pwrite

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



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

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
       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 cause of 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, POSIX.1-2008.

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.

   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
       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 5.03 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                             2017-09-15                          PREAD(2)