PREAD
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 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 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel differences
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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