gcc不会正确包含math.h
这是一个简单的例子，概述了我的问题
test.c的：
#include <stdio.h> #include <math.h> main () { fmod ( 3, 2 ); } 这里是我发布的编译test.c的命令
gcc -lm test.c -o test
这是我发出上述命令时得到的输出
/tmp/ccQmRk99.o: In function `main': test.c:(.text+0x3e): undefined reference to `fmod' collect2: ld returned 1 exit status
我得到相同的输出，而不是我使用cc 。 我正在使用以下版本的gcc
gcc-4.6.real (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
任何想法，为什么我的程序不会编译？
问题来自链接器， ld ，而不是gcc（因此退出状态消息）。 通常，ld需要在订单user supplier指定对象和库，其中user是使用库函数的对象， supplier是提供库的对象。
当你的test.c被编译成一个对象时，编译器指出fmod是一个未定义的引用
$ gcc -c test.c $ nm test.o U fmod 0000000000000000 T main
（nm列出了一个目标文件引用的所有function）
链接器将未定义的引用更改为已定义的引用，查找引用以查看它们是否在其他文件中提供。
$ gcc -lm test.o $ nm a.out 0000000000600e30 d _DYNAMIC 0000000000600fe8 d _GLOBAL_OFFSET_TABLE_ 00000000004006a8 R _IO_stdin_used w _Jv_RegisterClasses 0000000000600e10 d __CTOR_END__ ... 0000000000601018 D __dso_handle w __gmon_start__ ... U __libc_start_main@@GLIBC_2.2.5 0000000000601020 A _edata 0000000000601030 A _end 0000000000400698 T _fini 0000000000400448 T _init 0000000000400490 T _start 00000000004004bc t call_gmon_start 0000000000601020 b completed.7382 0000000000601010 W data_start 0000000000601028 b dtor_idx.7384 U fmod@@GLIBC_2.2.5 0000000000400550 t frame_dummy 0000000000400574 T main
其中大多数是指在main之前和之后运行的libc函数来设置环境。 你可以看到fmod现在指向glibc，它将由共享库系统解决。
我的系统默认设置为使用共享库。 如果我反而强制静态链接，我得到你看到的顺序依赖
$ gcc -static -lm test.o test.o: In function `main': test.c:(.text+0x40): undefined reference to `fmod' collect2: ld returned 1 exit status
将-lm稍后放在链接器命令中， 在 test.o 之后 ，允许它成功链接。 现在检查符号fmod应该被parsing为一个实际的地址，事实上是这样
$ gcc -static test.o -lm $ nm a.out | grep fmod 0000000000400480 T __fmod 0000000000402b80 T __ieee754_fmod 0000000000400480 W fmod
从gcc（1）手册页：“-l选项的位置很重要”。
特别：
-llibrary -l library Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.) It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with -L. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several directories.
