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Linux 2.6 - man page for pread (linux section 3posix)

READ(P) 			    POSIX Programmer's Manual				  READ(P)

NAME
       pread, read - read from a file

SYNOPSIS
       #include <unistd.h>

       ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
       ssize_t read(int fildes, void *buf, size_t nbyte);

DESCRIPTION
       The  read()  function  shall attempt to read nbyte bytes from the file associated with the
       open file descriptor, fildes, into the buffer pointed to by buf. The behavior of  multiple
       concurrent reads on the same pipe, FIFO, or terminal device is unspecified.

       Before  any action described below is taken, and if nbyte is zero, the read() function may
       detect and return errors as described below. In the absence of errors, or if error  detec-
       tion is not performed, the read() function shall return zero and have no other results.

       On  files  that support seeking (for example, a regular file), the read() shall start at a
       position in the file given by the file offset associated  with  fildes.	The  file  offset
       shall be incremented by the number of bytes actually read.

       Files  that  do	not  support  seeking-for example, terminals-always read from the current
       position. The value of a file offset associated with such a file is undefined.

       No data transfer shall occur past the current end-of-file. If the starting position is  at
       or  after  the  end-of-file,  0	shall be returned. If the file refers to a device special
       file, the result of subsequent read() requests is implementation-defined.

       If the value of nbyte is greater than {SSIZE_MAX}, the result is implementation-defined.

       When attempting to read from an empty pipe or FIFO:

	* If no process has the pipe open for writing, read() shall return 0 to indicate  end-of-
	  file.

	* If  some  process  has  the  pipe  open for writing and O_NONBLOCK is set, read() shall
	  return -1 and set errno to [EAGAIN].

	* If some process has the pipe open for writing and O_NONBLOCK	is  clear,  read()  shall
	  block  the  calling thread until some data is written or the pipe is closed by all pro-
	  cesses that had the pipe open for writing.

       When attempting to read a file (other than a pipe  or  FIFO)  that  supports  non-blocking
       reads and has no data currently available:

	* If O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].

	* If  O_NONBLOCK  is clear, read() shall block the calling thread until some data becomes
	  available.

	* The use of the O_NONBLOCK flag has no effect if there is some data available.

       The read() function reads data previously written to a file.  If any portion of a  regular
       file  prior  to the end-of-file has not been written, read() shall return bytes with value
       0.  For example, lseek() allows the file offset to be set beyond the end of existing  data
       in  the	file. If data is later written at this point, subsequent reads in the gap between
       the previous end of data and the newly written data shall return bytes with value 0  until
       data is written into the gap.

       Upon  successful  completion,  where nbyte is greater than 0, read() shall mark for update
       the st_atime field of the file, and shall return the number of  bytes  read.  This  number
       shall never be greater than nbyte. The value returned may be less than nbyte if the number
       of bytes left in the file is less than nbyte, if the read() request was interrupted  by	a
       signal,	or  if	the file is a pipe or FIFO or special file and has fewer than nbyte bytes
       immediately available for reading. For example, a read() from a	file  associated  with	a
       terminal may return one typed line of data.

       If  a  read() is interrupted by a signal before it reads any data, it shall return -1 with
       errno set to [EINTR].

       If a read() is interrupted by a signal after it has successfully read some data, it  shall
       return the number of bytes read.

       For regular files, no data transfer shall occur past the offset maximum established in the
       open file description associated with fildes.

       If fildes refers to a socket, read() shall be equivalent to recv() with no flags set.

       If the O_DSYNC and O_RSYNC bits have been set, read I/O operations on the file  descriptor
       shall complete as defined by synchronized I/O data integrity completion. If the O_SYNC and
       O_RSYNC bits have been set, read I/O operations on the file descriptor shall  complete  as
       defined by synchronized I/O file integrity completion.

       If  fildes refers to a shared memory object, the result of the read() function is unspeci-
       fied.

       If fildes refers to a typed memory object, the result of the read() function  is  unspeci-
       fied.

