B.1 Reading and Writing Data

The first I/O function you likely encountered when you first learned the C language was printf. This formats a text string and then prints it to standard output. The generalized version, fprintf, can print the text to a stream other than standard output. A stream is represented by a FILE* pointer. You obtain a FILE* pointer by opening a file with fopen. When you're done, you can close it with fclose. In addition to fprintf, you can use such functions as fputc, fputs, and fwrite to write data to the stream, or fscanf, fgetc, fgets, and fread to read data.

With the Linux low-level I/O operations, you use a handle called a file descriptor instead of a FILE* pointer. A file descriptor is an integer value that refers to a particular instance of an open file in a single process. It can be open for reading, for writing, or for both reading and writing. A file descriptor doesn't have to refer to an open file; it can represent a connection with another system component that is capable of sending or receiving data. For example, a connection to a hardware device is represented by a file descriptor (see Chapter 6, "Devices"), as is an open socket (see Chapter 5, "Interprocess Communication," Section 5.5, "Sockets") or one end of a pipe (see Section 5.4, "Pipes").

Include the header files <fcntl.h>, <sys/types.h>, <sys/stat.h>, and <unistd.h> if you use any of the low-level I/O functions described here.

B.1.1 Opening a File

To open a file and produce a file descriptor that can access that file, use the open call. It takes as arguments the path name of the file to open, as a character string, and flags specifying how to open it. You can use open to create a new file; if you do, pass a third argument that specifies the access permissions to set for the new file.

If the second argument is O_RDONLY, the file is opened for reading only; an error will result if you subsequently try to write to the resulting file descriptor. Similarly, O_WRONLY causes the file descriptor to be write-only. Specifying O_RDWR produces a file descriptor that can be used both for reading and for writing. Note that not all files may be opened in all three modes. For instance, the permissions on a file might forbid a particular process from opening it for reading or for writing; a file on a read-only device such as a CD-ROM drive may not be opened for writing.

You can specify additional options by using the bitwise or of this value with one or more flags. These are the most commonly used values:

·         Specify O_TRUNC to truncate the opened file, if it previously existed. Data written to the file descriptor will replace previous contents of the file.

·         Specify O_APPEND to append to an existing file. Data written to the file descriptor will be added to the end of the file.

·         Specify O_CREAT to create a new file. If the filename that you provide to open does not exist, a new file will be created, provided that the directory containing it exists and that the process has permission to create files in that directory. If the file already exists, it is opened instead.

·         Specify O_EXCL with O_CREAT to force creation of a new file. If the file already exists, the open call will fail.

If you call open with O_CREAT, provide an additional third argument specifying the permissions for the new file. See Chapter 10, "Security," Section 10.3, "File System Permissions," for a description of permission bits and how to use them.

For example, the program in Listing B.1 creates a new file with the filename specified on the command line. It uses the O_EXCL flag with open, so if the file already exists, an error occurs. The new file is given read and write permissions for the owner and owning group, and read permissions only for others. (If your umask is set to a nonzero value, the actual permissions may be more restrictive.)


When you create a new file with open, some permission bits that you specify may be turned off. This is because your umask is set to a nonzero value. A process's umask specifies bits that are masked out of all newly created files' permissions. The actual permissions used are the bitwise and of the permissions you specify to open and the bitwise complement of the umask.

To change your umask from the shell, use the umask command, and specify the numerical value of the mask, in octal notation. To change the umask for a running process, use the umask call, passing it the desired mask value to use for subsequent open calls.

For example, calling this line

umask (S_IRWXO | S_IWGRP); 

in a program, or invoking this command

% umask 027 

specifies that write permissions for group members and read, write, and execute permissions for others will always be masked out of a new file's permissions.

