GCC链接库进行编译 [英] GCC linked library for compile

问题描述

当源文件中已经以 #include 的形式包含信息时，为什么我们要告诉 gcc 要链接到哪个库?

Why do we have to tell gcc which library to link against when that information is already in source file in form of #include?

例如，如果我有一个使用线程并具有以下代码的代码:

For example, if I have a code which uses threads and has:

#include <pthread.h>

我仍然必须使用 gcc 中的 -pthread 选项进行编译:

I still have to compile it with -pthread option in gcc:

gcc -pthread test.c

如果我不提供 -pthread 选项，则会在查找线程函数定义时出现错误.

If I don't give -pthread option it will give errors finding thread function definitions.

我正在使用此版本:

gcc --version
gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4

推荐答案

这可能是使初学者接触C的最常见事物之一.

This may be one of the most common things that trip up beginners to C.

在C语言中，有两个不同的步骤来构建程序，即编译和链接.就您的问题而言，这些步骤将您的代码连接到两种不同类型的文件(标头和库).

In C there are two different steps to building a program, compilation and linking. For the purposes of your question, these steps connect your code to two different types of files, headers and libraries.

C代码中的 #include< pthread.h> 指令由编译器处理.编译器(实际上是预处理器)在将C文件转换为目标文件之前，实际上将 pthread.h 的内容粘贴到您的代码中.

The #include <pthread.h> directive in your C code is handled by the compiler. The compiler (actually preprocessor) literally pastes in the contents of pthread.h into your code before turning your C file into an object file.

pthread.h 是头文件，而不是库.它包含可以在库中找到的函数，它们采用哪些自变量以及返回什么的列表.标头可以不带库而存在，反之亦然.标头是一个文本文件，通常在Unix衍生的系统上的/usr/include 中找到.您可以像打开任何C文件一样打开它以读取内容.

pthread.h is a header file, not a library. It contains a list of the functions that you can expect to find in the library, what arguments they take and what they return. A header can exist without a library and vice-versa. The header is a text file, often found in /usr/include on Unix-derived systems. You can open it just like any C file to read the contents.

命令行 gcc -lpthread test.c 进行编译和链接.在过去，您首先要做的是 cc test.c ，然后是 ld -lpthread test.o .如您所见， -lpthread 实际上是链接器的一个选项.

The command line gcc -lpthread test.c does both compilation and linking. In the old days, you would first do something like cc test.c, then ld -lpthread test.o. As you can see, -lpthread is actually an option to the linker.

链接器对诸如C代码或标头之类的文本文件一无所知.它仅适用于编译的目标文件和现有库. -l <​​/code>标志告诉它要查找哪些库以查找您正在使用的功能.

The linker does not know anything about text files like C code or headers. It only works with compiled object files and existing libraries. The -l flag tells it which libraries to look in to find the functions you are using.

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