       A  read()  from	a  STREAMS file can read data in three different modes: byte-stream mode,
       message-nondiscard mode, and message-discard mode. The default shall be byte-stream  mode.
       This  can  be  changed using the I_SRDOPT ioctl() request, and can be tested with I_GRDOPT
       ioctl(). In byte-stream mode, read() shall retrieve data from the  STREAM  until  as  many
       bytes  as  were requested are transferred, or until there is no more data to be retrieved.
       Byte-stream mode ignores message boundaries.

       In STREAMS message-nondiscard mode, read() shall retrieve data until as many bytes as were
       requested  are  transferred,  or  until	a message boundary is reached. If read() does not
       retrieve all the data in a message, the remaining data shall be left on	the  STREAM,  and
       can  be retrieved by the next read() call.  Message-discard mode also retrieves data until
       as many bytes as were requested are transferred, or a message boundary is  reached.   How-
       ever,  unread data remaining in a message after the read() returns shall be discarded, and
       shall not be available for a subsequent read(), getmsg(), or getpmsg() call.

       How read() handles zero-byte STREAMS messages is determined by the current read mode  set-
       ting.  In  byte-stream  mode,  read()  shall accept data until it has read nbyte bytes, or
       until there is no more data to read, or until a zero-byte message  block  is  encountered.
       The  read()  function  shall then return the number of bytes read, and place the zero-byte
       message back on the STREAM to be retrieved by the next read(), getmsg(), or getpmsg().  In
       message-nondiscard  mode  or  message-discard mode, a zero-byte message shall return 0 and
       the message shall be removed from the STREAM. When a zero-byte  message	is  read  as  the
       first  message  on  a  STREAM, the message shall be removed from the STREAM and 0 shall be
       returned, regardless of the read mode.

       A read() from a STREAMS file shall return the data in the message  at  the  front  of  the
       STREAM head read queue, regardless of the priority band of the message.

       By  default, STREAMs are in control-normal mode, in which a read() from a STREAMS file can
       only process messages that contain a data part but do not  contain  a  control  part.  The
       read()  shall  fail  if	a  message containing a control part is encountered at the STREAM
       head. This default action can be changed by placing the STREAM in either control-data mode
       or  control-discard  mode  with the I_SRDOPT ioctl() command. In control-data mode, read()
       shall convert any control part to data and pass it to the application before  passing  any
       data  part  originally  present in the same message. In control-discard mode, read() shall
       discard message control parts but return to the process any data part in the message.

       In addition, read() shall fail if the STREAM head  had  processed  an  asynchronous  error
       before  the call. In this case, the value of errno shall not reflect the result of read(),
       but reflect the prior error. If a hangup occurs on the STREAM  being  read,  read()  shall
       continue  to  operate  normally	until the STREAM head read queue is empty. Thereafter, it
       shall return 0.

       The pread() function shall be equivalent to read(), except that it shall read from a given
       position  in  the  file	without  changing  the file pointer. The first three arguments to
       pread() are the same as read() with the addition of  a  fourth  argument  offset  for  the
       desired position inside the file.  An attempt to perform a pread() on a file that is inca-
       pable of seeking shall result in an error.

RETURN VALUE
       Upon successful completion, read()    and pread()  shall  return  a  non-negative  integer
       indicating the number of bytes actually read. Otherwise, the functions shall return -1 and
       set errno to indicate the error.

ERRORS
       The read() and	pread()  functions shall fail if:

       EAGAIN The O_NONBLOCK flag is set for  the  file  descriptor  and  the  process	would  be
	      delayed.

       EBADF  The fildes argument is not a valid file descriptor open for reading.

       EBADMSG
	      The  file is a STREAM file that is set to control-normal mode and the message wait-
	      ing to be read includes a control part.

       EINTR  The read operation was terminated due to the receipt of a signal, and no	data  was
	      transferred.

       EINVAL The  STREAM  or multiplexer referenced by fildes is linked (directly or indirectly)
	      downstream from a multiplexer.

