为什么strcpy（）在不包含的情况下工作<string.h>

Question

I am working with some C example, and using function strcpy() , but forgot to include <string.h> , though I have included <stdio.h> . To my surprise code ran successfully. Following is the code I am executing:

#include <stdio.h>

int main() {
   char message[10];
   int count, i;

   strcpy(message, "Hello, world!");

   printf("Repeat how many times? ");
   scanf("%d", &count);

   for(i=0; i < count; i++) {
      printf("%3d - %s\n", i, message);
   }
}

I am using gcc version 3.3.6 (Ubuntu 1:3.3.6 - 15ubuntu1)

I even have not received any compilation warnings.

Why my code is working without including <string.h> ?

Thanks in advance.

Answer 1

The compiler assumes int strcpy() and invokes ancient backwards compatibility behavior. Pointer arguments are one of the types for which this works so it's fine.

Don't depend on it. The safety rails are off.

Answer 2

As I recall, GCC 3 defaulted to C89 conformance plus GNU extensions. C89 permitted functions to be called without previously having been declared -- they were assumed to return int , and their parameter types were inferred from the argument types. If the resulting implicit type happened to match the called function's actual type (which was more likely in legacy C code than it is in modern code) then all was good. If not, then the behavior was undefined, but anything can happen with undefined behavior, even what the programmer who wrote the code expected to happen.

Additionally, although the standards don't specify that any of the standard headers include other headers, they are not forbidden from doing so, and in practice, in some C implementations, some of them do. If it happens that your implementation's stdio.h includes string.h or otherwise provides a compatible definition for strcpy() , then that serves just fine with that implementation . If that's what you're relying on, however, whether intentionally or not, then you are at risk of unexpected failure when you try to use your program with a different implementation.

Finally, do note that GCC 3.3 is very old. If at all possible, you should upgrade to something newer. Even the stodgy, long-term stability Linuxes tend to be using at least versions late in the 4.x series, and the latest is GCC 8.3.

Answer 3

The string.h include file tells the compiler how strcpy() is defined by giving a declaration of the function, but it doesn't provide the function itself, which instead sits in a library and will be automatically linked with your program.

If the function has not been declared when it reaches it, then the compiler will assume a declaration, based on defaults and how you use the function.

The compiler takes what you wrote, so the addresses of message and Hello, world! and assumes the function to return an int . You take risks by not telling the compiler how it has to compile the code thanks to the include . The outcome is undefined behavior.

By the way, Hello, world! takes 14 characters, 13 + the trailing \\0 , which is 4 more than in your declaration of message . This is undefined behavior, as the extra 4 chars will be written where they're not supposed to. Seemingly working, crash...

You cannot rely on a program showing an expected result. This is undefined behavior.

Always compile with the warnings options, like -Wall , and consider fixing all warnings.

Answer 4

The compiler knows some "built-in" functions ( printf() is another example for some compilers), and makes up for your mistake by providing it. It is a bad habit to rely on this because even if your today's compiler does this, nothing guarantees you that another one will do the same, so the compiled code won't be predictable.

Try compiling with -Wall -Wextra to make the compiler show more warnings, including the ones for your forgotten header : gcc -o input input.c -Wall -Wextra .

Also see here : warning: incompatible implicit declaration of built-in function 'xyz'