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Annotated edit history of pread(2) version 4, including all changes. View license author blame.
Rev Author # Line
1 perry 1 !!NAME
2 PerryLorier 2 pread, pwrite - read from or write to a file descriptor at a given offset
1 perry 3
4 !!SYNOPSIS
2 PerryLorier 5 __#define _XOPEN_SOURCE 500__
6 __#include <unistd.h>__
1 perry 7
2 PerryLorier 8 __ssize_t pread(int__ ''fd''__, void *__''buf''__, size_t__ ''count''__, off_t__ ''offset''__);__
1 perry 9
2 PerryLorier 10 __ssize_t pwrite(int__ ''fd''__, const void *__''buf''__, size_t__ ''count''__, off_t__ ''offset''__);__
1 perry 11
12 !!DESCRIPTION
2 PerryLorier 13 __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.
1 perry 14
2 PerryLorier 15 __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.
1 perry 16
2 PerryLorier 17 The file referenced by ''fd'' must be capable of seeking.
1 perry 18
19 !!RETURN VALUE
3 PerryLorier 20 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__),
2 PerryLorier 21 or -1 on error, in which case ''errno'' is set to indicate the error.
1 perry 22
23 !!ERRORS
2 PerryLorier 24 __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).
1 perry 25
26 !!CONFORMING TO
2 PerryLorier 27 Unix98
1 perry 28
29 !!HISTORY
2 PerryLorier 30 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
31 emulation on older kernels without the system calls) was added in glibc 2.1.
1 perry 32
33 !!SEE ALSO
2 PerryLorier 34 read(2), write(2), lseek(2)
4 RajinikanthSambasivan 35
36 __Why __Pread?
37 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
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