Listing B.1 (create-file.c) Create a New File
#include <fcntl.h> 
#include <stdio.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <unistd.h> 
int main (int argc, char* argv[]) 
  /* The path at which to create the new file.  */ 
  char* path = argv[1]; 
  /* The permissions for the new file.  */ 
  mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH; 
  /* Create the file.  */ 
  int fd = open (path, O_WRONLY | O_EXCL | O_CREAT, mode); 
  if (fd == -1) {
    /* An error occurred. Print an error message and bail.  */ 
    perror ("open"); 
    return 1; 
  return 0; 

Here's the program in action:

% ./create-file testfile 
% ls -l testfile 
-rw-rw-r--    1 samuel   users          0 Feb 1 22: 47 testfile 
% ./create-file testfile 
open: File exists 

Note that the length of the new file is 0 because the program didn't write any data to it.

B.1.2 Closing File Descriptors

When you're done with a file descriptor, close it with close. In some cases, such as the program in Listing B.1, it's not necessary to call close explicitly because Linux closes all open file descriptors when a process terminates (that is, when the program ends). Of course, once you close a file descriptor, you should no longer use it.

Closing a file descriptor may cause Linux to take a particular action, depending on the nature of the file descriptor. For example, when you close a file descriptor for a network socket, Linux closes the network connection between the two computers communicating through the socket.

Linux limits the number of open file descriptors that a process may have open at a time. Open file descriptors use kernel resources, so it's good to close file descriptors when you're done with them. A typical limit is 1,024 file descriptors per process. You can adjust this limit with the setrlimit system call; see Section 8.5, "getrlimit and setrlimit: Resource Limits," for more information.

B.1.3 Writing Data

Write data to a file descriptor using the write call. Provide the file descriptor, a pointer to a buffer of data, and the number of bytes to write. The file descriptor must be open for writing. The data written to the file need not be a character string; write copies arbitrary bytes from the buffer to the file descriptor.

The program in Listing B.2 appends the current time to the file specified on the command line. If the file doesn't exist, it is created. This program also uses the time, localtime, and asctime functions to obtain and format the current time; see their respective man pages for more information.

Listing B.2 (timestamp.c) Append a Timestamp to a File
#include <fcntl.h> 
#include <stdio.h> 
#include <string.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <time.h> 
#include <unistd.h> 
/* Return a character string representing the current date and time.  */ 
char* get_timestamp () 
  time_t now = time (NULL); 
  return asctime (localtime (&now)); 
int main (int argc, char* argv[]) 
  /* The file to which to append the timestamp.  */ 
  char* filename = argv[1]; 
  /* Get the current timestamp.  */ 
  char* timestamp = get_timestamp (); 
  /* Open the file for writing. If it exists, append to it; 
     otherwise, create a new file.  */ 
  int fd = open (filename, O_WRONLY | O_CREAT | O_APPEND, 0666); 
  /* Compute the length of the timestamp string.  */ 
  size_t length = strlen (timestamp); 
  /* Write the timestamp to the file.  */ 
  write (fd, timestamp, length); 
  /* All done.  */ 
  close (fd); 
  return 0; 

Here's how the timestamp program works:

% ./timestamp tsfile 
% cat tsfile 
Thu Feb  1 23:25:20 2001 
% ./timestamp tsfile 
% cat tsfile 
Thu Feb  1 23:25:20 2001 
Thu Feb  1 23:25:47 2001 

Note that the first time we invoke timestamp, it creates the file tsfile, while the second time it appends to it.

The write call returns the number of bytes that were actually written, or -1 if an error occurred. For certain kinds of file descriptors, the number of bytes actually written may be less than the number of bytes requested. In this case, it's up to you to call write again to write the rest of the data. The function in Listing B.2 demonstrates how you might do this. Note that for some applications, you may have to check for special conditions in the middle of the writing operation. For example, if you're writing to a network socket, you'll have to augment this function to detect whether the network connection was closed in the middle of the write operation, and if it has, to react appropriately.

