gcc/g++ 编译器出现 undefined reference to ‘这里是函数名‘,往往意味这这个函数没有定义

例如出现：

undefined reference to `read_card'      意思是      对"read_card"未定义的引用

r@r:~/coml_/c/10.5$ gcc function.c main.c -o 1231
/tmp/ccHuRgRq.o: In function `main':
main.c:(.text+0x5): undefined reference to `read_card'
main.c:(.text+0xa): undefined reference to `analyze_cards'
collect2: error: ld returned 1 exit status