pread和pwrite未定义? [英] pread and pwrite not defined?

问题描述

我正在尝试使用pread和pwrite，以便可以查找文件的开头并开始在一个原子操作中进行读取或写入.这两个函数都会为我做到这一点，但是，我遇到的问题是，即使我添加了 #define _XOPEN_SOURCE 500，编译器也向我发出警告:函数"pread"的隐式声明和#include< unistd.h> 就像手册页中所说的那样.我想念什么吗?下面是带有函数调用的两个语句.谢谢！

I am trying to use pread and pwrite so that I can lseek to the beginning of the file and start reading or writing in one atomic operation. Both of these functions will do that for me however, the issue I am having is that the compiler is giving me warning: implicit declaration of function ‘pread’ even after I added #define _XOPEN_SOURCE 500 and #include<unistd.h> like the man pages said. Am I missing something? The two statements with the function calls are below. Thanks!

 #include<unistd.h>



 #define _XOPEN_SOURCE 500
    int main (int argc, char *argv[])
    {



while ( (read_in += pread(source_fd, in_buf, in_buf_size,read_in) )  )
    {

if (write_out += pwrite(dest_fd, in_buf, read_in, write_out) == -1)
        {

推荐答案

您需要在包含以下内容之前定义 _XOPEN_SOURCE 500 :

You need to define _XOPEN_SOURCE 500 before your includes:

#define _XOPEN_SOURCE 500

#include<unistd.h>

否则， unistd.h 标头将看不到宏定义.

Otherwise, the unistd.h header won't see the macro definition.

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