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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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