Why the returns of strcmp is different? [duplicate

问题:

This question already has an answer here:

• Inconsistent strcmp() return value when passing strings as pointers or as literals 2 answers

Here is the C code and I compiled with gcc

char *a="a";
char *d="d";
printf("%d\n", strcmp("a", "d"));
printf("%d\n", strcmp(a, "d"));
printf("%d\n", strcmp(a, d));

When I compiled with -O the output is

-1
-3
-1

When I compiled without -O then output is

-1
-3
-3

Why the output is different and what is the code of strcmp?

回答1:

Why the output is different

Because all that matters is the sign (positive, negative or zero) of the return value. strcmp() is not required to return +1 or -1, nor does it have to return consistent values. I suspect that in the first and third case, the compiler optimizes away the call to strcmp() and puts -1 into the place of the return value. In the second case, I think the function is actually called.

what is the code of strcmp?

Deducing from the fact that it seemingly returns the difference between the character codes of the first differing character, I'd say this is glibc's strcmp():

int
 strcmp (p1, p2)
      const char *p1;
      const char *p2;
 {
   register const unsigned char *s1 = (const unsigned char *) p1;
   register const unsigned char *s2 = (const unsigned char *) p2;
   unsigned char c1, c2;

   do
     {
       c1 = (unsigned char) *s1++;
       c2 = (unsigned char) *s2++;
       if (c1 == '\0')
     return c1 - c2;
     }
   while (c1 == c2);

   return c1 - c2;
 }

Edit: @AndreyT doesn't believe me, so here's the assembly GCC 4.2 generated for me (OS X 10.7.5 64-bit Intel, default optimization level - no flags):

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:
Leh_func_begin1:
    pushq   %rbp
Ltmp0:
    movq    %rsp, %rbp
Ltmp1:
    subq    $32, %rsp
Ltmp2:
    leaq    L_.str(%rip), %rax
    movq    %rax, -16(%rbp)
    leaq    L_.str1(%rip), %rax
    movq    %rax, -24(%rbp)
    movl    $-1, %ecx             ; <- THIS!
    xorb    %dl, %dl
    leaq    L_.str2(%rip), %rsi
    movq    %rsi, %rdi
    movl    %ecx, %esi
    movq    %rax, -32(%rbp)
    movb    %dl, %al
    callq   _printf               ; <- no call to `strcmp()` so far!
    movq    -16(%rbp), %rax
    movq    %rax, %rdi
    movq    -32(%rbp), %rsi
    callq   _strcmp               ; <- strcmp()
    movl    %eax, %ecx
    xorb    %dl, %dl
    leaq    L_.str2(%rip), %rdi
    movl    %ecx, %esi
    movb    %dl, %al
    callq   _printf               ; <- printf()
    movq    -16(%rbp), %rax
    movq    -24(%rbp), %rcx
    movq    %rax, %rdi
    movq    %rcx, %rsi
    callq   _strcmp               ; <- strcmp()
    movl    %eax, %ecx
    xorb    %dl, %dl
    leaq    L_.str2(%rip), %rdi
    movl    %ecx, %esi
    movb    %dl, %al
    callq   _printf               ; <- printf()
    movl    $0, -8(%rbp)
    movl    -8(%rbp), %eax
    movl    %eax, -4(%rbp)
    movl    -4(%rbp), %eax
    addq    $32, %rsp
    popq    %rbp
    ret
Leh_func_end1:

    .section    __TEXT,__cstring,cstring_literals
L_.str:
    .asciz   "a"

L_.str1:
    .asciz   "d"

L_.str2:
    .asciz   "%d\n"

    .section    __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame0:
Lsection_eh_frame:
Leh_frame_common:
Lset0 = Leh_frame_common_end-Leh_frame_common_begin
    .long   Lset0
Leh_frame_common_begin:
    .long   0
    .byte   1
    .asciz   "zR"
    .byte   1
    .byte   120
    .byte   16
    .byte   1
    .byte   16
    .byte   12
    .byte   7
    .byte   8
    .byte   144
    .byte   1
    .align  3
Leh_frame_common_end:
    .globl  _main.eh
_main.eh:
Lset1 = Leh_frame_end1-Leh_frame_begin1
    .long   Lset1
Leh_frame_begin1:
Lset2 = Leh_frame_begin1-Leh_frame_common
    .long   Lset2
Ltmp3:
    .quad   Leh_func_begin1-Ltmp3
Lset3 = Leh_func_end1-Leh_func_begin1
    .quad   Lset3
    .byte   0
    .byte   4
Lset4 = Ltmp0-Leh_func_begin1
    .long   Lset4
    .byte   14
    .byte   16
    .byte   134
    .byte   2
    .byte   4
Lset5 = Ltmp1-Ltmp0
    .long   Lset5
    .byte   13
    .byte   6
    .align  3
Leh_frame_end1:


