CODEWITHSUNDEEP
cunixglibc
Simon Raily
Simon Raily

Reputation: 29

Problems with reading from file. read() syscall

When I try read data from file and print it, printf prints an empty string to terminal.

Use: Ubuntu 16.04.

gcc version 5.4.0.

kernel: 4.15.0-43-generic

Tried:

add fsync call after writing data.

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>

#define SIZE 6

int main()
{
   int ret = -1;
   char buffer[SIZE] = { 0 };
   int fd = open("data.txt", O_CREAT | O_RDWR, 0666);

   if (fd < 0)
   {
   perror("open()");
   goto Exit;
   }

   if (write(fd, "Hello", 5) < 0)
   {
       perror("write()");
   goto Exit;
   }

   fsync(fd);

   if (read(fd, buffer, SIZE - 1) < 0)
   {
   perror("read()");
   goto Exit;
   }

   printf("%s\n", buffer);
   ret = 0;

   Exit:
        close(fd);
        return ret;
}

Expected: should write and read data from/to file.

Actual: data writes to file. After reading data, printf prints an empty string.

Upvotes: 1

Views: 240

Answers (1)

Maxim Egorushkin
Maxim Egorushkin

Reputation: 136425

After writing you need to rewind the file.

Fix:

lseek(fd, 0, SEEK_SET);

Note that generally you do not need to zero-initialize your read buffers, that is a waste of time. You should rather use the return value of read/recv to determine the length of the received data and zero-terminate it manually, if necessary.

Fix:

ssize_t r = read(fd, buffer, SIZE - 1);
if (r < 0)
    // handle error
buffer[r] = 0; // zero-terminate manually.
printf("%s\n", buffer);

Upvotes: 2

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