Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
PREAD(2)		   Linux Programmer's Manual		      PREAD(2)

NAME
       pread, pwrite - read from or write to a file descriptor at a given off-
       set

SYNOPSIS
       #include	<unistd.h>

       ssize_t pread(int fd, void *buf,	size_t count, off_t offset);

       ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       pread(),	pwrite():
	   _XOPEN_SOURCE >= 500
	   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

DESCRIPTION
       pread() reads up	to count bytes from file descriptor fd at offset  off-
       set  (from the start of the file) into the buffer starting at buf.  The
       file offset is not changed.

       pwrite()	writes up to count bytes from the buffer starting  at  buf  to
       the  file  descriptor  fd  at  offset  offset.	The file offset	is not
       changed.

       The file	referenced by fd must be capable of seeking.

RETURN VALUE
       On success, the number of bytes read or written is returned (zero indi-
       cates  that  nothing  was  written,  in the case	of pwrite(), or	end of
       file, in	the case of pread()), or -1 on error, in which case  errno  is
       set to indicate the error.

ERRORS
       pread()	can  fail  and set errno to any	error specified	for read(2) or
       lseek(2).  pwrite() can fail and	set errno to any error	specified  for
       write(2)	or lseek(2).

VERSIONS
       The  pread()  and  pwrite() system calls	were added to Linux in version
       2.1.60; the entries in the i386 system call table were added in 2.1.69.
       C  library support (including emulation using lseek(2) on older kernels
       without the system calls) was added in glibc 2.1.

CONFORMING TO
       POSIX.1-2001.

NOTES
       The pread() and pwrite()	system calls are especially useful  in	multi-
       threaded	 applications.	 They allow multiple threads to	perform	I/O on
       the same	file descriptor	without	being affected by changes to the  file
       offset by other threads.

       On  Linux,  the	underlying  system  calls  were	renamed	in kernel 2.6:
       pread() became pread64(), and pwrite() became pwrite64().   The	system
       call numbers remained the same.	The glibc pread() and pwrite() wrapper
       functions transparently deal with the change.

       On some 32-bit architectures, the calling signature  for	 these	system
       calls differ, for the reasons described in syscall(2).

BUGS
       POSIX  requires	that opening a file with the O_APPEND flag should have
       no effect on the	location at which pwrite() writes data.	  However,  on
       Linux,  if a file is opened with	O_APPEND, pwrite() appends data	to the
       end of the file,	regardless of the value	of offset.

SEE ALSO
       lseek(2), read(2), readv(2), write(2)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2013-06-21			      PREAD(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=pwrite&sektion=2&manpath=Debian+8.1.0>

home | help