4.4.1. echo.c

echo receives incoming data from an HTML form and returns an HTML document listing the field names and the values entered into the fields by the customer. To avoid any confusion with the Unix utility echo, we renamed ours to myecho. It is worth looking at myecho.c, because it shows that the process is easier than it sounds:

#include <stdio.h>
#include <stdlib.h>
#define MAX_ENTRIES 10000
typedef struct
    {
    char *name;
    char *val;
    } entry;

char *makeword(char *line, char stop);
char *fmakeword(FILE *f, char stop, int *len);
char x2c(char *what);
void unescape_url(char *url);
void plustospace(char *str);

int main(int argc, char *argv[])
    {
    entry entries[MAX_ENTRIES];
    register int x,m=0;
    int cl;
    char mbuf[200];

The next line:

printf("Content-type: text/html\n\n");

supplies the HTML header. We can have any MIME type here. It must be followed by a blank line, hence the \n\n. The line:

if(strcmp(getenv("REQUEST_METHOD"),"POST"))

checks that we have the right sort of input method. There are normally only two possibilities in a CGI script: GET and POST. In both cases the data is formatted very simply:

fieldname1=value&fieldname2=value&...

If the method is GET, the data is written to the environment variable QUERY_STRING. If the method is POST, the data is written to the standard input and can be read character by character with fgetc( ) (see echo2.c in Appendix B, "The echo Program ").

The next section returns the length of date to come:

{
        printf("This script should be referenced with a METHOD of POST.\n");
        exit(1);
        }
    if(strcmp(getenv("CONTENT_TYPE"),"application/x-www-form-urlencoded")) 
        {
        printf("This script can only be used to decode form results. \n");
        exit(1);
        }
cl = atoi(getenv("CONTENT_LENGTH"));

The following snippet reads in the data, breaking at the & symbols:

for(x=0;cl && (!feof(stdin));x++) 
    {
    m=x;
    entries[x].val = fmakeword(stdin,'&',&cl);
    plustospace(entries[x].val);
    unescape_url(entries[x].val);
    entries[x].name = makeword(entries[x].val,'=');
    }

The next line displays the top of the return HTML document:

printf("<H1>Query Results</H1>");

The final section lists the fields in the original form with the values filled in by the customer:

printf("You submitted the following name/value pairs:<p>%c",10);
    printf("<ul>%c",10);

    for(x=0; x <= m; x++)
        printf("<li> <code>%s = %s</code>%c",entries[x].name,
               entries[x].val,10);
    printf("</ul>%c",10);
    }

We compile myecho.c and copy the result to mycgi [38] to see it in action next time we run the form. The result on the client machine is something like this (depending on how the form was filled in):

[38]Of course, we could have changed the form to use myecho instead.

QUERY RESULTS
You submitted the following name/value pairs:
* 2315_order=20
* 2316_order=10
* 2317_order=
* 2318_order=
* card_type=Amex
* card_num=1234567

Clearly, it's not difficult to modify myecho.c to return another form, presenting the data in a more user-friendly fashion and asking the customer to hit a button to signify agreement. The second form activates another script/program, process_orders, which turns the order into delivered business. However, we will leave these pleasures as an exercise for the reader.