Overriding a standard function behaves unexpectedly - c

Question

[Update] 2016.07.02
main.c
#include <stdio.h>
#include <string.h>
size_t strlen(const char *str) {
printf("%s\n", str);
return 99;
}
int main() {
const char *str = "AAA";
size_t a = strlen(str);
strlen(str);
size_t b = strlen("BBB");
return 0;
}
The expected output is
AAA
AAA
BBB
Compile with gcc -O0 -o main main.c :
AAA
Compile with gcc -O3 -o main main.c
AAA
AAA
The corresponding asm code with flag -O0
000000000040057c <main>:
40057c: 55 push %rbp
40057d: 48 89 e5 mov %rsp,%rbp
400580: 48 83 ec 20 sub $0x20,%rsp
400584: 48 c7 45 e8 34 06 40 movq $0x400634,-0x18(%rbp)
40058b: 00
40058c: 48 8b 45 e8 mov -0x18(%rbp),%rax
400590: 48 89 c7 mov %rax,%rdi
400593: e8 c5 ff ff ff callq 40055d <strlen>
400598: 48 89 45 f0 mov %rax,-0x10(%rbp)
40059c: 48 c7 45 f8 03 00 00 movq $0x3,-0x8(%rbp)
4005a3: 00
4005a4: b8 00 00 00 00 mov $0x0,%eax
4005a9: c9 leaveq
4005aa: c3 retq
4005ab: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
and with -O3 :
0000000000400470 <main>:
400470: 48 83 ec 08 sub $0x8,%rsp
400474: bf 24 06 40 00 mov $0x400624,%edi
400479: e8 c2 ff ff ff callq 400440 <puts#plt>
40047e: bf 24 06 40 00 mov $0x400624,%edi
400483: e8 b8 ff ff ff callq 400440 <puts#plt>
400488: 31 c0 xor %eax,%eax
40048a: 48 83 c4 08 add $0x8,%rsp
40048e: c3 retq
With the flag -O0, why is the second and third call to strlen did not call the user defined strlen ?
And with -O3, why is the third call to strlen been optimized out?

Answer 1

GCC recognizes strlen() has a builtin and replaced it builtins. Hence, your version of strlen() isn't called.
I compiled your code with -fno-builtin which disables the builtins and I get the Log in strlen output twice from the 1st and 3rd strlen() calls. Probably the second strlen() gets optimized away as it's return value isn't used. This probably happens before GCC recognizes that it can't use the builtin for strlen(). Otherwise, it can't optimize away 2nd strlen() call because it has the side effect of printing the message.
Similarly, if store the result of 2nd strlen() call with something like:
size_t b = strlen(str); // call 2
then I see, "Log in strlen()" getting printed 3 times.
If I compile with -O3 (either with or without -fno-builtin), I get no output at all because, as said before, GCC optimizes away the whole
program.
This is not an issue with GCC because redefining a standard function is technically undefined behaviour and hence GCC has the liberty handle it in anyway.

Answer 2

Probably build in function enabled by default. If you wise call user define function, then change return type of strlen function.So,
in strlen_adder.h
int strlen(const char*);
in strlen_adder.c
int strlen(const char *s)
Also remove #include<string.h> in main.c file.