gcc默认在Mac OS X上的C语言中链接数学库吗? [英] gcc link the math library by default in C on mac OS X?
问题描述
从这个问题开始:为什么必须链接C语言中的数学库?
我知道C数学库(libm)与C标准库(libc)是分开的，并且默认情况下未链接.
I know that C math library (libm) is separated from C standard library (libc), and is not linked in by default.
但是当我在Mac OS X 10.11.1上使用不带-lm的gcc filename.c编译以下代码时
:
But when I compiled the code below using gcc filename.c without -lm on mac osx 10.11.1
:
#include <math.h>
#include <stdio.h>
int
main (void)
{
double x = sqrt (2.0);
printf ("The square root of 2.0 is %f\n", x);
return 0;
}
没有链接错误，并且输出可执行文件正常工作.
There's no link error and the output executable file works correctly.
然后我尝试了otool -L output:
output:
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1225.1.1)
/opt/local/lib/libgcc/libgcc_s.1.dylib (compatibility version 1.0.0, current version 1.0.0)
我想知道Mac上是否存在某些库结构差异?
I wonder to know is there some library structure differences on mac?
还是gcc 5.2.0的新功能?
Or it's the new feature for gcc 5.2.0?
非常感谢！
更新:
我通过以下方式更改了代码
I changed the code with:
double in = 0;
scanf("%lf", &in);
double x = sqrt(in);
，它仍然不需要-lm.
然后我用otool -vVt反汇编代码:
(__TEXT,__text) section
_main:
0000000100000eed pushq %rbp
0000000100000eee movq %rsp, %rbp
0000000100000ef1 subq $0x10, %rsp
0000000100000ef5 pxor %xmm0, %xmm0
0000000100000ef9 movsd %xmm0, -0x10(%rbp)
0000000100000efe leaq -0x10(%rbp), %rax
0000000100000f02 movq %rax, %rsi
0000000100000f05 leaq 0x82(%rip), %rdi ## literal pool for: "%lf"
0000000100000f0c movl $0x0, %eax
0000000100000f11 callq 0x100000f54 ## symbol stub for: _scanf
0000000100000f16 movq -0x10(%rbp), %rax
0000000100000f1a movd %rax, %xmm0
0000000100000f1f callq 0x100000f5a ## symbol stub for: _sqrt
0000000100000f24 movd %xmm0, %rax
0000000100000f29 movq %rax, -0x8(%rbp)
0000000100000f2d movq -0x8(%rbp), %rax
0000000100000f31 movd %rax, %xmm0
0000000100000f36 leaq 0x55(%rip), %rdi ## literal pool for: "The square root of 2.0 is %f\n"
0000000100000f3d movl $0x1, %eax
0000000100000f42 callq 0x100000f4e ## symbol stub for: _printf
0000000100000f47 movl $0x0, %eax
0000000100000f4c leave
0000000100000f4d retq
似乎已调用sqrt.那么为什么Mac上的情况会有所不同?
It seems sqrt is called. So why things go different on mac?
更新:
我在以下问题中找到了结论: C std库似乎未在目标文件中链接
I found the conclusion in this question: C std library don't appear to be linked in object file
它表示在OS X上，数学库是libSystem的一部分:
$ ls -l /usr/lib/libm.dylib
lrwxr-xr-x 1 root wheel 15 3 Jun 01:39 /usr/lib/libm.dylib@ -> libSystem.dylib
推荐答案
OSX上没有单独的数学库.虽然许多系统在单独的数学库中的标准C math.h标头中提供函数，但OSX却不这样做，它是libSystem库的一部分，该库始终链接在其中.
There's no separate math library on OSX. While a lot of systems ship functions in the standard C math.h header in a separate math library, OSX does not do that, it's part of the libSystem library, which is always linked in.
除此之外，如果编译器可以在编译时执行计算，那么它可能会优化掉此类调用.
In addition to that, a compiler might optimize away any such call if it can perform the computation at compile time.
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