NAME

       pread, pwrite - read from or write to a file descriptor at a given offset

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       ssize_t pread(int fd, void buf[.count], size_t count,
                     off_t offset);
       ssize_t pwrite(int fd, const void buf[.count], size_t count,
                     off_t offset);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pread(), pwrite():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

DESCRIPTION

       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.

RETURN VALUE

       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 error.

ERRORS

       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).

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

       Added in Linux 2.1.60; the entries in the i386 system call table were added in Linux 2.1.69.   C  library
       support (including emulation using lseek(2) on older kernels without the system calls) was added in glibc
       2.1.

   C library/kernel differences
       On Linux, the underlying system calls were renamed in Linux 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).

NOTES

       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.

BUGS

       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.

SEE ALSO

       lseek(2), read(2), readv(2), write(2)