pread

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

NAME
       pread, pwrite - read from or write to a file descriptor at a given off-
       set

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  off-
       set  (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 writ-
       ten.

       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.

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 multi-
       threaded 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 5.05 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/.

Linux                             2017-09-15                          PREAD(2)