       EIO    The process is a member of a background process attempting to read  from	its  con-
	      trolling	terminal,  the process is ignoring or blocking the SIGTTIN signal, or the
	      process group is orphaned. This error may also  be  generated  for  implementation-
	      defined reasons.

       EISDIR The fildes argument refers to a directory and the implementation does not allow the
	      directory to be read using read() or pread(). The readdir() function should be used
	      instead.

       EOVERFLOW
	      The  file  is  a	regular  file,	nbyte is greater than 0, the starting position is
	      before the end-of-file, and the starting position is greater than or equal  to  the
	      offset maximum established in the open file description associated with fildes.

       The read() function shall fail if:

       EAGAIN or EWOULDBLOCK

	      The  file  descriptor is for a socket, is marked O_NONBLOCK, and no data is waiting
	      to be received.

       ECONNRESET
	      A read was attempted on a socket and the connection  was	forcibly  closed  by  its
	      peer.

       ENOTCONN
	      A read was attempted on a socket that is not connected.

       ETIMEDOUT
	      A read was attempted on a socket and a transmission timeout occurred.

       The read() and	pread()  functions may fail if:

       EIO    A physical I/O error has occurred.

       ENOBUFS
	      Insufficient resources were available in the system to perform the operation.

       ENOMEM Insufficient memory was available to fulfill the request.

       ENXIO  A request was made of a nonexistent device, or the request was outside the capabil-
	      ities of the device.

       The pread() function shall fail, and the file pointer shall remain unchanged, if:

       EINVAL The offset argument is invalid. The value is negative.

       EOVERFLOW
	      The file is a regular file and an attempt was made to read at or beyond the  offset
	      maximum associated with the file.

       ENXIO  A request was outside the capabilities of the device.

       ESPIPE fildes is associated with a pipe or FIFO.

       The following sections are informative.

EXAMPLES
   Reading Data into a Buffer
       The following example reads data from the file associated with the file descriptor fd into
       the buffer pointed to by buf.

	      #include <sys/types.h>
	      #include <unistd.h>
	      ...
	      char buf[20];
	      size_t nbytes;
	      ssize_t bytes_read;
	      int fd;
	      ...
	      nbytes = sizeof(buf);
	      bytes_read = read(fd, buf, nbytes);
	      ...

APPLICATION USAGE
       None.

RATIONALE
       This volume of IEEE Std 1003.1-2001 does not specify the value of the file offset after an
       error  is returned; there are too many cases. For programming errors, such as [EBADF], the
       concept is meaningless since no file is involved. For errors  that  are	detected  immedi-
       ately,  such  as  [EAGAIN],  clearly  the pointer should not change. After an interrupt or
       hardware error, however, an updated value would be very useful and is the behavior of many
       implementations.

       Note  that  a  read() of zero bytes does not modify st_atime.  A read() that requests more
       than zero bytes, but returns zero, shall modify st_atime.

       Implementations are allowed, but not  required,	to  perform  error  checking  for  read()
       requests of zero bytes.

   Input and Output
       The  use  of  I/O  with	large  byte  counts has always presented problems.  Ideas such as
       lread() and lwrite() (using and returning longs) were considered at one time. The  current
       solution  is  to  use abstract types on the ISO C standard function to read() and write().
       The abstract types can be declared so that  existing  functions	work,  but  can  also  be
       declared so that larger types can be represented in future implementations. It is presumed
       that whatever constraints limit the maximum  range  of  size_t  also  limit  portable  I/O
       requests to the same range. This volume of IEEE Std 1003.1-2001 also limits the range fur-
       ther by requiring that the byte count be limited so that a  signed  return  value  remains
       meaningful.  Since the return type is also a (signed) abstract type, the byte count can be
       defined by the implementation to be larger than an int can hold.

       The standard developers considered adding atomicity requirements to a pipe  or  FIFO,  but
       recognized  that due to the nature of pipes and FIFOs there could be no guarantee of atom-
       icity of reads of {PIPE_BUF} or any other size that would be an aid to applications porta-
       bility.

