Differences between current version and predecessor to the previous major change of pread(2).
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| Newer page: | version 4 | Last edited on Tuesday, May 6, 2003 6:28:27 am | by RajinikanthSambasivan | |
| Older page: | version 1 | Last edited on Tuesday, June 4, 2002 12:23:44 am | by perry | Revert |
@@ -1,83 +1,37 @@
-PREAD
-!!!PREAD
-NAME
-SYNOPSIS
-DESCRIPTION
-RETURN VALUE
-ERRORS
-CONFORMING TO
-HISTORY
-SEE ALSO
-----
!!NAME
+pread, pwrite - read from or write to a file descriptor at a given offset
-
-pread, pwrite - read from or write to a file descriptor at a given offset
!!SYNOPSIS
+ __#define _XOPEN_SOURCE 500__
+ __#include <unistd.h>__
+ __ssize_t pread(int__ ''fd''__, void *__''buf''__, size_t__ ''count''__, off_t__ ''offset''__);__
-__#define
_XOPEN
_SOURCE 500
__
+
__ssize
_t pwrite(int
__ ''fd''__, const void *__''buf''__, size_t__ ''count''__, off_t__ ''offset''__);
__
-
-__#include __
-
-
-__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''__);__
!!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.
-__pread()__ reads up to ''count'' bytes from
file
-descriptor
''fd'' at offset ''offset'' (from the start
-the file) into the buffer starting at ''buf''. The
-file offset is not changed
.
+The
file referenced by
''fd'' must be capable
of seeking
.
-
-__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.
-
-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).
-
-__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).
!!CONFORMING TO
+Unix98
-
-Unix98
!!HISTORY
+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. The libc support (including
+emulation on older kernels without the system calls) was added in glibc 2.1.
-
-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. The libc support (including
-emulation on older kernels without the system calls) was
-added in glibc 2.1.
!!SEE ALSO
+read(2), write(2), lseek(2)
-
-read
(2
), write
(2
),
-2
)
-----
+__Why __Pread?
+The use of the pread
()/pwrite
() functions is by faster by almost a factor of two than the use of a
lseek()/read() or lseek()/write() combination, since only one system call has to be performed, thus saving unecessary user/kernel mode transitions
