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

PREAD(2)                  Linux Programmer's Manual                 PREAD(2)

NAME         top

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

SYNOPSIS         top

       #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         top

       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         top

       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 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 cause of
       the error.

ERRORS         top

       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         top

       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         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       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.

   C library/kernel differences
       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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.07 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
       https://www.kernel.org/doc/man-pages/.

Linux                            2015-07-23                         PREAD(2)