C Math sqrt和Gcc -lm选项 [英] C Math sqrt and Gcc -lm option
问题描述
一些善良的灵魂可以帮助我理解为什么我必须使用-lm
gcc选项(像这样
gcc main.c -o main -lm
)在情况(1)中没有,而在情况(2)中没有?
Could some good soul help me understand why do I have to use -lm
gcc option (like so
gcc main.c -o main -lm
) in case (1) and do not have in case (2)?
案例(1)
#include <math.h>
int main() {
float x = 4;
sqrt(x);
return 0;
}
案例(2)
#include <math.h>
int main() {
sqrt(4);
return 0;
}
我正在使用带有gcc的Ubuntu 16.04.
我知道为什么我应该使用-lm
,但是我不知道为什么没有它的情况(2).
I'm using Ubuntu 16.04, with gcc.
I know why I should use -lm
, but I don't know why the case (2) works without it.
推荐答案
让我们看一下几个函数.这是第一个:
Let's have a look at a couple of functions. Here's the first one:
#include <math.h>
double func(double num) {
return sqrt(num);
}
使用gcc -O3 -c so.c
进行编译,并使用objdump -d so.o
进行反汇编:
Compile with gcc -O3 -c so.c
and disassemble with objdump -d so.o
:
0000000000000000 <func>:
0: f2 0f 51 c8 sqrtsd %xmm0,%xmm1
4: 66 0f 2e c9 ucomisd %xmm1,%xmm1
8: 7a 05 jp f <func+0xf>
a: 66 0f 28 c1 movapd %xmm1,%xmm0
e: c3 retq
f: 48 83 ec 08 sub $0x8,%rsp
13: e8 00 00 00 00 callq 18 <func+0x18>
18: 48 83 c4 08 add $0x8,%rsp
1c: c3 retq
因此,我们有许多FPU寄存器操作和一个调用.让我们尝试看看该调用所调用的内容.运行nm so.o
:
So we have a bunch of FPU register operations, and a call. Let's try to see what that call calls. Run nm so.o
:
0000000000000000 T func
U sqrt
因此,我们正在定义(T
)一个名为"func"的函数,并导入(U
)一个名为sqrt
的符号.最终的程序必须与-lm
链接,因为实际上是在其中定义sqrt
的地方.
So we are defining (T
) a function called "func", and importing (U
) a symbol called sqrt
. The eventual program will have to be linked with -lm
, as that's where sqrt
is actually defined.
现在让我们尝试第二个功能:
Now let's try a second function:
#include <math.h>
double func() {
return sqrt(4);
}
再次使用gcc -O3 -c so.c
进行编译,并使用objdump -d so.o
进行反汇编:
Again, compile with gcc -O3 -c so.c
and disassemble with objdump -d so.o
:
0000000000000000 <func>:
0: f2 0f 10 05 00 00 00 movsd 0x0(%rip),%xmm0 # 8 <func+0x8>
7: 00
8: c3 retq
没有FPU寄存器操作.没有函数调用.让我们用nm so.o
验证:
No FPU register operations. No function calls. Let's verify with nm so.o
:
0000000000000000 T func
0000000000000000 r .LC0
是的.不导入符号.
如果省略#include <math.h>
,您将了解这里发生的事情:
You get a hint of what's going on here if you omit #include <math.h>
:
$ gcc -O3 -g -c so.c
so.c: In function ‘func’:
so.c:2:10: warning: incompatible implicit declaration of built-in function ‘sqrt’
return sqrt(4);
^
sqrt
是内置函数.编译器不会将其视为某些不透明的构造.它了解它的作用.在第二种情况下,编译器理解它不需要在运行时对函数进行调用.它知道编译期间的答案.它只是将答案放在返回寄存器中,并称之为一天.由于运行时不包含对sqrt
的调用,因此无需与-lm
链接.
sqrt
is a built-in function. The compiler does not treat it as some opaque construct. It understands what it does. In the second case, the compiler understands that it does not need to place a call to the function at run time. It knows what the answer is during compilation. It simply places the answer in the return registers and calls it a day. Since the run time does not contain a call to sqrt
, there is no need to link with -lm
.
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