Listing B.3 (write-all.c) Write All of a Buffer of Data
/* Write all of COUNT bytes from BUFFER to file descriptor FD. 
   Returns -1 on error, or the number of bytes written.  */ 
ssize_t write_all (int fd, const void* buffer, size_t count) 
  size_t left_to_write = count; 
  while (left_to_write > 0) {
    size_t written = write (fd, buffer, count); 
    if (written == -1) 
      /* An error occurred; bail.  */ 
      return -1; 
      /* Keep count of how much more we need to write.  */ 
      left_to_write -= written; 
  /* We should have written no more than COUNT bytes!   */ 
  assert (left_to_write == 0); 
  /* The number of bytes written is exactly COUNT.  */ 
  return count; 

B.1.4 Reading Data

The corresponding call for reading data is read. Like write, it takes a file descriptor, a pointer to a buffer, and a count. The count specifies how many bytes are read from the file descriptor into the buffer. The call to read returns -1 on error or the number of bytes actually read. This may be smaller than the number of bytes requested, for example, if there aren't enough bytes left in the file.

Reading DOS/Windows Text Files

After reading this book, we're positive that you'll choose to write all your programs for GNU/Linux. However, your programs may occasionally need to read text files generated by DOS or Windows programs. It's important to anticipate an important difference in how text files are structured between these two platforms.

In GNU/Linux text files, each line is separated from the next with a newline character. A newline is represented by the character constant '\n', which has ASCII code 10. On Windows, however, lines are separated by a two-character combination: a carriage return character (the character '\r, ' which has ASCII code 13), followed by a newline character.

Some GNU/Linux text editors display ^M at the end of each line when showing a Windows text file—this is the carriage return character. Emacs displays Windows text files properly but indicates them by showing (DOS) in the mode line at the bottom of the buffer. Some Windows editors, such as Notepad, display all the text in a GNU/Linux text file on a single line because they expect a carriage return at the end of each line. Other programs for both GNU/Linux and Windows that process text files may report mysterious errors when given as input a text file in the wrong format.

If your program reads text files generated by Windows programs, you'll probably want to replace the sequence '\r\n' with a single newline. Similarly, if your program writes text files that must be read by Windows programs, replace lone newline characters with '\r\n' combinations. You must do this whether you use the low-level I/O calls presented in this appendix or the standard C library I/O functions.

Listing B.4 provides a simple demonstration of read. The program prints a hexadecimal dump of the contents of the file specified on the command line. Each line displays the offset in the file and the next 16 bytes.

Listing B.4 (hexdump.c) Print a Hexadecimal Dump of a File
#include <fcntl.h> 
#include <stdio.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <unistd.h> 
int main (int argc, char* argv[]) 
  unsigned char buffer[16]; 
  size_t offset = 0; 
  size_t bytes_read; 
  int i; 
  /* Open the file for reading.  */ 
  int fd = open (argv[1], O_RDONLY); 
  /* Read from the file, one chunk at a time. Continue until read 
     "comes up short", that is, reads less than we asked for. 
     This indicates that we've hit the end of the file.  */ 
  do {
    /* Read the next line's worth of bytes.  */ 
    bytes_read = read (fd, buffer, sizeof (buffer)); 
    /* Print the offset in the file, followed by the bytes themselves.  */ 
    printf ("0x%06x : ", offset); 
    for (i = 0; i < bytes_read; ++i) 
      printf ("%02x ", buffer[i]); 
    printf ("\n"); 
    /* Keep count of our position in the file.  */ 
    offset += bytes_read; 
  while (bytes_read == sizeof (buffer)); 
  /* All done.  */ 
  close (fd); 
  return 0; 

Here's hexdump in action. It's shown printing out a dump of its own executable file:

% ./hexdump hexdump 
0x000000 : 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
0x000010 : 02 00 03 00 01 00 00 00 c0 83 04 08 34 00 00 00 
0x000020 : e8 23 00 00 00 00 00 00 34 00 20 00 06 00 28 00 
0x000030 : 1d 00 1a 00 06 00 00 00 34 00 00 00 34 80 04 08 

Your output may be different, depending on the compiler you used to compile hexdump and the compilation flags you specified.

