pread, pwrite - read from or write to a file descriptor at a given offset
#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
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 cause of 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).
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.
POSIX.1-2001, POSIX.1-2008.
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.
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).
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)