-lm鏈接外部函數庫

#include <math.h> #include <stdio.h> int main(int argc, char** argv) { float pi=3.1415; printf("sin(pi/2)=%f/n",sin(pi/2)); return 0; }

zhang@ubuntu:~/code$ gcc -Wall math.c -o math /tmp/cc1qOJ4d.o: In function `main': math.c:(.text+0x22): undefined reference to `sin' collect2: ld returned 1 exit status zhang@ubuntu:~/code$ gcc -Wall math.c -o math -lm zhang@ubuntu:~/code$ ./math sin(pi/2)=1.000000

可見加上-lm選項之後即可編譯成功！