       This volume of IEEE Std 1003.1-2001 requires that no action be taken for read() or write()
       when nbyte is zero. This is not intended to take precedence over detection of errors (such
       as  invalid buffer pointers or file descriptors). This is consistent with the rest of this
       volume of IEEE Std 1003.1-2001, but the phrasing here could be misread to  require  detec-
       tion  of the zero case before any other errors. A value of zero is to be considered a cor-
       rect value, for which the semantics are a no-op.

       I/O is intended to be atomic to ordinary files and pipes and FIFOs.  Atomic means that all
       the  bytes  from  a  single  operation  that started out together end up together, without
       interleaving from other I/O operations. It is a known attribute of terminals that this  is
       not  honored,  and  terminals are explicitly (and implicitly permanently) excepted, making
       the behavior unspecified. The behavior for other device types is  also  left  unspecified,
       but the wording is intended to imply that future standards might choose to specify atomic-
       ity (or not).

       There were recommendations to add format parameters to read() and write() in order to han-
       dle  networked  transfers  among heterogeneous file system and base hardware types. Such a
       facility may be required for support by the OSI presentation of layer  services.  However,
       it was determined that this should correspond with similar C-language facilities, and that
       is beyond the scope of this volume of IEEE Std 1003.1-2001.  The concept was suggested  to
       the developers of the ISO C standard for their consideration as a possible area for future
       work.

       In 4.3 BSD, a read() or write() that is interrupted by a signal	before	transferring  any
       data  does  not by default return an [EINTR] error, but is restarted. In 4.2 BSD, 4.3 BSD,
       and the Eighth Edition, there is an additional function, select(),  whose  purpose  is  to
       pause  until  specified	activity (data to read, space to write, and so on) is detected on
       specified file descriptors. It is common in applications written  for  those  systems  for
       select()  to  be used before read() in situations (such as keyboard input) where interrup-
       tion of I/O due to a signal is desired.

       The issue of which files or file types are interruptible is considered  an  implementation
       design issue. This is often affected primarily by hardware and reliability issues.

       There are no references to actions taken following an "unrecoverable error". It is consid-
       ered beyond the scope of this volume of IEEE Std 1003.1-2001 to describe what  happens  in
       the case of hardware errors.

       Previous versions of IEEE Std 1003.1-2001 allowed two very different behaviors with regard
       to the handling of interrupts. In order	to  minimize  the  resulting  confusion,  it  was
       decided	that  IEEE Std 1003.1-2001 should support only one of these behaviors. Historical
       practice on AT&T-derived systems was to have read() and write() return -1 and set errno to
       [EINTR]	when  interrupted  after some, but not all, of the data requested had been trans-
       ferred. However, the U.S. Department of Commerce FIPS 151-1 and	FIPS  151-2  require  the
       historical  BSD	behavior, in which read() and write() return the number of bytes actually
       transferred before the interrupt. If -1 is returned when any data is  transferred,  it  is
       difficult  to recover from the error on a seekable device and impossible on a non-seekable
       device.	Most  new  implementations  support  this  behavior.  The  behavior  required  by
       IEEE Std 1003.1-2001 is to return the number of bytes transferred.

       IEEE Std 1003.1-2001  does  not specify when an implementation that buffers read()ss actu-
       ally moves the data into the user-supplied buffer, so an implementation may  chose  to  do
       this at the latest possible moment. Therefore, an interrupt arriving earlier may not cause
       read() to return a partial byte count, but rather to return -1 and set errno to [EINTR].

       Consideration was also given to combining the two previous options, and setting	errno  to
       [EINTR]	while  returning  a  short count. However, not only is there no existing practice
       that implements this, it is also contradictory to the idea that when  errno  is	set,  the
       function responsible shall return -1.

FUTURE DIRECTIONS
       None.

SEE ALSO
       fcntl()	,  ioctl() , lseek() , open() , pipe() , readv() , the Base Definitions volume of
       IEEE Std 1003.1-2001, Chapter 11, General Terminal  Interface,  <stropts.h>,  <sys/uio.h>,
       <unistd.h>

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  READ(P)


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