чтение или запись в файловый дескриптор по заданному смещению (read from or write to a file descriptor at a given offset)
Имя (Name)
pread, pwrite - read from or write to a file descriptor at a
given offset
Синопсис (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
Описание (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.
Возвращаемое значение (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 error.
Ошибки (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).
Версии (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.
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008.
Примечание (Note)
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).
Ошибки (баги) (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.
Смотри также (See also)
lseek(2), read(2), readv(2), write(2)
