pread(2) - Linux manual page (original) (raw)

pread(2) System Calls Manual pread(2)

NAME top

   pread, pwrite - read from or write to a file descriptor at a given
   offset

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <unistd.h>**

   **ssize_t pread(int** _fd_**, void** _buf_**[.**_count_**], size_t** _count_**,**
                 **off_t** _offset_**);**
   **ssize_t pwrite(int** _fd_**, const void** _buf_**[.**_count_**], 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 top

   **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 top

   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 it is not an error for a successful call to transfer
   fewer bytes than requested (see [read(2)](../man2/read.2.html) and [write(2)](../man2/write.2.html)).

   On error, -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **pread**() can fail and set _[errno](../man3/errno.3.html)_ to any error specified for [read(2)](../man2/read.2.html)
   or [lseek(2)](../man2/lseek.2.html).  **pwrite**() can fail and set _[errno](../man3/errno.3.html)_ to any error
   specified for [write(2)](../man2/write.2.html) or [lseek(2)](../man2/lseek.2.html).

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001.

   Added in Linux 2.1.60; the entries in the i386 system call table
   were added in Linux 2.1.69.  C library support (including
   emulation using [lseek(2)](../man2/lseek.2.html) on older kernels without the system
   calls) was added in glibc 2.1.

C library/kernel differences On Linux, the underlying system calls were renamed in Linux 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)](../man2/syscall.2.html).

NOTES top

   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.

BUGS top

   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 top

   [lseek(2)](../man2/lseek.2.html), [read(2)](../man2/read.2.html), [readv(2)](../man2/readv.2.html), [write(2)](../man2/write.2.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 pread(2)

Pages that refer to this page:ps(1), fcntl(2), io_uring_enter2(2), io_uring_enter(2), read(2), readv(2), syscall(2), syscalls(2), write(2), off_t(3type), cpuid(4), socket(7), spufs(7), user_namespaces(7), xfs_io(8)