Basic syntax in C

[heras]

Hi,
Behold my ignorance. I've got the following code that works:

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

int main(void)
{
	FILE *text = fopen("/home/heras/code/text", "r");
	// stuff that works as expected

But even with tons of brackets I can't get this to compile:

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

int main(void)
{
	if (FILE *text = fopen("/home/heras/code/text", "r") == NULL) {
		perror("fopen: ");
		exit(EXIT_FAILURE);
	}

## I get varying versions of the below, depending on where I put brackets: 
readfile.c: In function ‘main’:
readfile.c:7: error: expected ‘)’ before ‘text’
readfile.c:7: error: expected expression before ‘==’ token
readfile.c:16: error: ‘text’ undeclared (first use in this function)

This FAQ shows a working solution, but I don't understand why mine doesn't. In another program I've got such a construction that does work:

    if ((sockfd = socket(PF_INET, SOCK_STREAM, 0)) == -1) {
        perror("socket: ");
        exit(EXIT_FAILURE);
    }

I must be making some basic syntax mistake(s) or something. What is wrong with my attempt at error handling?
Thanks,
heras

[laserlight]

You could write:

FILE *text;
if ((text = fopen("/home/heras/code/text", "r")) == NULL) {
    perror("fopen: ");
    exit(EXIT_FAILURE);
}

You could also write:

FILE *text = fopen("/home/heras/code/text", "r");
if (text == NULL) {
    perror("fopen: ");
    exit(EXIT_FAILURE);
}

[heras]

Hi laserlight, thanks for your reply.
I've found that second solution in one of the FAQs of this board and it works. But what I don't understand is why my socket program accepts this construction and my readfile program doesn't. Is there some difference in the types of the return values of sockfd() vs fopen() ?

[Elysia]

There are two errors with the example of yours.
Firstly, C89 will not allowed you to define variables in statements like that. All variables must be defined at the beginning of a function. Always. C99 and C++ allows this, but then the rule below would make it that you can only use it inside your if, so using it to check for error would be bad.
Secondly, since it's defined within your if, the variable will also be destructed within the if due to block rules.
So you can assign within and if, but you can't actually define a variable inside.

/* Invalid in C89; valid in C99 and C++ */
if (int i = 0 == 0)
	i = 1; /* Valid, because i exists within the if scope */
i = 0; /* Invalid, because i no longer exists. */

[heras]

All variables must be defined at the beginning of a function. Always. C99 and C++ allows this, but then the rule below would make it that you can only use it inside your if, so using it to check for error would be bad.
Secondly, since it's defined within your if, the variable will also be destructed within the if due to block rules.
So you can assign within and if, but you can't actually define a variable inside.

Ah, I think I understand, thanks! I've noticed that compiling with gcc -std=c99 makes warnings go away when the previously undeclared i is used as follows:

    for (i = 0; i < 10; i++) {
    ...

It may work but probably bad form as well if I understand correctly.

edit: and obviously the sockfd from the snippet it my first post was already declared as opposed to FILE *text.

[Elysia]

You're not actually defining any variable either.
But if you did...

for (int i = 0; i < 10; i++)
{
	/* i is accessible throughout the for scope */
}

Then this is possible. The variable "i" can be accessed within the i scope, but will be destroyed when you leave the loop.
Again, only works with C99 & C++.

[heras]

   for (int i = 0; ...

Oops, yes, that's what I meant to type. I'll stick to your suggestion though of always defining them first
Thanks,
heras