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!!NAME 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''__);__ __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. 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. !!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). !!CONFORMING TO 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. !!SEE ALSO read(2), write(2), lseek(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
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pread(2)
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