Input/output

François Trahay

In this lecture, we mainly talk about files, as this is the easiest example of I/O to manipulable. However, note that the content of the first 3 sections apply to I/O other than files (eg sockets).

Reminder on files:

A file is a series of contiguous bytes stored in a medium (for example, a disk) under a name (the “name of the file”).
We distinguish several types of the files:
- text: containing bytes that can be displayed on the screen. This type of files consists of lines identified by the character end of line (on Unix, ASCII code character 10 while on Windows, ASCII code character 10 followed by a character of ASCII code 13);
- binary: containing bytes that cannot be displayed on the screen.

On Unix, the commands hexdump -C filename, bless filename or xxd filename show the exact content of a file. Use them to

compare the contents of helloWorldUnix.c and helloWorldWindows.c
see that the file default_names_fichierIssuDuTP10DuModuleCSC4103.txt is not quite a text file (and, see also how are the accented characters stored in a file)

When you “open” a file, the operating system provides a notion of current position (sometimes called offset in the rest of this course) for reading or writing.
- This current position determines which byte in the file will be read/written during the next I/O operation.
- This offset advances each time a read or write operation is performed.
- The operating system provides the user with primitives to explicitly change this position (without reading or writing bytes).
The “end of a file” corresponds to the location behind the last byte of the file. When a program reaches the end of file, it cannot read bytes anymore. On the other hand, the program can write bytes (depending on the mode in which the file was opened).
There are 3 ways to access a file:
- Sequential: the bytes are read one after the others from the beginning of the file.
- Direct: you can set the offset without reading bytes before offset.
- Indexed sequential: the file contains records, each record being identified by a key (unique or no). Using the key, you can position the offset at the start of a recording. You can also read the recordings in the order defined by their key.

The Linux system and the C library provide sequential and direct access modes. For an indexed sequential access mode, other libraries are required (Unix NDBM, GDBM, Oracle Berkeley DB, …).

Buffered / non-buffered IO

Buffered I/O
- Write operations are grouped in a buffer which is written to disc from time to time
- When reading, a data block is loaded from disk to buffer
→ a buffered I/O ≠ an operation on the disk
- eg. fopen, fread, fscanf, fwrite, fprintf, etc.
- Data stream identified by an opaque pointer FILE*
Unbuffered I/O
- an unbuffered I/O = an operation on the disk †
- eg. open, read, write, etc.
- Open file identified by a file descriptor of type int

I/O primitives

File open / close

int open(const char *path, int flags, mode_t mode)
- returns f_id
- flags can take one of the following values:
  - O_RDONLY: read only
  - O_WRONLY: write only
  - O_RDWR: read and write
- Additional flags:
  - O_APPEND: append data (write at the end of the file)
  - O_TRUNC: truncate (empty) the file when opening it
  - O_CREAT: creation if the file does not exist. The permissions are (mode & ∼umask)
  - O_SYNC: open file in synchronous write mode
  - O_NONBLOCK (ot O_NDELAY): open and subsequent operations performed on the descriptor will be non-blocking.
int close(int desc)

About the O_SYNC option in open:

To improve performance, by default, during a write operation, the operating system does not physically write the bytes on disk (they are stored in a kernel cache, waiting to be writen to disk)
Therefore, in the event of a sudden stop of the machine (example: power outage):
- data thought to have been written to disk may to be lost because they were in fact in memory;
- there is also a risk of inconsistency in the data on the disk.
Solutions to synchronize file data in memory with the disc:
- implicit synchronization (i.e. on each write): adding the O_SYNC option when opening the file;
- explicit synchronization (i.e. the application decides) via the int fsync(int fd) primitive

Note that we can also create a file using the creat primitive:

int creat(const char *path, mode_t mode) : return value = f_id
- which is equivalent to the following call to open:
- open(path, O_WRONLY|O_CREAT|O_TRUNC, mode).

Reading on a file descriptor

ssize_t read(int fd, void *buf, size_t count)
- returns the number of bytes successfully read
- When read returns, the buf zone contains the read data;
- In the case of a file, the number of bytes read may not be be equal to count:
  - We reached the end of the file
  - We did a non-blocking read and the data was exclusively locked

Writing on a file descriptor

ssize_t write(int fd, const void *buf, size_t count)
- return the number of bytes written
- In the case of a file, the return value (without error) of the write operation means that:
  - Bytes were written to kernel caches unless O_SYNC was specify at file open;
  - Bytes have been written to disk if O_SYNC was specified.
- In the case of a file, a number of bytes written that is different from count means an error (e.g. No space left on device)

Writing to disk is atomic: if two processes P₁ and P₂ simultaneously write to the same file in the same location, when the two processes have finished their writing, we will find:

either the data written by P₁,
either the data written by P₂,
but never a mix of the data written by P₁ and P₂.

