PREAD(2) Linux Programmer's Manual PREAD(2)
pread, pwrite - read from or write to a file descriptor at a given offset
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)):
_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, the number of bytes read or written is returned (zero indicates that nothing
was written, in the case of pwrite(), or end of file, in the case of pread()), or -1 on
error, in which case 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).
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 emula-
tion using lseek(2) on older kernels without the system calls) was added in glibc 2.1.
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 affect 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 release 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-06-21 PREAD(2)