为什么gcc / clang默认知道链接到libc？

Question

When I run clang/gcc to compile a .c file, I don't need to explicitly link to libc. But it still works as libc and two additional libraries are automatically linked. Why does gcc/clang know to link automatically? Where is this behavior mentioned?

$ cat main.c 
/* vim: set noexpandtab tabstop=2: */
#include <stdio.h>

int main() {
    puts("Hello World!");
    return 0;
}
$ clang -o main.exe main.c # or gcc
$ ./main.exe 
Hello World!
$ nm -D /lib/x86_64-linux-gnu/libc-2.27.so | grep -w puts
00000000000809c0 W puts
$ ldd main.exe 
    linux-vdso.so.1 (0x00007ffe743ba000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f397ce7b000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f397d26c000)

Answer 1

Why does gcc/clang know to link automatically?

The GCC developers built this into GCC as a convenience. Which libraries are linked by default is partly affected by the language being compiled, which is deduced from the file names and may be controlled with the -x switch.

Where is this behavior mentioned?

This page in the GCC documentation mentions there are some libraries linked in by default and says you can disable or modify this behavior with -nostdlib and other switches, but I do not see an explicit list of the libraries that are linked in by default. It might vary by system/platform as well as by language. You can use the -v switch to ask GCC to show you the commands it is executing, and the link command (using ld ) should reveal the libraries.