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