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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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pread(2) System Calls Manual pread(2)
pread, pwrite - read from or write to a file descriptor at a given
offset
Standard C library (libc, -lc)
#include <unistd.h>
ssize_t pread(size_t count;
int fd, void buf[count], size_t count,
off_t offset);
ssize_t pwrite(size_t count;
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
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.
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.
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).
POSIX.1-2008.
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).
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.
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.
lseek(2), read(2), readv(2), write(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-06-28 pread(2)
Pages that refer to this page: ps(1), F_NOTIFY(2const), io_uring_enter2(2), io_uring_enter(2), read(2), readv(2), syscall(2), syscalls(2), write(2), off_t(3type), cpuid(4), socket(7), spufs(7), user_namespaces(7), xfs_io(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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