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Web App With C & FastCGI - Form Data (#3)
How to access form data.
Updated Dec 22, 2022

Form Data

We can access form data by reading bytes from the Request.in input stream:

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

int main() {

	printf("Hello.\n");

	FCGX_Init();
	int Socket = FCGX_OpenSocket("127.0.0.1:2016", 1024);
	FCGX_Request Request;
	FCGX_InitRequest(&Request, Socket, 0);

	while(FCGX_Accept_r(&Request) >= 0) {
		printf("Request accepted.\n");
		
		char *Path =
			FCGX_GetParam(
				"SCRIPT_NAME",
				Request.envp
				);

		char Title[64] = {0};
		char Status[64] = "200 OK";
		
		if(strcmp(Path, "/") == 0) {
			strcpy(Title, "Home");
		} else if(strcmp(Path, "/about/") == 0) {
			strcpy(Title, "About");
		} else {
			strcpy(Title, "404 Not Found");
			strcpy(Status, "404 Not Found");
		}

		// form handling

		char *Method =
			FCGX_GetParam(
				"REQUEST_METHOD",
				Request.envp
				);

		char Form[512] = {0};
		
		if(strcmp(Method, "GET") == 0) {
			strcpy(Form,
				     "<form method='POST' action=''>"
					   "<input id='username' name='username' type='text'></input>"
					   "<input id='password' name='password' type='password'></input>"
					   "<input type='submit' value='Submit'>"
					   "</form>"
				);
		} else if(strcmp(Method, "POST") == 0) {

			// read form data
			
			int ContentLength =
				atoi(FCGX_GetParam("CONTENT_LENGTH", Request.envp));

			char *ContentBuffer = malloc((ContentLength + 1) * sizeof(*ContentBuffer));

			FCGX_GetStr(ContentBuffer, ContentLength, Request.in);
			ContentBuffer[ContentLength] = '\0';
			strcpy(Form, ContentBuffer);
			free(ContentBuffer);
		}
		
		FCGX_FPrintF(
			Request.out,
			"Status: %s\r\n"
			"Content-type: text/html\r\n\r\n"
			"<h1>%s</h1>"
			"<p>Path: %s</p>"
			"<div>Form: <br>%s</div>",
			Status,
			Title,
			Path,
			Form
			);
		FCGX_Finish_r(&Request);
	}
	
	return 0;
}
  • FCGX_GetParam() returns a pointer to the given FCGI parameter or NULL.
  • atoi() creates an int out of a string.
  • malloc((ContentLength + 1) allocates ContentLength bytes + 1 for the NULL byte.
  • FCGX_GetStr reads bytes from the input stream.