Note that when the file is opened with the option O_APPEND, if P₁ and P₂ write simultaneously (at the end of the file, because of O_APPEND), when the two processes will have finished their writing, we will find at the end of file:

either the data written by P₁ followed by the one written by P₂,
or the data written by P₂ followed by the one written by P₁.

No writing is therefore lost! Attention, this concurrent write at the end of file is not equivalent to two processes simultaneously performing the following operations:

lseek(fd,0,SEEK_END); /* move the cursor to the end of file */
write(fd,data,taille);

In fact, in the latter case, one of the written data may by overwritten by the other.

The copy.c file on the next page illustrates the use of open, read, write and close.

/************/
/* copy.c */
/************/
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>

#define USAGE "USAGE: copy src dest\n"
#define WRITE_ERROR "write error (no space left on device ?)\n"

int source, dest;
int buf;
int nb_read, nb_written;

int main(int argc, char *argv[]) {
  if (argc != 3) {
    write(STDERR_FILENO, USAGE, strlen(USAGE));
    return EXIT_FAILURE;
  }
  source = open(argv[1], O_RDONLY);
  if (source < 0) {
    perror(argv[1]);
    return EXIT_FAILURE;
  }
  dest = open(argv[2],
              O_WRONLY|O_CREAT|O_TRUNC,
              S_IRWXU|S_IRWXG|S_IRWXO);
  if (dest < 0) {
    perror(argv[2]);
    return EXIT_FAILURE;
  }
  while ((nb_read = read(source, (void*)&buf, sizeof(buf))) > 0) {
    nb_written = write(dest, (void*)&buf, nb_read);
    if (nb_written <= 0) {
      if (nb_written == 0) {
        write(STDERR_FILENO, WRITE_ERROR, strlen(WRITE_ERROR));
      }
      else {
        perror("write");
      }
      return EXIT_FAILURE;
    }
  }
  if (nb_read < 0) {
    perror("read");
    return EXIT_FAILURE;
  }
  if (close(source) < 0) {
    perror(argv[1]);
    return EXIT_FAILURE;
  }
  if (close(dest) < 0) {
    perror(argv[2]);
    return EXIT_FAILURE;
  }
  return EXIT_SUCCESS;
}

This operation of copying the contents of one file to another descriptor is an operation frequently performed in web servers. Indeed, these servers must in particular send the content of files to client who have requested them. This is why the linux system offers the sendfile primitive (ssize_t sendfile (int out_fd, int in_fd, off_t * offset, size_t count)). It reads count bytes of in_fd and write them to out_fd (which must match an socket). sendfile is more more efficient than the combination read / write.

The fallocate function is the Linux specific version of the portable function posix_fallocate.

File descriptor duplication

Mechanism mainly used to perform redirection of the three standard I/O files.
int dup(int old_fd)
- return a new file descriptor new_fd
- associates the smallest available file descriptor of the calling process the same entry in the open files table as the descriptor old_fd
int dup2(int old_fd, int new_fd)
- force the file descriptor new_fd to become a synonym of the old_fd descriptor. If the descriptor new_fd is not available, the system first closes close(new_fd)

I/O and concurrence

Locking a file

struct flock {
  short l_type;
  short l_whence;
  off_t l_start;
  off_t l_len;
};

int fcntl(int fd, F_SETLK, struct flock*lock);

Locks are attached to an inode. So locking a file affects all file descriptors (and therefore all open files) corresponding to this inode
A lock is the property of a process: this process is the only one authorized to modify or remove it
Locks have a scope of [integer1:integer2] or [integer:∞]
Locks have a type:
- F_RDLCK: allows concurrent read access
- F_WRLCK: exclusive access

The exclusive-lock.c file illustrates exclusive file locking:

/***********/
/* exclusive_lock.c */
/***********/
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

int main(){
  int fd;
  struct flock lock;

  fd = open("/tmp/ficTest",O_RDWR|O_CREAT, S_IRWXU|S_IRWXG|S_IRWXO);
  if (fd < 0) {
    perror("open");
    exit(EXIT_FAILURE);
  }

  /* Exclusive lock on the 15th byte */
  lock.l_type = F_WRLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 15;
  lock.l_len = 1;