B.1.5 Moving Around a File

A file descriptor remembers its position in a file. As you read from or write to the file descriptor, its position advances corresponding to the number of bytes you read or write. Sometimes, however, you'll need to move around a file without reading or writing data. For instance, you might want to write over the middle of a file without modifying the beginning, or you might want to jump back to the beginning of a file and reread it without reopening it.

The lseek call enables you to reposition a file descriptor in a file. Pass it the file descriptor and two additional arguments specifying the new position.

·         If the third argument is SEEK_SET, lseek interprets the second argument as a position, in bytes, from the start of the file.

·         If the third argument is SEEK_CUR, lseek interprets the second argument as an offset, which may be positive or negative, from the current position.

·         If the third argument is SEEK_END, lseek interprets the second argument as an offset from the end of the file. A positive value indicates a position beyond the end of the file.

The call to lseek returns the new position, as an offset from the beginning of the file. The type of the offset is off_t. If an error occurs, lseek returns -1. You can't use lseek with some types of file descriptors, such as socket file descriptors.

If you want to find the position of a file descriptor in a file without changing it, specify a 0 offset from the current position—for example:

off_t position = lseek (file_descriptor, 0, SEEK_CUR); 

Linux enables you to use lseek to position a file descriptor beyond the end of the file. Normally, if a file descriptor is positioned at the end of a file and you write to the file descriptor, Linux automatically expands the file to make room for the new data. If you position a file descriptor beyond the end of a file and then write to it, Linux first expands the file to accommodate the "gap" that you created with the lseek operation and then writes to the end of it. This gap, however, does not actually occupy space on the disk; instead, Linux just makes a note of how long it is. If you later try to read from the file, it appears to your program that the gap is filled with 0 bytes.

Using this behavior of lseek, it's possible to create extremely large files that occupy almost no disk space. The program lseek-huge in Listing B.5 does this. It takes as command-line arguments a filename and a target file size, in megabytes. The program opens a new file, advances past the end of the file using lseek, and then writes a single 0 byte before closing the file.

Listing B.5 (lseek-huge.c) Create Large Files with lseek
#include <fcntl.h> 
#include <stdlib.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <unistd.h> 
int main (int argc, char* argv[]) 
  int zero = 0; 
  const int megabyte = 1024 * 1024; 
  char* filename = argv[1]; 
  size_t length = (size_t) atoi (argv[2]) * megabyte; 
  /* Open a new file.  */ 
  int fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0666); 
  /* Jump to 1 byte short of where we want the file to end.  */ 
  lseek (fd, length - 1, SEEK_SET); 
  /* Write a single 0 byte.  */ 
  write (fd, &zero, 1); 
  /* All done.  */ 
  close (fd); 
  return 0; 

Using lseek-huge, we'll make a 1GB (1024MB) file. Note the free space on the drive before and after the operation.

% df -h . 
Filesystem            Size  Used Avail Use% Mounted on 
/dev/hda5             2.9G  2.1G  655M  76% / 
% ./lseek-huge bigfile 1024 
% ls -l bigfile 
-rw-r-----    1 samuel   samuel   1073741824 Feb  5 16:29 bigfile 
% df -h . 
Filesystem            Size  Used Avail Use% Mounted on 
/dev/hda5             2.9G  2.1G  655M  76% / 

No appreciable disk space is consumed, despite the enormous size of bigfile. Still, if we open bigfile and read from it, it appears to be filled with 1GB worth of 0s. For instance, we can examine its contents with the hexdump program of Listing B.4.

% ./hexdump bigfile | head -10 
0x000000 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
0x000010 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
0x000020 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
0x000030 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
0x000040 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
0x000050 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

If you run this yourself, you'll probably want to kill it with Ctrl+C, rather than watching it print out 230 0 bytes.

Note that these magic gaps in files are a special feature of the ext2 file system that's typically used for GNU/Linux disks. If you try to use lseek-huge to create a file on some other type of file system, such as the fat or vfat file systems used to mount DOS and Windows partitions, you'll find that the resulting file does actually occupy the full amount of disk space.

Linux does not permit you to rewind before the start of a file with lseek.