g++ special handling of printf

Question

Anyone knows why the following code works under g++ 4.7.2? If I change the name printf to another name such as f, it has compiler error saying constexpr can't contain non-const function calls (which I think is the correct behavior).

[hidden]$ cat d.cpp 
extern "C" { extern int printf(const char* s, ...); }
constexpr int g() { return printf(""), 0; }
template <int N> struct X { const static int value = N; };
int n = X<g()>::value;
[hidden]$ g++ -std=c++11 -c d.cpp
[hidden]$ g++ -v |& tail -1
gcc version 4.7.2 20121109 (Red Hat 4.7.2-8) (GCC)

Note I don't include any header files.

As I said, if I change the `printf` to any other name, it has the compiler error. So generally it shouldn't work. Here it works only because gcc has special handling for a name called `printf`. This is weird. — Kan Li, May 17 '13 at 21:05
This is probably a bug, g++ 4.6 as well as clang++ 3.1 reject this. — n. m. could be an AI, May 17 '13 at 21:06

score 4 · Accepted Answer · answered May 17 '13 at 21:19

4

printf() is handled as a builtin function by GCC/g++ in many cases (though I think this behavior is still a bug). From http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html:

The ISO C90 functions ... printf ... are all recognized as built-in functions unless -fno-builtin is specified (or -fno-builtin-function is specified for an individual function)

You get the correct diagnostic if you use the -fno-builtin option.

The bug appears to be fixed in 4.8.0.

answered May 17 '13 at 21:19

Michael Burr

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Good to hear it is fixed in 4.8.0 – Kan Li May 17 '13 at 21:25
`gcc 4.8.2` still allows `printf` and other [builtins in a constexpr](http://stackoverflow.com/questions/22176777/why-can-i-call-a-non-constexpr-function-inside-a-constexpr-function). My gut says this is not strictly conformant unless they document this as an extension. – Shafik Yaghmour Mar 04 '14 at 18:37

furas · Answer 2 · 2013-05-18T00:54:55.897

I think stdio.h is included by default

I try it with puts and works for me [gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1)]

extern "C" { extern int puts(const char* s); }
constexpr int g() { return puts(""), 0; }
template <int N> struct X { const static int value = N; };
int n = X<g()>::value;

Edit:

@Keith Thompson

Before I wrote "included by default" I tried the code below without #include <stdio.h>. It compiles with some warnings but runs - so (for some reason) printf, scanf, puts works without #include <stdio.h>. Maybe stdio.h is not included by default, maybe library with printf, scanf, puts is linked by default.

int main()
{
    char one;

    printf("Hello ");
    scanf("%c", &one);
    puts("world!");
}

I don't believe *any* header is included by default. Try declaring `FILE *foo` without `#include `; you'll find that the identifier `FILE` is not visible. — Keith Thompson, May 17 '13 at 22:15

g++ special handling of printf

2 Answers2