pread, pwrite - read from or write to a file descriptor at a given offset
#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() 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, 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 cause of the error.
POSIX.1-2001, POSIX.1-2008.
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).
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