trusty (2) pwrite64.2.gz

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, 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.

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.

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.

       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 affect 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 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.