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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 affect 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 3.53 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2013-06-21                          PREAD(2)