.subsections_via_symbols

And the original source code:

#include <stdio.h>
#include <string.h>

int main()
{
    const char *a = "a";
    const char *d = "d";
    printf("%d\n", strcmp("a", "d"));
    printf("%d\n", strcmp(a, "d"));
    printf("%d\n", strcmp(a, d));

    return 0;
}

And the output it generated (screenshot for having a better proof):

回答2:

The C standard allows the implementation to return any negative value. It also allows the implementation to do optimizations of library function calls as long as the result obeys the standard ... thus, implementations can optimize functions like strcmp by generating inline machine instructions instead of calling a function. Extra optimizations are possible when arguments are constants. So the reason the results are different is because the optimizer happens to generate different code for some of the cases. A conforming program is not allowed to care which negative value is returned.

Edit:

On my system at the moment, the output is

-1
-3
-3

Here is the code the compiler generated that produced those results (obtained with gcc -S):

    movl    $-1, 4(%esp)
    movl    $LC2, (%esp)
    call    _printf
    movl    $LC1, 4(%esp)
    movl    28(%esp), %eax
    movl    %eax, (%esp)
    call    _strcmp
    movl    %eax, 4(%esp)
    movl    $LC2, (%esp)
    call    _printf
    movl    24(%esp), %eax
    movl    %eax, 4(%esp)
    movl    28(%esp), %eax
    movl    %eax, (%esp)
    call    _strcmp
    movl    %eax, 4(%esp)

As you can see, there are only two strcmp calls. The -1 result for the first comparison is produced at compile-time, because the compiler knows that "a" is less than "d". If I use -O, it produces this code:

    movl    $-1, 4(%esp)
    movl    $LC0, (%esp)
    call    _printf
    movl    $-1, 4(%esp)
    movl    $LC0, (%esp)
    call    _printf
    movl    $-1, 4(%esp)
    movl    $LC0, (%esp)
    call    _printf

回答3:

I'm getting

 -1
 -3
 -1

output for optimized (-O4) build with GCC 4.1.2 on Linux. Here's the code that the compiler generates for main

main:
.LFB25:
        subq    $8, %rsp
.LCFI0:
        movl    $-1, %esi
        xorl    %eax, %eax
        movl    $.LC0, %edi
        call    printf
        movzbl  .LC1(%rip), %edx
        movzbl  .LC2(%rip), %eax
        movl    %edx, %esi
        subl    %eax, %esi
        jne     .L2
        movzbl  .LC1+1(%rip), %esi
        movzbl  .LC2+1(%rip), %eax
        subl    %eax, %esi
.L2:
        movl    $.LC0, %edi
        xorl    %eax, %eax
        call    printf
        movl    $-1, %esi
        movl    $.LC0, %edi
        xorl    %eax, %eax
        call    printf
        xorl    %eax, %eax
        addq    $8, %rsp
        ret

which means that the first and the last comparisons were actually optimized out, while the middle comparison was actually implemented intrinsically through subtraction (which is why it produced -3). I don't see any logic in this selective behavior, so it is probably just a quirk of the optimizer.

BTW, without optimization the same GCC 4.1.2 produces

 -1
 -1
 -1

output because it calls strcmp. strcmp in this standard library is implemented as

<strcmp>           mov    (%rdi),%al
<strcmp+2>         cmp    (%rsi),%al
<strcmp+4>         jne    <strcmp+19>
<strcmp+6>         inc    %rdi
<strcmp+9>         inc    %rsi
<strcmp+12>        test   %al,%al
<strcmp+14>        jne    <strcmp>
<strcmp+16>        xor    %eax,%eax
<strcmp+18>        retq
<strcmp+19>        mov    $0x1,%eax
<strcmp+24>        mov    $0xffffffff,%ecx
<strcmp+29>        cmovb  %ecx,%eax
<strcmp+32>        retq

which means that it is intentionally implemented to return -1, 0 or +1, even if it might be seen as suboptimal.

回答4:

strcmp returns < 0 if strings are not equal.
It indicates that second string has higher value for the first character that does not match in the strings. The exact precise value is Unspecified.
The only thing defined is whether the output is:

• Zero or
• Positive or
• Negative