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pread64(2)


NAME

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

SYNOPSIS

   #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

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 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.

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).

VERSIONS

   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 emulation using lseek(2) on older  kernels
   without the system calls) was added in glibc 2.1.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008.

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.

   C library/kernel differences
   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).

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)

COLOPHON

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.



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