PREAD(2) Linux Programmer's Manual PREAD(2)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)Linux 2.2.0-pre9 1999-01-21 PREAD(2)
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PREAD(2) Linux Programmer's Manual PREAD(2)NAME
pread, pwrite - read from or write to a file descriptor at a given offset
SYNOPSIS
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
pread(), pwrite():
Since glibc 2.12:
_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.12:
_XOPEN_SOURCE >= 500
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).
VERSIONS
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. C library support (including emulation using lseek(2) on older kernels without the system calls) was added in glibc 2.1.
CONFORMING TO
POSIX.1-2001.
SEE ALSO lseek(2), read(2), write(2), feature_test_macros(7)COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-20 PREAD(2)
I have used SAR -b to get some Unix cache / buffer metrics and the results are confusing me a bit.
The pread/s & pwrit/s are showing 0. However the lread/s and lwrit/s are showing figures. I note also that the bread/s and bwrit/s are showing figures. I believe that pread/s and pwrit/s is not... (3 Replies)
Hello,
Please can any one explain about the parameters to the write systemcalls??
How are they passed?? and how is the address of the user buffer is handled by the kenel??
for ex: write(fd,buf,count);
How does the kernel handles this user buffer address??
After write does the kernel write... (1 Reply)
Hi Experts,
If I understand correctly in posix standard system calls read/write or pread/pwrite a file descriptor should be sent.
How it works in case of raw disk? Are there something like files on raw devices? Or these disks are read just using block coordinates?
I want to understand how... (2 Replies)