Redefining functions lin libC

Question

Assume the following simple C code:

file1.c

#include <stdio.h>

char* gets(char* i){
  return i;
}

which is redefining the libC native function gets.

This compiles fine with gcc file1.c.

My question is howcome the linker isn't complaining about duplicate symbols as this function is also defined in libC itself?

@chux Linker warns with `warning: the `gets' function is dangerous and should not be used` when using it. I use C11. — Some Name, Mar 20 '19 at 03:26
Alex, be sure to try `gcc -std=c11 -Wall -Wextra -Wconversion file1.c` — chux - Reinstate Monica, Mar 20 '19 at 03:34
the warning is a different matter and has to do with overflow protection — Alex, Mar 20 '19 at 03:37
the warning about `gets()` is NOT about overflow protection but rather about the function `gets()` has been depreciated for years and completely removed in the last two versions of the C programing language — user3629249, Mar 20 '19 at 05:47

score 1 · Answer 1 · answered Mar 20 '19 at 02:57

1

Because you can override functions from standard libraries in C, check this

answered Mar 20 '19 at 02:57

Nguyen Cong

174
1
13

2

Overriding standard library functions is undefined behaviour, although it often (but not always) "works as expected". See [section 7.1.3 (Reserved Identifiers)](https://port70.net/~nsz/c/c11/n1570.html#7.1.3) of the C standard, particularly paragraph 2. – rici Mar 20 '19 at 05:50

score 1 · Answer 2 · answered Mar 20 '19 at 06:43

add option -whole-archive in the link phase, as below:

gcc -c -ofile1.o file1.c
gcc -ofile1 -Wl,--whole-archive -lc file1.o -Wl,--no-whole-archive

result:

file1.o: In function `gets':
file1.c:(.text+0x0): multiple definition of `gets'
file1.o: In function `main':
file1.c:(.text+0x18): undefined reference to `gets@@GLIBC_2.2.5'
collect2: error: ld returned 1 exit status

Some Name · Answer 3 · 2019-03-20T03:53:46.593

You ran into undefined behavior see N1570 6.9(p5) External definition:

If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof or _Alignof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.161)

As can be seen. The Standard allows 2 possibilities:

exactly one definition
no more than one definition.

This might depend on the linkage of libc which itself is system dependent. In case you link libc statically the linker will complain. In case of dynamic linking it have no reasong of complaining.

If you take a look at objdump you will find plt section like

898:   e8 b3 fe ff ff          callq  750 <gets@plt>

Anyway the behavior is undefined.

Redefining functions lin libC

3 Answers3