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Answer 1

+5 A:

try

printf("%s", (char*) buffer);

;-)

tkotitan 2009-07-22 19:45:49

The question rises with the fact that I was able to run the same code on some other machine... I suspect a difference in gcc version might cause a problem like this?

Lawrence 2009-07-22 19:49:35

well it sounds like that other machine isn't standard C

tkotitan 2009-07-22 19:51:55

printf(%s) is bad practice as you're very likely to trash the stack if your string contains any special characters (%d, %s, etc.). The first argument of printf should always be a string literal. I'm guessing different versions of GCC are more strict about whether or not they'll allow you to do it.

Falaina 2009-07-22 19:53:46

They have to allow it, or they aren't conforming C compilers. They don't have to like it.

David Thornley 2009-07-22 20:00:19

They allow it, they just give a warning when that happens (and they are in their right as a conformant implementation). And he is most likely compiling with "treat warnings as errors".

Pavel Minaev 2009-07-22 20:02:45

But suppose they start not liking odd numbers?

anon 2009-07-22 20:06:50

It was a compiler-related issue; please see my comment above.

Lawrence 2009-07-22 20:08:22

Answer 2

A:

This is a warning for your safety, not an error. This new compiler is apparently being more strict about it. I don't think it's actually illegal in C, so the compiler should have an option to disable treating this as an error.

However, you pretty much never want to pass anything other than a string literal as the first argument to printf. The reason that doing so is such a horrible idea that the compiler has a special built-in check to warn you about it is this: Suppose the non-literal string that you pass as the first argument to printf happens to contain printf formatting characters. printf is then going to try to access second, third, fourth, etc, arguments that you didn't actually pass in, and may well crash your program by trying to do so. If the non-literal first argument is actually user-supplied, then the problem is even worse since a malicious user could crash your program at will.

Tyler McHenry 2009-07-22 19:54:16

Usually, you can do what you want to do with a string literal for a format string, particularly since the rest of the statement is fixed at compile time. There may be cases where it's preferable to pass in a non-literal string, but as Tyler points out this is a dangerous practice, and you should never under any circumstances use user input this way.

David Thornley 2009-07-22 19:59:37

Answer 3

+4 A:

Newer GCC versions try to parse the format string passed to printf and similar functions and determine if the argument list correctly matches the format string. It can't do this because you've passed it a buffer for the first argument, which would normally be a format string.

Your code is not incorrect C, it's just a poor usage of C. As others mentioned you should use "%s" as a format string to print a single string. This protects you from a class of errors that involve percentage signs in your string, if you don't control the input. It's a best practice to never pass anything but a string literal as the first argument to the printf or sprintf family of functions.

Dan Olson 2009-07-22 20:00:02

very good, clear answer

anon 2009-07-22 20:04:12

I think you nailed the problem :)

Lawrence 2009-07-22 20:16:33

Answer 4

+1 A:

This warning is generated by gcc if

-Wformat-nonliteral

is set. It's not part of -Wall or -Wextra (at least for version 4.4.0), so just drop it if you want the code to compile warning-free.

Christoph 2009-07-22 20:08:42

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