  /* Because of the F_SETLKW parameter, we get stuck on the fcntl if */
  /* the lock cannot be acquired                                   */
  printf("attempt to acquire an exclusive lock by process %d...\n",
	 getpid());
  if (fcntl(fd, F_SETLKW, &lock) < 0){
    perror("Acquiring lock");
    exit(EXIT_FAILURE);
  }
  printf("... Exclusive lock acquired by process %d\n", getpid());

  /* Here we could do the processing that needed to be protected */
  /* by the lock                                                 */
  sleep(10);

  /* Release the lock */
  printf("Releasing the lock by process %d...\n", getpid());
  lock.l_type = F_UNLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 15;
  lock.l_len = 1;
  if (fcntl(fd, F_SETLK, &lock) < 0){
    perror("Releasing lock");
    exit(EXIT_FAILURE);
  }
  printf("...OK\n");

  return EXIT_SUCCESS;
}

The shared-lock.c file illustrates the shared locking:

/*****************/
/* shared_lock.c */
/*****************/
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

int main(){
  int fd;
  struct flock lock;

  fd = open("/tmp/ficTest",O_RDWR|O_CREAT, S_IRWXU|S_IRWXG|S_IRWXO);
  if (fd < 0) {
    perror("open");
    exit(EXIT_FAILURE);
  }

  /* Shared lock on the 15th byte */
  lock.l_type = F_RDLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 15;
  lock.l_len = 1;

  /* Because of the F_SETLKW parameter, we get stuck on the fcntl if */
  /* the lock cannot be acquired                                   */
  printf("attempt to acquire a shared lock by process %d...\n",
	 getpid());
  if (fcntl(fd, F_SETLKW, &lock) < 0){
    perror("Acquiring lock");
    exit(EXIT_FAILURE);
  }
  printf("... shared lock acquired by process %d\n", getpid());

  /* Here we could do the processing that needed to be protected */
  /* by the lock                                                 */
  sleep(10);

  /* Release the lock */
  printf("Releasing the lock by process %d...\n", getpid());
  lock.l_type = F_UNLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 15;
  lock.l_len = 1;
  if (fcntl(fd, F_SETLK, &lock) < 0){
    perror("Releasing lock");
    exit(EXIT_FAILURE);
  }
  printf("...OK\n");

  return EXIT_SUCCESS;
}

If we run exclusive-lock first, running exclusive-lock or shared-lock wait before locking.
If we run shared-lock first, another shared-lock can set the (shared) lock. On the other hand, a exclusive-lock must wait to be able to lock.
Note that exclusive_lock may suffer starvation:
- start a 1st shared_lock.
- start exclusive_lock: it wait
- start a 2nd shared_lock. The 1st shared_lock ends. But as the 2nd shared_lock is running, exclusive_lock is still waiting.
- start a 3rd shared_lock. The 2nd shared_lock ends. But as the 3rd shared_lock is running, exclusive_lock is still waiting.
- we see that as long as shared_lock starts while the previous shared_lock has not finished running, exclusive_lock must wait: exclusive_lock may face starvation.
To prevent this starvation, we must add a mutual exclusion.

Offset manipulation

off_t lseek(int fd, off_t unOffset, int origine)
- return the new offset
- allows to handle the offset of the file
Warning ! Race condition if several threads manipulate the file
Solutions:
- Handling of the file in mutual exclusion
- Using pread or pwrite instead of lseek + read or lseek + write

Improving the I / O performance

Giving advices to the kernel

int posix_fadvise(int fd, off_t offset, off_t len, int advice)
- examples of advice: POSIX_FADV_SEQUENTIAL, POSIX_FADV_RANDOM, POSIX_FADV_WILLNEED
- return value = 0 if OK, error number otherwise
- allows you to tell the kernel how the programm will access a file, which allows the kernel to optimize accordingly

Asynchronous I/O

int aio_read(struct aiocb *aiocbp);
int aio_write(struct aiocb *aiocbp);

Starts an asynchronous read / write operation
Returns immediately

int aio_suspend(const struct aiocb * const aiocb_list[],
                int nitems,
                const struct timespec *timeout);

Waits for the end of an asynchronous operation

int aio_error(const struct aiocb *aiocbp);

Tests the end of an asynchronous operation

mmap

void *mmap(void *addr, 
           size_t length,
           int prot,
           int flags,
           int fd,
           off_t offset);

“map” a file in memory
memory accesses to the buffer are transformed into disk operations

int munmap(void *addr, size_t length);

“unmap” a buffer