引用math.h的时候，数学函数报错为未定义的符号

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本文介绍了在使用GCC编译器时遇到的数学函数链接错误问题，如log10、floor和pow等函数未定义的参考。文章提供了具体的解决方案，即在编译命令中加入-lm选项来链接数学库。

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undefined reference to `log10'
undefined reference to `floor'
undefined reference to `pow‘

搜了一下，原来是在编译的是没有没有引用数学函数的函数库导致的，在gcc yoursourcefile.c 的时候加上' -lm' 就好了 "gcc -lm yoursourcefile.c" 编译通过

参考：http://stackoverflow.com/questions/6534191/undefined-reference-to-only-some-math-h-functions

The linker isn't complaining about pow((double) 2, (double) 3) because the compiler is replacing it with a constant 8.0. You shouldn't depend on this behavior; instead, you should always use the -lm option properly. (BTW, that's more clearly written as pow(2.0, 3.0).

Consider the following program:

#include <stdio.h>
#include <math.h>
int main(void) {
    double x = 0.1;
    printf("%g\n", pow(2.0, 3.0));
    printf("%g\n", asin(x));
    return 0;
}

When I compile and link it on my system using

gcc c.c -o c

I get:

/tmp/ccXx8ZRL.o: In function `main':
c.c:(.text+0x36): undefined reference to `asin'
collect2: ld returned 1 exit status

Note that it complains about asin but not about pow.

If I change the pow call to pow(x, 3.0), I get:

/tmp/ccOeSaBK.o: In function `main':
c.c:(.text+0x24): undefined reference to `pow'
c.c:(.text+0x52): undefined reference to `asin'
collect2: ld returned 1 exit status

Normally if you want to call a standard math library function, you need to have #include <math.h> at the top of the source file (I presume you already have that) and you need to pass the -lm option to the compiler after the file that needs it. (The linker keeps track of references that haven't been resolved yet, so it needs to see the object file that refers to asin first, so it can resolve it when it sees the math library.)

The linker isn't complaining about the call to pow(2.0, 3.0) because gcc is clever enough to resolve it to a constant 8.0. There's no call to the pow function in the compiled object file, so the linker doesn't need to resolve it. If I change pow(2.0, 3.0) to pow(x, 3.0), the compiler doesn't know what the result is going to be, so it generates the call.