Message Board

**lruc** · 03-31-2009

I am working on a simple message board written in C using CGI, but I keep getting an internal error. I have reviewed my code 100 times and still cant find it.

Code:

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

void append(void)
{
	FILE *log = fopen("log.txt","a");
	char m[101];
	char *qs = getenv("QUERY_STRING");
	if(qs != NULL)
	{
		if(sscanf(qs,"m=%100s",m) == 1)
		{
			fputs(m,log);
		}
	}
	fclose(log);
	return;
}

void recreate(void)
{
	FILE *log;
	char m[101];
	log = fopen("log.txt","r");
	printf("Content-Type: text/html\n\n<html><head><title>Message Board</title></head><body>");
	while(!feof(log))
	{
		fgets(m,sizeof m,log);
		printf("%s</br>",m);
	}
	printf("<form action=\"http://98.243.228.110/cgi-bin/mb.cgi\" method=\"get\"><div><label><input name=\"m\"></label></div></form></body></html>");
	fclose(log);
	return;
}

int main(void)
{
	append();
	recreate();
	return EXIT_SUCCESS;
}

Can anybody spot the problem?

**Elysia** · 03-31-2009

Well, basics... does your fopen calls succeed (check for NULL)?

**lruc** · 03-31-2009

Originally Posted by Elysia

Well, basics... does your fopen calls succeed (check for NULL)?

What do you mean? The fopen call comes before the check for NULL if thats what your asking. Besides, the check for NULL would be irrelevant to that because it checks a string from the environment variable QUERY_STRING not from a file.

**Elysia** · 03-31-2009

>> FILE *log = fopen("log.txt","a");
You do not check if log == NULL. Calling file functions on an unopened file is undefined behavior.

**matsp** · 03-31-2009

Originally Posted by lruc

What do you mean? The fopen call comes before the check for NULL if thats what your asking. Besides, the check for NULL would be irrelevant to that because it checks a string from the environment variable QUERY_STRING not from a file.

What elysia meant is that you need to check if the file actually was possible to open, and you didn't get NULL back from fopen(). If your code is running with a current directory that you can not write into, then it would fail to create the file, and crash when you try to do feof() or fgets() or some such.

--
Mats

**lruc** · 03-31-2009

Ok heres some code with checks thrown in(and some moderate obfuscation):

Code:

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

void append(void)
{
	FILE *log = fopen("log.txt","a");
	char m[101];
	char *qs = getenv("QUERY_STRING");
	if(log != NULL && qs != NULL)
	{
		if(sscanf(qs,"m=%100s",m) == 1)
		{
			fputs(m,log);
		}
	}
	fclose(log);
	return;
}

void recreate(void)
{
	FILE *log = fopen("log.txt","r");
	char m[101];
	printf("Content-Type: text/html\n\n<html><head><title>Message Board</title></head><body>");
	if(log != NULL)
	{
		while(!feof(log))
		{
			fgets(m,sizeof m,log);
			printf("%s</br>",m);
		}
	}
	else
	{
		printf("Error");
	}
	printf("<form action=\"http://98.243.228.110/cgi-bin/mb.cgi\" method=\"get\"><div><label><input name=\"m\"></label></div></form></body></html>");
	fclose(log);
	return;
}

int main(void)
{
	append();
	recreate();
	return EXIT_SUCCESS;
}

**matsp** · 03-31-2009

And what erorr are you getting?

--
Mats

**Elysia** · 03-31-2009

Calling fclose on a non-opened file may very well result in a crash too.

**matsp** · 03-31-2009

And you may want to either use fflush(stdout) or at least add a newline after printing the error!

--
Mats

**dwks** · 03-31-2009

FWIW, fputs() doesn't append a newline like puts() does, so your log as generated by append() might have every message on one line . . . .

**lruc** · 03-31-2009

Originally Posted by matsp

And what erorr are you getting?

--
Mats

The strange thing is nothing when I run the program from the terminal. The only error I get is when I run it through the server, which gives no details to the error.

**lruc** · 03-31-2009

Originally Posted by Elysia

Calling fclose on a non-opened file may very well result in a crash too.

So if the fopen returns NULL its ok to not close the file? Some more revisions:

Code:

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

void append(void)
{
	FILE *log = fopen("log.txt","a");
	char m[101];
	char *qs = getenv("QUERY_STRING");
	if(log != NULL && qs != NULL)
	{
		if(sscanf(qs,"m=%100s",m) == 1)
		{
			fprintf(log,"%s\n",m);
		}
	}
	fclose(log);
	return;
}

void recreate(void)
{
	FILE *log = fopen("log.txt","r");
	char m[101];
	printf("Content-Type: text/html\n\n<html><head><title>Message Board</title></head><body>");
	if(log != NULL)
	{
		while(!feof(log))
		{
			fgets(m,sizeof m,log);
			printf("%s</br>",m);
		}
	}
	else
	{
		puts("Error");
	}
	printf("<form action=\"http://98.243.228.110/cgi-bin/mb.cgi\" method=\"get\"><div><label><input name=\"m\"></label></div></form></body></html>");
	fclose(log);
	return;
}

int main(void)
{
	append();
	recreate();
	return EXIT_SUCCESS;
}

**matsp** · 03-31-2009

Not just OK, but it's CORRECT to not close a file that you didn't succeed in opening.

--
Mats

**laserlight** · 03-31-2009

Originally Posted by lruc

So if the fopen returns NULL its ok to not close the file?

The file is not open, so it cannot be closed.

**lruc** · 03-31-2009

OK I'm one step closer. The file is outputted but the form isn't appended correctly. Also the last line in the file is printed twice. Should I use a do loop instead of a while loop?

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