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


   #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():
       _XOPEN_SOURCE >= 500
       || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L


   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

   The file referenced by fd must be capable of seeking.


   On success, pread() returns the number of bytes read (a return of  zero
   indicates  end  of  file)  and  pwrite()  returns  the  number of bytes

   Note that is not an error for a successful call to transfer fewer bytes
   than requested (see read(2) and write(2)).

   On  error, -1 is returned and errno is set to indicate the cause of the


   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).


   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.


   POSIX.1-2001, POSIX.1-2008.


   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.

   C library/kernel differences
   On Linux, the underlying system  calls  were  renamed  in  kernel  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).


   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.


   lseek(2), read(2), readv(2), write(2)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.