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

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       http://www.kernel.org/doc/man-pages/.