#define _GNU_SOURCE

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

#include "mail.h"
#include "booking.h"

int
main()
{
    char to[20], name[20];
    //char *year, *mon, *day, *hour, *min;
    char date[20];
    char num_guest[4], phone[10];

    while (FCGI_Accept() >= 0) {
        printf("Content-type: text/html\n\n");
        printf("<h1>Welcome</h1>");

        char *method = getenv("REQUEST_METHOD");
        char form[2048] = {0};
        char output[2048] = {0};

        if(strcmp(method, "GET") == 0) {
            strcpy(form,
                "<form method='POST' action=''>"
                "<input id='name' name='name' type='text'></input>"
                "<br>"
                "<input id='to' name='to' type='email'></input>"
                "<br>"
                "<input id='date' type='datetime-local' name='date' />"
                "<br>"
                "<input type='tel' "
                    "id='phone' name='phone' size='20' "
                    "minlenght='9' maxlenght='14' required>"
                "<br>"
                "<input type='number' id='num_guest' name='num_guest'" 
                    "min='1' max='18'>"
                "<br>"
                "<input type='submit' value='Submit'>"
                "</form>"
            );
        } else if (strcmp(method, "POST") == 0) {
            int ilen = atoi(getenv("CONTENT_LENGTH"));
            //char *bufp = malloc(ilen);
            char *bufp = (char *) malloc(ilen * sizeof(char));
            fread(bufp, ilen, 1, stdin);
            printf("<h3>Thank you for your reservation!</h3>");
            strcpy(output, bufp);
            free(bufp);

                        /* declare data variables */
            sscanf(output, 
                "name=%[^&]&to=%[^&]&date=%[^&]&phone=%[^&]&num_guest=%[^&]", 
                name, to, date, phone, num_guest);

            replace(date, "%3A", ":"); // makes the date readable
            replace(to, "%40", "@");   // makes the mail readable
        }

        printf(
            "<div>"
            "%s"
            "</div>",
            form
        );
    }

    return 0;
}

The code above works fine, but I would like to call these functions after clicking the Submit button.

book_for(to, name, num_guest, phone, date);
send_mail("addr", "confirmation.mail");

Those functions alone and declaring before the arguments as char *name = 'some name'; work fine too sending the mail to the recipient.

Thanks for your help :)