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14.1.3 File Descriptor Operations


14.1.3 File Descriptor Operations

These functions operate on I/O streams referenced using file descriptors.

File descriptors are small integers corresponding to a file that has been opened by the current process. For example, standard input is usually file descriptor 0, standard output is 1, and standard error is 2. Further files opened by a process will then be assigned 3, 4, 5, and so forth. The name ``file descriptor'' is slightly deceptive; on Unix platforms, sockets and pipes are also referenced by file descriptors.

close( fd)
Close file descriptor fd. Availability: Macintosh, Unix, Windows.

Note: This function is intended for low-level I/O and must be applied to a file descriptor as returned by open() or pipe(). To close a ``file object'' returned by the built-in function open() or by popen() or fdopen(), use its close() method.

dup( fd)
Return a duplicate of file descriptor fd. Availability: Macintosh, Unix, Windows.

dup2( fd, fd2)
Duplicate file descriptor fd to fd2, closing the latter first if necessary. Availability: Macintosh, Unix, Windows.

fdatasync( fd)
Force write of file with filedescriptor fd to disk. Does not force update of metadata. Availability: Unix.

fpathconf( fd, name)
Return system configuration information relevant to an open file. name specifies the configuration value to retrieve; it may be a string which is the name of a defined system value; these names are specified in a number of standards (POSIX.1, Unix 95, Unix 98, and others). Some platforms define additional names as well. The names known to the host operating system are given in the pathconf_names dictionary. For configuration variables not included in that mapping, passing an integer for name is also accepted. Availability: Macintosh, Unix.

If name is a string and is not known, ValueError is raised. If a specific value for name is not supported by the host system, even if it is included in pathconf_names, an OSError is raised with errno.EINVAL for the error number.

fstat( fd)
Return status for file descriptor fd, like stat(). Availability: Macintosh, Unix, Windows.

fstatvfs( fd)
Return information about the filesystem containing the file associated with file descriptor fd, like statvfs(). Availability: Unix.

fsync( fd)
Force write of file with filedescriptor fd to disk. On Unix, this calls the native fsync() function; on Windows, the MS _commit() function.

If you're starting with a Python file object f, first do f.flush(), and then do os.fsync(f.fileno()), to ensure that all internal buffers associated with f are written to disk. Availability: Macintosh, Unix, and Windows starting in 2.2.3.

ftruncate( fd, length)
Truncate the file corresponding to file descriptor fd, so that it is at most length bytes in size. Availability: Macintosh, Unix.

isatty( fd)
Return True if the file descriptor fd is open and connected to a tty(-like) device, else False. Availability: Macintosh, Unix.

lseek( fd, pos, how)
Set the current position of file descriptor fd to position pos, modified by how: 0 to set the position relative to the beginning of the file; 1 to set it relative to the current position; 2 to set it relative to the end of the file. Availability: Macintosh, Unix, Windows.

open( file, flags[, mode])
Open the file file and set various flags according to flags and possibly its mode according to mode. The default mode is 0777 (octal), and the current umask value is first masked out. Return the file descriptor for the newly opened file. Availability: Macintosh, Unix, Windows.

For a description of the flag and mode values, see the C run-time documentation; flag constants (like O_RDONLY and O_WRONLY) are defined in this module too (see below).

Note: This function is intended for low-level I/O. For normal usage, use the built-in function open(), which returns a ``file object'' with read() and write() methods (and many more). To wrap a file descriptor in a ``file object'', use fdopen().

openpty( )
Open a new pseudo-terminal pair. Return a pair of file descriptors (master, slave) for the pty and the tty, respectively. For a (slightly) more portable approach, use the pty module. Availability: Macintosh, Some flavors of Unix.

pipe( )
Create a pipe. Return a pair of file descriptors (r, w) usable for reading and writing, respectively. Availability: Macintosh, Unix, Windows.

read( fd, n)
Read at most n bytes from file descriptor fd. Return a string containing the bytes read. If the end of the file referred to by fd has been reached, an empty string is returned. Availability: Macintosh, Unix, Windows.

Note: This function is intended for low-level I/O and must be applied to a file descriptor as returned by open() or pipe(). To read a ``file object'' returned by the built-in function open() or by popen() or fdopen(), or sys.stdin, use its read() or readline() methods.

tcgetpgrp( fd)
Return the process group associated with the terminal given by fd (an open file descriptor as returned by open()). Availability: Macintosh, Unix.

tcsetpgrp( fd, pg)
Set the process group associated with the terminal given by fd (an open file descriptor as returned by open()) to pg. Availability: Macintosh, Unix.

ttyname( fd)
Return a string which specifies the terminal device associated with file-descriptor fd. If fd is not associated with a terminal device, an exception is raised. Availability:Macintosh, Unix.

write( fd, str)
Write the string str to file descriptor fd. Return the number of bytes actually written. Availability: Macintosh, Unix, Windows.

Note: This function is intended for low-level I/O and must be applied to a file descriptor as returned by open() or pipe(). To write a ``file object'' returned by the built-in function open() or by popen() or fdopen(), or sys.stdout or sys.stderr, use its write() method.

The following data items are available for use in constructing the flags parameter to the open() function. Some items will not be available on all platforms. For descriptions of their availability and use, consult open(2).

O_RDONLY
O_WRONLY
O_RDWR
O_APPEND
O_CREAT
O_EXCL
O_TRUNC
Options for the flag argument to the open() function. These can be bit-wise OR'd together. Availability: Macintosh, Unix, Windows.

O_DSYNC
O_RSYNC
O_SYNC
O_NDELAY
O_NONBLOCK
O_NOCTTY
O_SHLOCK
O_EXLOCK
More options for the flag argument to the open() function. Availability: Macintosh, Unix.

O_BINARY
Option for the flag argument to the open() function. This can be bit-wise OR'd together with those listed above. Availability: Windows.

O_NOINHERIT
O_SHORT_LIVED
O_TEMPORARY
O_RANDOM
O_SEQUENTIAL
O_TEXT
Options for the flag argument to the open() function. These can be bit-wise OR'd together. Availability: Windows.

SEEK_SET
SEEK_CUR
SEEK_END
Parameters to the lseek() function. Their values are 0, 1, and 2, respectively. Availability: Windows, Macintosh, Unix. New in version 2.5.

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