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 read(2)							     read(2)




 NAME
      read, readv, pread - read from file

 SYNOPSIS
      #include <&lt&lt&lt;unistd.h>&gt&gt&gt;

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

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

      #include <&lt&lt&lt;sys/uio.h>&gt&gt&gt;

      ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);

 DESCRIPTION
      The read() function attempts to read nbyte bytes from the file
      associated with the open file descriptor, fildes, into the buffer
      pointed to by buf.

      If nbyte is 0, read() will return 0 and have no other results.

      On files that support seeking (for example, a regular file), the
      read() starts at a position in the file given by the file offset
      associated with fildes.  The file offset is 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 will occur past the current end- of-file.  If the
      starting position is at or after the end-of-file, 0 will be returned.
      If the file refers to a device special file, the result of subsequent
      read() requests is implementation-dependent.

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

      When attempting to read from an empty pipe or FIFO:

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

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

	   +  If some process has the pipe open for writing and O_NONBLOCK
	      is clear, read() will block until some data is written or the
	      pipe is closed by all processes that had the pipe open for
	      writing.




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 read(2)							     read(2)




      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() will return a -1 and set errno to
	      EAGAIN.

	   +  If O_NONBLOCK is clear, read() will block 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() returns 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
      will return bytes with value 0 until data is written into the gap.

      Upon successful completion, where nbyte is greater than 0, read() will
      mark for update the st_atime field of the file, and return the number
      of bytes read. This number will 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
      will return -1 with errno set to [EINTR].

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

      A read() from a STREAMS file can read data in three different modes:
      byte-stream mode, message-ondiscard mode, and message-discard mode.
      The default is byte-stream mode. This can be changed using the
      I_SRDOPT ioctl() request, and can be tested with the I_GRDOPT ioctl().
      In byte-stream mode, read() retrieves 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() retrieves 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 is 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



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 read(2)							     read(2)




      message boundary is reached. However, unread  data remaining in a
      message after the read() returns is discarded, and is not	 available
      for  a  subsequent read(), readv(), or getmsg() call.

      How read() handles zero-byte STREAMS messages is determined by the
      current read mode setting. In byte-stream mode, read() accepts 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
      then returns the number of bytes read, and places the zero-byte
      message back on the STREAM to be retrieved by the next read(),
      readv(), or getmsg().  In message-nondiscard mode or message-discard
      mode, a zero-byte message returns 0 and the message is removed from
      the STREAM.  When a zero-byte message is read as the first message on
      a STREAM, the message is removed from the STREAM and 0 is returned,
      regardless of the read mode.

      A read() from a STREAMS file returns 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() fails 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() converts any control part to
      data and passes it to the application before passing any data part
      originally present in the same message. In control-discard mode,
      read() discards message control parts but returns to the process any
      data part in the message.

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

      The readv() function is equivalent to read(), but places the input
      data into the iovcnt buffers specified by the members of the iov
      array: iov[0], iov[1], ..., iov[iovcnt-1].  The iovcnt argument is
      valid if greater than 0 and less than or equal to {IOV_MAX}.

      Each iovec entry specifies the base address and length of an area in
      memory where data should be placed. The readv() function always fills
      an area completely before proceeding to the next.

      Upon successful completion, readv() marks for update the st_atime
      field of the file.




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 read(2)							     read(2)




      The pread() function performs the same action as read(), except that
      it reads from a given position in the file without changing the file
      pointer.	The first three arguments of 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
      incapable of seeking results in an error.

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

 ERRORS
      The read(), pread() and readv() functions will fail if:

	   [EAGAIN]	  The O_NONBLOCK flag is set for the file descriptor
			  and the process would be delayed in read(),
			  pread() or readv().

	   [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 waiting 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 starting file offset associated with fildes is
			  greater than the maximum supported file size.

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

	   [EIO]	  A physical I/O error has occurred.

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

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




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 read(2)							     read(2)




      The readv() function will fail if:

	   [EINVAL]	  The sum of the iov_len values in the iov array
			  overflowed an ssize_t.

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

	   [ENXIO]	  A request was made of a non-existent device, or
			  the request was outside the capabilities of the
			  device.

      The readv() function may fail if:

	   [EINVAL]	  The iovcnt argument was less than or equal to 0,
			  or greater than {IOV_MAX}.

      The pread() function will fail and the file pointer remains 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.

	   [ESPIPE]	  fildes is associated with a pipe or FIFO.

 SEE ALSO
      fcntl(2), ioctl(2), lseek(2), open(2), pipe(2), <stropts.h>,
      <sys/uio.h>, <unistd.h>, XBD Specification, Chapter 9, General
      Terminal Interface.

 CHANGE HISTORY
      First released in Issue 1.  Derived from Issue 1 of the SVID.

 Issue 4
      The following changes are incorporated for alignment with the ISO
      POSIX-1 standard:

	   +  The type of the argument buf is changed from char * to void*,
	      and the type of the argument nbyte is changed from unsigned to
	      size_t.

	   +  The DESCRIPTION section now states that the result is
	      implementation-dependent if nbyte is greater than {SSIZE_MAX}.
	      This limit was defined by the constant {INT_MAX} in Issue 3.

