c - 为什么如果我重定向文件上的 STD_OUT 并写入 STD_OUT 文件仍然为空？

Question

我无法用标准输出解决问题 笑，我在 Unix 操作系统上，所以 STD_IN = 0，STD_OUT = 1，STD_ERR = 2 的文件描述符，基本上我试图关闭与标准输出，然后用我要写入的文件覆盖它，但是当我打开文件时它是空的。代码:

#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
/*special file's descriptor  --use it or constants in unistd.h-- */
#define STD_IN 0 /*standard input stream*/
#define STD_OUT 1 /*standard output stream*/
#define STD_ERR 2 /*standard output-error stream*/

int main(unsigned int num_of_args, char** args)
{
    if(num_of_args != 3)
    {
        write(STD_ERR, "Few argouments.\nThe use  = ./executable <message> <file>\n", 69);
        return -1;
    }

    int file_des= open(args[2],  O_CREAT,  0640);

    if(file_des < 0)
    {
        write(STD_ERR, "Error, we couldn't open file.\n", 31);
        return -1;
    }

    //I close the descriptor associated with STD_OUT
    close(STD_OUT);

    //I copy the descriptor associated with fil_des on the first available descriptor(so STD_OUT)
    dup(file_des);

    write(STD_OUT, args[1], sizeof(args[1]));

    close(file_des);

    return 0;
}

应该没问题，我没有看到任何错误，但是当我打开文件时，我没有找到写入的消息。因为？帮帮我

Answer 1

此代码不正确:

int file_des= open(args[2],  O_CREAT,  0640);

根据 the POSIX documentation for open() (注意粗体部分 - 我的粗体):

SYNOPSIS

#include <sys/stat.h>
#include <fcntl.h>

int open(const char *path, int oflag, ...);
int openat(int fd, const char *path, int oflag, ...);

DESCRIPTION ...

Values for oflag are constructed by a bitwise-inclusive OR of flagsfrom the following list, defined in <fcntl.h>. Applications shallspecify exactly one of the first five values (file access modes) belowin the value of oflag:

O_EXEC
    Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.
O_RDONLY
    Open for reading only.
O_RDWR
    Open for reading and writing. The result is undefined if this flag is applied to a FIFO.
O_SEARCH
    Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.
O_WRONLY
    Open for writing only.

你的 open()电话需要包括 O_WRONLY或 O_RDWR当您写入文件时:

int file_des= open(args[2],  O_CREAT | O_WRONLY,  0640);