read - read from a file descriptor


   #include <unistd.h>

   ssize_t read(int fd, void *buf, size_t count);


   read()  attempts to read up to count bytes from file descriptor fd into
   the buffer starting at buf.

   On files that support seeking, the read operation commences at the file
   offset, and the file offset is incremented by the number of bytes read.
   If the file offset is at or past the end of file, no  bytes  are  read,
   and read() returns zero.

   If count is zero, read() may detect the errors described below.  In the
   absence of any errors, or if read() does not check for errors, a read()
   with a count of 0 returns zero and has no other effects.

   If count is greater than SSIZE_MAX, the result is unspecified.


   On success, the number of bytes read is returned (zero indicates end of
   file), and the file position is advanced by this number.  It is not  an
   error  if  this  number  is smaller than the number of bytes requested;
   this may happen for example because fewer bytes are actually  available
   right  now  (maybe  because we were close to end-of-file, or because we
   are reading from a pipe, or from a terminal),  or  because  read()  was
   interrupted by a signal.  See also NOTES.

   On  error,  -1  is  returned,  and errno is set appropriately.  In this
   case, it is  left  unspecified  whether  the  file  position  (if  any)


   EAGAIN The  file descriptor fd refers to a file other than a socket and
          has been marked nonblocking (O_NONBLOCK),  and  the  read  would
          block.  See open(2) for further details on the O_NONBLOCK flag.

          The  file  descriptor  fd refers to a socket and has been marked
          nonblocking   (O_NONBLOCK),   and   the   read   would    block.
          POSIX.1-2001  allows  either error to be returned for this case,
          and does not require these constants to have the same value,  so
          a portable application should check for both possibilities.

   EBADF  fd is not a valid file descriptor or is not open for reading.

   EFAULT buf is outside your accessible address space.

   EINTR  The  call  was interrupted by a signal before any data was read;
          see signal(7).

   EINVAL fd is attached to an object which is unsuitable for reading;  or
          the  file  was  opened  with  the  O_DIRECT flag, and either the
          address specified in buf, the value specified in count,  or  the
          file offset is not suitably aligned.

   EINVAL fd  was  created  via  a call to timerfd_create(2) and the wrong
          size buffer was  given  to  read();  see  timerfd_create(2)  for
          further information.

   EIO    I/O  error.  This will happen for example when the process is in
          a background process group, tries to read from  its  controlling
          terminal,  and  either it is ignoring or blocking SIGTTIN or its
          process group is orphaned.  It may also occur when  there  is  a
          low-level I/O error while reading from a disk or tape.

   EISDIR fd refers to a directory.

   Other errors may occur, depending on the object connected to fd.  POSIX
   allows a read() that is interrupted after reading some data  to  return
   -1  (with  errno set to EINTR) or to return the number of bytes already


   SVr4, 4.3BSD, POSIX.1-2001.


   The types size_t and ssize_t are,  respectively,  unsigned  and  signed
   integer data types specified by POSIX.1.

   On  Linux,  read()  (and  similar  system  calls) will transfer at most
   0x7ffff000  (2,147,479,552)  bytes,  returning  the  number  of   bytes
   actually  transferred.   (This  is  true  on  both  32-bit  and  64-bit

   On NFS filesystems, reading small  amounts  of  data  will  update  the
   timestamp only the first time, subsequent calls may not do so.  This is
   caused by client side attribute caching, because most if  not  all  NFS
   clients  leave  st_atime (last file access time) updates to the server,
   and client side reads satisfied from the client's cache will not  cause
   st_atime updates on the server as there are no server-side reads.  UNIX
   semantics can be obtained by disabling client-side  attribute  caching,
   but in most situations this will substantially increase server load and
   decrease performance.


   According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
   with Regular File Operations"):

       All of the following functions shall be atomic with respect to each
       other in the effects specified in POSIX.1-2008 when they operate on
       regular files or symbolic links: ...

   Among  the APIs subsequently listed are read() and readv(2).  And among
   the effects that should be atomic across threads  (and  processes)  are
   updates  of  the  file  offset.  However, on Linux before version 3.14,
   this was not the case:  if  two  processes  that  share  an  open  file
   description  (see  open(2))  perform a read() (or readv(2)) at the same
   time, then the I/O operations were not atomic with respect updating the
   file  offset, with the result that the reads in the two processes might
   (incorrectly) overlap in the blocks of data that they  obtained.   This
   problem was fixed in Linux 3.14.


   close(2),  fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2),
   readlink(2), readv(2), select(2), write(2), fread(3)


   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.