pread, pwrite - read from or write to a file descriptor at a given offset
fd, void *
fd, const void *
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.
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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