C带有指定字节长度的读写问题

Question

I have a while loop in which I am reading from an archive file and extracting the first file.

  int fd = open(argv[2], O_RDWR | O_CREAT, 0666);
  char name_buffer[16];
  char size_buffer[10];

  // go to the first header
  lseek(fd, SARMAG, SEEK_CUR);

  // store the number of bits read in a struct current_header
  // until its size equal to the size of the entire
  // header, or in other words, until the entire
  // header is read
  while ((num_read = read(fd, (char*) &current_header, 
    sizeof(struct ar_hdr))) == sizeof(struct ar_hdr))
  {

    // scans the current string in header and stores
    // in nameStr array
    sscanf(current_header.ar_name, "%s", name_buffer);
    sscanf(current_header.ar_date, "%s", date_buffer);
    sscanf(current_header.ar_uid, "%s", uid_buffer);
    sscanf(current_header.ar_gid, "%s", gid_buffer);

    int mode;
    sscanf(current_header.ar_mode, "%o", &mode);
    sscanf(current_header.ar_size, "%s", size_buffer);
    sscanf(current_header.ar_fmag, "%s", fmag_buffer);

    new_file_fd = open(name_buffer, O_WRONLY | O_CREAT | O_TRUNC);
    int size = atoi(size_buffer);
    char buf[size];
    size_t count = size;

    while ((n_read = read(fd, buf, size)) > 0)
    {
      n_write = 0;
      do {
        n = write(new_file_fd, &buf[n_write], n_read - n_write);
        n_write += n;
      } while (n_write < n_read);

    }
    close(new_file_fd);
   }
   lseek(fd, size + (size%2), SEEK_CUR);

For a given archive file with the structure:

!<arch>
File1             1382142494  501   20    100644  29        `
Testing 123

File2             1382142504  501   20    100644  23        `
Testing 246

The expected output should be a file "File1" that contains only the contents "Testing123". Instead, I get File1 with the contents: Testing 123

File2             1382142504  501   20    100644  23        `
Testing 246

For some reason, even though I've specified the amount of bits to read and write (parameter 3 is "size" which returns 29 -- the size of File1) it keeps reading and writing past 29 bytes.

Any ideas why this might be happening?

Answer 1

while ((n_read = read(fd, buf, size)) > 0)

You need to update size within the loop.

size -= n_read

Otherwise you're just going to continue looping until you get to the end of the file. The reason you need to loop a call to read() is just that it doesn't guarantee it will read the specified number of bytes on the first call, it only guarantees it won't exceed that. So you need to keep calling it until you've read all the bytes you want. But you do need to update the bytes parameter since the file descriptor fd will keep advancing to the end of the file otherwise.