printf @ plt和puts @ plt之间的区别 [英] Difference between printf@plt and puts@plt

问题描述

我一直在通过反汇编一些C代码来学习汇编语言.当我使用GDB反汇编此基本C代码时:

I've been learning assembly language by disassembling some C code. When I disassemble this basic C code with GDB:

#include <stdio.h>
void main(void) {
    printf("Hello World\n");
}

在汇编代码中，它给出了这一行:

Among assembly code, it gives this line:

0x08048424 <+25>:   call   0x80482e0 <puts@plt>

但是，当我反汇编以下在printf函数中具有整数的代码时:

However, when I disassemble below code which has an integer in printf function:

#include <stdio.h>
void main(void) {
    int a = 1;
    printf("Hello Word %d\n", a);
}

它给出了这一行:

0x0804842e <+35>:   call   0x80482e0 <printf@plt>

printf @ plt和puts @ plt有什么区别?

What is the difference between printf@plt and puts@plt?

为什么反汇编程序在没有整数参数的情况下无法识别printf函数?

Why disassembler does not recognize printf function without an integer parameter?

推荐答案

在GCC中， printf 和 puts 是内置函数.这意味着编译器完全了解其语义.在这种情况下，如果编译器认为会产生更好(更快和/或更紧凑)的代码，则可以将对一个函数的调用替换为对另一个函数的等效调用.

In GCC printf and puts are built-in functions. Which means that the compiler has full knowledge of their semantics. In such cases the compiler is free to replace a call to one function with an equivalent call to another, if it thinks that it will produce better (faster and/or more compact) code.

puts 是通常更有效的函数，因为它不必解析和解释格式字符串.

puts is a generally more efficient function since it does not have to parse and interpret a format string.

这正是您的情况.您第一次拨打 printf 并不需要任何特定于 printf 的功能.您提供给 printf 的格式字符串很简单:它没有转换说明符.编译器认为，等效调用 put 可以更好地满足您对 printf 的首次调用.

This is exactly what happened in your case. Your first call to printf does not really need any printf-specific features. The format string you supplied to printf is trivial: it has no conversion specifiers in it. The compiler thought that your first call to printf is better served with an equivalent call to puts.

与此同时，您第二次调用 printf 时，会轻而易举地使用 printf 格式字符串，即它依赖于 printf 特定的功能.

Meanwhile, your second call to printf makes non-trivial use of printf format string, i.e. it relies on printf-specific features.

(从2005年开始对此问题进行了相当彻底的研究: http://www.ciselant.de/projects/gcc_printf/gcc_printf.html )

(Some rather thorough research of this specific matter from 2005: http://www.ciselant.de/projects/gcc_printf/gcc_printf.html)

这篇关于printf @ plt和puts @ plt之间的区别的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！