Chapter 4 File I/O

File I/O

• 6 types of files.

• 4 types of file syscall

  • open
  • read
  • write
  • close

A program to copy a file

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>

// __POSIX_C_SOURCE needed for *strtok, strcat


const int FILENAME_LIMIT = 100;
const int BUFFER_SIZE = 10;


int main(int argc, char *argv[]) {

   if (argc != 2) {
      printf("exactly one arguement expected\n");
      return 1;
   }
   char filename[FILENAME_LIMIT]; // support only 100 chars in filename
   strcpy(filename, argv[1]);
   int fd = open(filename, O_RDONLY);

   char* file = strtok(filename, ".");
   char* ext = strtok(NULL, ".");
   char copied_file_name[FILENAME_LIMIT];

   if (sizeof(ext) == 0) {
      // no extension in file
      strcat(copied_file_name, file);
      strcat(copied_file_name,  "_copy");


   } else {

      strcat(copied_file_name, file);
      strcat(copied_file_name,  "_copy");
      strcat(copied_file_name,  ".");
      strcat(copied_file_name,  ext);
   }




   int fd_cpy = open(copied_file_name, O_RDWR | O_CREAT, 0664 );


   if (fd == -1 || fd_cpy == -1) {
      perror("could not open file");
      return 1;
   }

   int buf[BUFFER_SIZE] ; // allocate this on stack so that freeing is not required
   int read_status = 0;
      while( (read_status = read(fd, buf, BUFFER_SIZE)) > 0) {
        int write_status =  write(fd_cpy, buf, read_status);
        if (write_status == -1) {
         perror("write");
        }
        memset(buf, '\0',BUFFER_SIZE);
      }
      if (read_status != 0) {
         perror("read");
         return 0;
      }
      int write_status =  write(fd_cpy, buf, read_status);
        if (write_status == -1) {
         perror("write");
      }
      close(fd);
      close(fd_cpy);

   return 0;
}

• SUSv3 specifies that if open() succeeds, it is guaranteed to use the lowest-numbered unused file descriptor for the process.

• O_DIRECT flag Allow file I/O to bypass the buffer cache. This feature is described in Sec- tion 13.6. The _GNU_SOURCE feature test macro must be defined in order to make this constant definition available from .

• creat() syscall is old syscall wgen O_CREATE could not be passed to open

• read does not add '\0' char at the end explicitly

• For a disk file, possible reasons for such a partial write are that the disk was filled or that the process resource limit on file sizes was reached. (The rele- vant limit is RLIMIT_FSIZE

• close FD so that we do not run out of FDs

• lseek moves the offset to read and write by a specified number. It does not work for

  • pipes
  • sockets
  • FIFO
  • terminal

but works for tapes and disks

file holes

e. Somewhat surprisingly, it is possible to write bytes at an arbitrary point past the end of the file. The space in between the previous end of the file and the newly written bytes is referred to as a file hole. From a programming point of view, the bytes in a hole exist, and reading from the hole returns a buffer of bytes containing 0 (null bytes).

• writing to holes increases disk size but not file size

• holes do not consume disk space but appear in file size

• ioctl can be used to write control bytes to a device or file when its not possible using read and write