HTML form and C - checking for a unique username - html

Question

I'm currently writing my own website and I'm trying to make sure that when someone is making an account the username is unique. I'm doing the back-end in C (since I don't know php/js) and I've been running in a bit of a problem. Right now I'm getting the environment variables in a file newuser.txt (this file has only unique usernames) as such:
fullname=test
description=test
username=test
password=test
I know that at lines 3, 7, 11 etc. in my newusers.txt file I will get my username so I thought of adding all the usernames to another file (which also hosts the incoming data) and then check to see if the incoming username is unique and if it is then I want to add all the data (so fullname, username etc) to newusers.txt. Here's my code:
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(int argc, char *argv[])
{
int currentLine = 1;
char fileLine[100];
int searchLine= 3;
char input[200];
int i=0;
int n = atoi(getenv("CONTENT_LENGTH"));
fgets(input,n+1,stdin); //get the input from the form
printf("Content-Type:text/html\n\n");
printf("<TITLE>Account Creation Query</TITLE>\n");
if (n == 0)
{
printf("<p> Error. Please try again.</p>");
}
FILE *f = fopen("newusers.txt", "ab");
FILE *g = fopen("incoming.txt", "ab");
if (f == NULL)
{
printf("<p> Error in opening the file. Check if the file exists</p>");
printf("<p>Login Page</p>");
printf("<p>Homepage</p>");
}
else
{
while(fgets(fileLine, 100, f)) /*searching for the usernames and adding them to the incoming.txt file */
{
if(searchLine == currentLine)
{
fputs(fileLine, g);
searchLine = searchLine + 4;
}
currentLine++;
}
char *token = strtok(input, "&"); /*tokenizing the incoming data and adding it to the incoming.txt file */
while(token!=NULL)
{
fputs(token, g);
fputs("\n", g);
token = strtok(NULL, "&");
}
}
printf("<p> Account created successfully. You can now login!</p>");
printf("<p>Login Page</p>");
fclose(f);
fclose(g);
return 0;
}
Ideally at this point my incoming.txt file would look like this:
firstname=bla
description=bla
username=bla
password=bla
username=u1
username=u2
username=u3
...
Right now I'm stuck at comparing the incoming username to the other usernames and then copying the data back into newusers.txt. Any help would be appreciated!

Answer 1

use system() call to invoke grep binary to search instead writing searching code in C.
as below
//concat strings to get "grep filename username | cut -d'=' -f2" into cmdstring and then
// read the contents
in=popen(cmdstring, "r");
if(in==NULL)
{
perror("Shell execution error");
exit(EXIT_FAILURE);
}
fgets(buf,3000,in)!=NULL)
here buf contains name of the new user then it is already exist.
otherwise he chose unique name.

Answer 2

I'd strongly recommend that you learn a scripting language for this project. PHP, Perl, Python, Ruby, Javascript… there's plenty of choices, and any one of them will be much more suitable for web programming than C.
That being said, what you need here is a database. Consider using SQLite or Berkeley DB; both are easy to interact with from C, and will allow you to perform lookups and insertions much more easily than would be possible with a flat file (as you're trying to do here).