printf becomes puts

Firstly, let's read following simple code:

#include <stdio.h>
int input_number(void)
{
        printf("please input numbers:\n");
        return 0;
}
int output_number(void)
{
        printf("check my implementation");
        return 0;
}

int main(int argc, char** argv)
{
        input_number();
        output_number();
}

in function input_number() and output_number(), printf() are called.

After build, will them still be the printf()? Let's dump it.

0000000000001169 <input_number>:
    1169:       f3 0f 1e fa             endbr64
    116d:       55                      push   %rbp
    116e:       48 89 e5                mov    %rsp,%rbp
    1171:       48 8d 3d 8c 0e 00 00    lea    0xe8c(%rip),%rdi        # 2004 <_IO_stdin_used+0x4>
    1178:       e8 e3 fe ff ff          callq  1060 <[email protected]>
    117d:       b8 00 00 00 00          mov    $0x0,%eax
    1182:       5d                      pop    %rbp
    1183:       c3                      retq

0000000000001184 <output_number>:
    1184:       f3 0f 1e fa             endbr64
    1188:       55                      push   %rbp
    1189:       48 89 e5                mov    %rsp,%rbp
    118c:       48 8d 3d 87 0e 00 00    lea    0xe87(%rip),%rdi        # 201a <_IO_stdin_used+0x1a>
    1193:       b8 00 00 00 00          mov    $0x0,%eax
    1198:       e8 d3 fe ff ff          callq  1070 <[email protected]>
    119d:       b8 00 00 00 00          mov    $0x0,%eax
    11a2:       5d                      pop    %rbp
    11a3:       c3                      retq

Look, in input_number(), address 1178 callq puts.

in output_number(), address 1198 callq printf.

Interesting.

How do I notice it. The whole story is:

we build FreeRTOS with Linux GCC(for some reason). And we found Linux GCC GLibc printf goes to tty. So we write a printf() in FreeRTOS. But when we use printf() in main(), GCC reports link error of missing puts(). 

Other tips:

1. -nostdlib to avoid use GLibc.

2. implement our own libc functions.

3. remove relocation section(.rela.dyn). So all the libs we developed need to avoid dynamically link by removing cflags -fPIE -fPIC and link flags -pie.  

Refer to: https://blog.csdn.net/alex_mianmian/article/details/50582833