[英]sqrt from math.h causes linker error “undefined reference to sqrt” only when the argument is not a constant
I created a small program, as follows: 我创建了一个小程序,如下所示:
#include <math.h>
#include <stdio.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
int i;
double tmp;
double xx;
for(i = 1; i <= 30; i++) {
xx = (double) i + 0.01;
tmp = sqrt(xx);
printf("the square root of %0.4f is %0.4f\n", xx,tmp);
sleep(1);
xx = 0;
}
return 0;
}
When I try to compile this with the following command, I get a compiler error. 当我尝试使用以下命令对此进行编译时,出现编译器错误。
gcc -Wall calc.c -o calc
returns: 返回:
/tmp/ccavWTUB.o: In function `main':
calc.c:(.text+0x4f): undefined reference to `sqrt'
collect2: ld returned 1 exit status
If I replace the variable in the call to sqrt(xx) with a constant like sqrt(10.2), it compiles just fine. 如果我将sqrt(xx)调用中的变量替换为sqrt(10.2)之类的常量,则编译就可以了。 Or, if I explicitly link like the following:
或者,如果我明确链接如下:
gcc -Wall -lm calc.c -o calc
It also works just fine. 它也可以正常工作。 Can anyone tell me what's causing this?
谁能告诉我是什么原因造成的? I've been a C programmer for a long time (and I've written similar small programs using math.h) and I have never seen anything like this.
我很长时间以来一直是C程序员(并且我已经使用math.h编写了类似的小程序),但从未见过这样的事情。
My version of gcc follows: 我的gcc版本如下:
$ gcc --version
gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$
If you look at the output of the compiler in the case where you used sqrt(10.2)
, I'll bet you see that a call to sqrt()
isn't actually made. 如果您在使用
sqrt(10.2)
的情况下查看编译器的输出,我敢打赌,您会发现实际上并未调用sqrt()
。
This happens because GCC recognizes several functions that it can treat specially. 发生这种情况是因为GCC可以识别几种可以特殊处理的功能。 This gives it the ability to do certain optimizations, in this case Constant folding .
这样,它就可以进行某些优化,在本例中为“ 常量折叠” 。 Such special functions are called Built-ins .
这样的特殊功能称为内置功能。
In the case where it must link to the math library (because you're calling it with a variable), you need to link it explicitly. 如果必须将其链接到数学库(因为要使用变量对其进行调用),则需要显式链接。 Some operating systems/compilers do it for you, which is why you might not have noticed in the past.
一些操作系统/编译器会为您执行此操作,这就是为什么您过去可能没有注意到的原因。
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