为什么我可以在gcc -std = c11中使用gets（）？

Question

The gets() function has been removed from the C language. No such function exists in the standard.

Yet I compile the following code:

#include <stdio.h>

int main (void)
{
  (void) gets (NULL);
}

using

gcc -std=c11 -pedantic-errors -Wall -Wextra

and it compiles without giving any errors or warnings. Similarly,

#include <stdio.h>

int gets;

int main (void)
{}

will not compile (error: 'gets' redeclared as different kind of symbol).

In the standard 4. Conformance §6 we can read:

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program

Given the above I don't think gcc is standard-compliant, even in pedantic mode. Is there a reason for this? Is this intentional or is it a bug?

GCC version 4.9.1.

Edit:

gcc --version
gcc (x86_64-win32-seh-rev1, Built by MinGW-W64 project) 4.9.1

Answer 1

gcc is just the compiler, not the entire implementation.

On my system (Linux Mint 17.3, gcc 4.8.4, GNU libc 2.19), I get:

$ gcc -std=c11 -pedantic-errors -Wall -Wextra -c c.c
c.c: In function ‘main’:
c.c:5:3: error: implicit declaration of function ‘gets’ [-Wimplicit-function-declaration]
   (void) gets (NULL);
   ^

To correctly diagnose the error, the implementation needs to be conforming. That means both the compiler (which never provided gets in the first place) and the library.

You're using a library that still provides the gets function. Because of that the implementation as a whole (which consists of the compiler gcc, the library, and a few other pieces) does not conform to C11.

Bottom line: This is not a gcc issue, and there's not much that gcc can do about it. (Well, it could issue a special-case diagnostic for gets , but then it would have to determine that it's not a valid call to a user-defined function with the same name.)

Answer 2

The key line of your code is:

#include <stdio.h>

Did you update your system's C library and headers? They're also part of the C implementation, along with the compiler.

Answer 3

update this may not be an answer to the question, I try to make it informational.

I happened to find that gcc mentioned gets is not following C11 standard for some library issue glibc 2.16 .

See gcc supporting status of C11: https://gcc.gnu.org/wiki/C11Status

But I cannot find the definition of "library issue" and current status for other versions of glibc.

So I tried on my machine ubuntu16.04 with gcc version 5.3.1 20160413 , glibc version Ubuntu GLIBC 2.23 We can get enough warning on compile time, but it's still OK to execute the output object file for "Backwards compatibility".

warning: implicit declaration of function ‘gets’ [-Wimplicit-function-declaration]
warning: the `gets' function is dangerous and should not be used.