Understanding the strcmp function of gnu libc

Question

Here is the strcmp function that i found in the glibc:

int
STRCMP (const char *p1, const char *p2)
{
  const unsigned char *s1 = (const unsigned         


        

Answer 1

You're correct of course. One of the casts should be sufficient. Especially if the pointer is cast, casting the retrieved value is a no-op.

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Here is the x86-64 compiled with gcc -O3 for the one with unnecessary cast:

STRCMP:
.L4:
        addq    $1, %rdi
        movzbl  -1(%rdi), %eax
        addq    $1, %rsi
        movzbl  -1(%rsi), %edx
        testb   %al, %al
        je      .L7
        cmpb    %dl, %al
        je      .L4
        subl    %edx, %eax
        ret
.L7:
        movzbl  %dl, %eax
        negl    %eax
        ret

and here's the one without the unnecessary cast:

STRCMP:
.L4:
        addq    $1, %rdi
        movzbl  -1(%rdi), %eax
        addq    $1, %rsi
        movzbl  -1(%rsi), %edx
        testb   %al, %al
        je      .L7
        cmpb    %dl, %al
        je      .L4
        subl    %edx, %eax
        ret
.L7:
        movzbl  %dl, %eax
        negl    %eax
        ret

They're identical

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However there is one gotcha, that is now mostly of historical interest. If char is signed and the signed representation is not two's complement,

*(const unsigned char *)p1

and

(unsigned char)*p1

are not equivalent. The former reinterprets the bit pattern, while the latter converts the value using modulo arithmetic. This is of only historical interest since not even GCC supports any architecture that doesn't have 2's complement signed representation. And it is the compiler with most ports.