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
_XOPEN_SOURCE >= 500
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.
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
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
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.
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
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.
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