pread, pwrite - read from or write to a file descriptor at
a given offset
#define _XOPEN_SOURCE 500
ssize_t pread(int fd, void *buf, size_t count, off_t off
ssize_t pwrite(int fd, const void *buf, size_t count,
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
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. The libc support (including emula
tion on older kernels without the system calls) was added
in glibc 2.1.
read(2), write(2), lseek(2)