pread(2) — Linux manual page

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 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         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 5.11 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                          2021-03-22                       PREAD(2)

Pages that refer to this page: fcntl(2)read(2)readv(2)syscall(2)syscalls(2)write(2)cpuid(4)proc(5)socket(7)spufs(7)system_data_types(7)user_namespaces(7)xfs_io(8)