mirror of https://github.com/mkerrisk/man-pages
168 lines
4.1 KiB
Groff
168 lines
4.1 KiB
Groff
.\" Copyright (C) 1999 Joseph Samuel Myers.
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH PREAD 2 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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pread, pwrite \- read from or write to a file descriptor at a given offset
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
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", off_t " offset );
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.BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
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", off_t " offset );
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.fi
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR pread (),
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.BR pwrite ():
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.nf
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_XOPEN_SOURCE >= 500
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|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
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.fi
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.SH DESCRIPTION
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.BR pread ()
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reads up to
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.I count
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bytes from file descriptor
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.I fd
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at offset
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.I offset
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(from the start of the file) into the buffer starting at
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.IR buf .
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The file offset is not changed.
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.PP
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.BR pwrite ()
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writes up to
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.I count
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bytes from the buffer starting at
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.I buf
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to the file descriptor
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.I fd
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at offset
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.IR offset .
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The file offset is not changed.
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.PP
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The file referenced by
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.I fd
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must be capable of seeking.
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.SH RETURN VALUE
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On success,
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.BR pread ()
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returns the number of bytes read
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(a return of zero indicates end of file)
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and
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.BR pwrite ()
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returns the number of bytes written.
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.PP
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Note that it is not an error for a successful call to transfer fewer bytes
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than requested (see
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.BR read (2)
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and
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.BR write (2)).
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.PP
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On error, \-1 is returned and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.BR pread ()
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can fail and set
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.I errno
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to any error specified for
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.BR read (2)
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or
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.BR lseek (2).
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.BR pwrite ()
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can fail and set
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.I errno
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to any error specified for
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.BR write (2)
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or
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.BR lseek (2).
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.SH VERSIONS
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The
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.BR pread ()
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and
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.BR pwrite ()
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system calls were added to Linux in
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version 2.1.60; the entries in the i386 system call table were added
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in 2.1.69.
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C library support (including emulation using
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.BR lseek (2)
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on older kernels without the system calls) was added in glibc 2.1.
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008.
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.SH NOTES
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The
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.BR pread ()
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and
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.BR pwrite ()
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system calls are especially useful in multithreaded applications.
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They allow multiple threads to perform I/O on the same file descriptor
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without being affected by changes to the file offset by other threads.
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.\"
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.SS C library/kernel differences
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On Linux, the underlying system calls were renamed
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in kernel 2.6:
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.BR pread ()
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became
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.BR pread64 (),
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and
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.BR pwrite ()
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became
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.BR pwrite64 ().
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The system call numbers remained the same.
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The glibc
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.BR pread ()
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and
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.BR pwrite ()
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wrapper functions transparently deal with the change.
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.PP
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On some 32-bit architectures,
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the calling signature for these system calls differ,
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for the reasons described in
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.BR syscall (2).
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.SH BUGS
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POSIX requires that opening a file with the
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.BR O_APPEND
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flag should have no effect on the location at which
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.BR pwrite ()
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writes data.
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However, on Linux, if a file is opened with
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.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=43178
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.BR O_APPEND ,
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.BR pwrite ()
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appends data to the end of the file, regardless of the value of
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.IR offset .
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.SH SEE ALSO
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.BR lseek (2),
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.BR read (2),
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.BR readv (2),
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.BR write (2)
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