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PREAD(2)                            Linux Programmer's Manual                            PREAD(2)

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

       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).

       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.  C library support (including emula‐
       tion using lseek(2) on older kernels without the system calls) was added in glibc 2.1.


       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).

       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.

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

       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2013-06-21                                   PREAD(2)

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