NAME

       pread, pwrite - read from or write to a file descriptor at a given offset

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       ssize_t pread(int fd, void buf[.count], size_t count,
                     off_t offset);
       ssize_t pwrite(int fd, const void buf[.count], 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 offset (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,  pread()  returns the number of bytes read (a return of zero indicates end of
       file) and pwrite() returns the number of bytes written.

       Note that it is not an error for a successful call to transfer fewer bytes than  requested
       (see read(2) and write(2)).

       On error, -1 is returned and 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).

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

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

   C library/kernel differences
       On Linux, the underlying system calls were renamed in Linux 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).

NOTES

       The pread() and pwrite() system calls are especially useful in multithreaded 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.

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)