How come I am able to override printf in C language?

Question

Consider the next piece of code:

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

int printf(const char *format, ...)
{
    va_list args;

    return 1;
}

int main()
{
    printf("Hello there");

    return 0;
}

And indeed, printf prints nothing. Meaning, the compiler 'prefer' this function over the one declared in <stdio.h>. But C doesn't allow function overriding, and from my understanding, it should throw an error of conflicting errors.

Can anyone explain this behavior?

Answer 1

The C standard says (7.1.3 Reserved identifiers):

• All identifiers with external linkage in any of the following subclauses (including the future library directions) and errno are always reserved for use as identifiers with external linkage.
• Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.
• If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.

So when <stdio.h> is included, printf is reserved as an identifier with file scope in the name space of ordinary identifiers. In addition, printf is always reserved as an identifier with external linkage, even if <stdio.h> is not included.

7.1.4 Use of library functions does not allow to define a function with a reserved name.

In most implementations such definition will silently replace the library function, but the standard does not guarantee anything.