来自 unistd.h 的 linux read() function 对我不起作用:(

Question

I tried everything i could think of but for some reason it doesn't store the data from the file to "data", but the file has the written data.

#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

int main()
{
   char data[69]=" ";
   int fd = open("./MyFile.txt", O_RDWR | O_CREAT | O_SYNC | O_RSYNC);
   write(fd, "HELLO", 5);
   read(fd, data, 5);

   cout << data << endl;

return 0;
}

Could you guys please help me. I'm trying to learn file I/O and I don't know if it's the O_RDWR or whatever that is wrong here.

Answer 1

From man write :

For a seekable file (ie, one to which lseek(2) may be applied, for example, a regular file) writing takes place at the current file offset, and the file offset is incremented by the number of bytes actually written.

From man read :

On files that support seeking, the read operation commences at the current file offset , and the file offset is incremented by the number of bytes read.

You need to seek back to the start of the file after your write , if you want to then read what you just wrote:

lseek(fd, 0, SEEK_SET);

Always read and study the documentation for the functions that you use, particularly when they're not doing what you thought they would do.

Answer 2

The file descriptor position is at the end after the write(2) . To read(2) from the beginning, you need rewind the fd back to the beginning.

You can do with lseek :

   write(fd, "HELLO", 5);
   lseek(fd, 0, SEEK_SET);
   read(fd, data, 5);

Also you should add error checks for all these system calls (open, read, write, lseek).