global asm block有什么用？

asm ("movl $1, %ebx\n");
int main()
{
}

这种asm有什么用？


glibc中看见个使用
.c可能会被直接生成汇编代码...

/* This file is compiled into assembly code which is then munged by a sed
   script into two files: crti.s and crtn.s.

   * crti.s puts a function prologue at the beginning of the
   .init and .fini sections and defines global symbols for
   those addresses, so they can be called as functions.

   * crtn.s puts the corresponding function epilogues
   in the .init and .fini sections. */