pread, pwrite - read from or write to a file descriptor at a given offset
#define _XOPEN_SOURCE 500 #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);
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).
Unix98
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 emulation on older kernels without the system calls) was added in glibc 2.1.
Why Pread? The use of the pread()/pwrite() functions is by faster by almost a factor of two than the use of a lseek()/read() or lseek()/write() combination, since only one system call has to be performed, thus saving unecessary user/kernel mode transitions
