/in/foo.c: In function 'main':
/in/foo.c:24:5: warning: implicit declaration of function 'println' [-Wimplicit-function-declaration]
     println("请输入第一个字符串：");
     ^~~~~~~
/tmp/cco2dkKW.o: In function `main':
foo.c:(.text.startup+0x13): undefined reference to `println'
foo.c:(.text.startup+0x3a): undefined reference to `println'
foo.c:(.text.startup+0xe9): undefined reference to `println'
foo.c:(.text.startup+0xf9): undefined reference to `println'
collect2: error: ld returned 1 exit status