UNIX Input/Output

UNIX Input/Output
Universal File I/O

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Files in UNIX:
- A file is a sequence of bytes
- All I/O devices are modeled as files
- A device driver is the code that allows for (the standard set of) operations to be performed on an I/O device
File Descriptors:
- A (usually small) nonnegative integer that an application can use to identify a file

Universal Operations

Open:
- State the intent to use a file
Read:
- Copy \(n > 0\) bytes from a file (at the current position) to memory and update the current position (possibly triggering an end-of-file condition)
Write:
- Copy \(n > 0\) bytes from memory to a file (at the current position) and update the current position
Close:
- State that the device will no longer be used

Opening a File: open()

int open(char *pathname, int flags, mode_t mode);

Purpose:

Open (and possibly create) a file.

Details:

`pathname`	The path to the file
`flags`	A bit mask specifying how the file will be accessed
`mode`	An optional (except when creating) access permission
Return	A file descriptor (or -1 if an error occurs)

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

Opening a File: Common flags

`O_RDONLY`	Read only
`O_WRONLY`	Write only
`O_RDWR`	Read and write
`O_CREAT`	Create the file if it doesn't exist
`O_APPEND`	Writes are always appended to the end of the file

Opening a File: Some Values of errno

`EACCES`	The file permissions don't allow the mode specified by flags
`EISDIR`	Attempted to open a directory for writing
`EMFILE`	Too many open files (for the process)
`ENFILE`	Too many open files (for the system)

Closing a File: close()

int close(int fd);

Purpose:

Dissociate a file descriptor from its file and make it available for reuse

Details:

`fd`	The (open) file descriptor to close
Return	0 on success; -1 on error

#include <unistd.h>

Closing a File: Some Values of errno

`EBADF`	`fd` isn't a valid open file descriptor
`EIO`	An I/O error occurred

Reading from a File: read()

ssize_t read(int fd, void *buffer, size_t count)

Purpose:

(Attempt to) copy a sequence of bytes from a file into memory

Details:

`fd`	The file descriptor to read from
`buffer`	The ultimate location of the bytes in memory
`count`	The maximum number of bytes to copy
Return	The number of bytes copied; 0 on EOF; -1 on error

#include <unistd.h>

Reading from a File (cont.)

The Buffer:
- Must be at least count bytes long
- Must have been allocated before the call
The Number of Bytes Read:
- May be less than the requested number (e.g., if EOF is encountered, if the file is line-oriented and an LF is encounted)
What is Read:
- A sequence of bytes (NOT a C string so it will not be null-terminated)

Reading from a File: Some Values of errno

`EBADF`	`fd` is invalid or not open for reading
`EIO`	I/O error
`EISDIR`	`fd` refers to a directory

Writing to a File: write()

ssize_t write(int fd, const void *buffer, size_t count);

Purpose:

Copy bytes from memory to a file

Details:

`fd`	The file descriptor to copy to
`buffer`	The memory to copy from
`count`	The maximum number of bytes to copy
Return	The number of bytes copied; -1 on error

#include <unistd.h>

Writing to a File: Some Values of errno

`EBADF`	`fd` is invalid or not open for reading
`EDQUOT`	The user's disk quota has been exhausted
`EIO`	I/O error
`ENOSPC`	The device has no room for the data

An Example: The Error Handler

unixexamples/io/error_handler.h

#ifndef ERROR_HANDLER_H
#define ERROR_HANDLER_H

void
handle_error();

#endif

unixexamples/io/error_handler.c

#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include "error_handler.h"


void
handle_error()
{
  perror("Error");
  exit(EXIT_FAILURE);
}

An Example: The Application

unixexamples/io/coursewriter.c

#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include "error_handler.h"

#define  LENGTH 5

int
main(int argc, char **argv)
{
  char       level;
  char       id[LENGTH];
  int        fd;
  ssize_t    n;

  level = '0';
  fd = open("courses.txt", O_RDONLY);
  if (fd <0) handle_error();

  
  while ((n = read(fd, id, LENGTH)) == LENGTH)
    {
      if(id[2] != level)
        {
          level = id[2];
          write(STDOUT_FILENO, "\n", 1);
        }
      write(STDOUT_FILENO, id, LENGTH);
      write(STDOUT_FILENO, "\n", 1);
    }

  close(fd);
}