      The following change is incorporated for alignment with the FIPS
      requirements:




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 read(2)							     read(2)




	   +  The last paragraph of the DESCRIPTION section now states that
	      if read() is interrupted by a signal after it has successfully
	      read some data, it will return the number of bytes read.	In
	      Issue 3 it was optional whether read() returned the number of
	      bytes read, or whether it returned -1 with errno set to EINTR.

      Other changes are incorporated as follows:

	   +  The header <&lt&lt&lt;unistd.h>&gt&gt&gt; is added to the SYNOPSIS section.

	   +  The DESCRIPTION section is rearranged for clarity and to align
	      more closely with the ISO POSIX-1 standard. No functional
	      changes are  made other than as noted elsewhere in this CHANGE
	      HISTORY section.

	   +  In the ERRORS section in previous issues, generation of the
	      EIO error depended on whether or not an implementation
	      supported Job Control. This functionality is now defined as
	      mandatory.

	   +  The ENXIO error is marked as an extension.

	   +  The APPLICATION USAGE section is removed.

	   +  The description of EINTR is amended.

 Issue 4, Version 2
      The following changes are incorporated for X/OPEN UNIX conformance:

	   +  The readv() function is added to the SYNOPSIS.

	   +  The DESCRIPTION is updated to describe the reading of data
	      from STREAMS files. An operational description of the readv()
	      function is also added.

	   +  References to the readv() function are added to the RETURN
	      VALUE and ERRORS sections in appropriate places.

	   +  The ERRORS section has been restructured to describe errors
	      that apply generally (that is, to both read() and readv()),
	      and to describe those that apply to readv() specifically. The
	      EBADMSG, EINVAL, and EISDIR errors are also added.












				    - 6 -	  Formatted:  August 2, 2006






 read(2)							     read(2)




				 HP-UX EXTENSIONS



 DESCRIPTION
      For readv(), the iovec structure is defined in /usr/include/sys/uio.h.

      For ordinary files, if the O_RSYNC|O_DSYNC file status flag is set,
      the calling process blocks until the data being read and all file
      attributes required to retrieve the data are the same as their image
      on disk.	Writes pending on the data to be read are executed before
      returning to the calling process.	 If the O_RSYNC|O_SYNC file status
      flag is set, the behavior is identical to that for O_RSYNC|O_DSYNC
      with this addition: all file attributes changed by the read operation
      (including access time, modification time and status change time) must
      also be the same as their image on disk.	For block special files, if
      either the O_RSYNC|O_DSYNC or O_RSYNC|O_SYNC status flag is set, the
      calling process blocks until the data being read is an image of the
      data on the disk.	 Writes pending on the data to be read are executed
      before returning to the calling process.

      When attempting to read from a regular file with enforcement-mode file
      and record locking set (see chmod(2)), and the segment of the file to
      be read is blocked by a write lock owned by another process, the
      behavior is determined by the O_NDELAY and O_NONBLOCK file status
      flags:

	   +  If O_NDELAY or O_NONBLOCK is set, read() returns -1 and errno
	      is set to [EAGAIN].

	   +  If O_NDELAY and O_NONBLOCK are clear, read() does not return
	      until the blocking write lock is removed.

      When attempting to read from an empty pipe (or FIFO):

	   +  If no process has the pipe open for writing, the read returns
	      a 0.

	   +  If some process has the pipe open for writing and O_NONBLOCK
	      is set, the read returns -1 and errno is set to [EAGAIN].

	   +  If O_NDELAY is set, the read returns a 0.

	   +  If some process has the pipe open for writing and O_NDELAY and
	      O_NONBLOCK are clear, the read blocks until data is written to
	      the file or the file is no longer open for writing.

      When attempting to read a file associated with a tty that has no data
      currently available:





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 read(2)							     read(2)




	   +  If O_NDELAY is set, the read returns 0.

	   +  If O_NDELAY and O_NONBLOCK are clear, the read blocks until
	      data becomes available.

 RETURN VALUE
      Upon successful completion, read() returns the number of bytes
      actually read and placed in the buffer; this number may be less than
      nbyte if:

	   +  The file is associated with a communication line (see ioctl(2)
	      and termio(7)), or

	   +  The number of bytes left in the file is less than nbyte bytes.

	   +  read() was interrupted by a signal after it had successfully
	      read some, but not all of the data requested.

      When an end-of-file is reached, a value of 0 is returned.	 Otherwise,
      a -1 is returned and errno is set to indicate the error.

 ERRORS
      read() fails if any of the following conditions are encountered:

	   [EBADF]	  fildes is not a valid file descriptor open for
			  reading.

	   [EINTR]	  A signal was caught before any data was
			  transferred (see sigvector(2)).

	   [EAGAIN]	  Enforcement-mode file and record locking is set,
			  O_NDELAY or O_NONBLOCK is set, and there is a
			  blocking write lock.

	   [EDEADLK]	  A resource deadlock would occur as a result of
			  this operation (see lockf(2) and fcntl(2)).

	   [EFAULT]	  buf points outside the allocated address space.
			  Reliable detection of this error is implementation
			  dependent.

	   [ENOLCK]	  The system record lock table is full, preventing
			  the read from sleeping until the blocking write
			  lock is removed.

      In addition, readv() can return one of the following errors:

	   [EFAULT]	  iov_base or iov points outside of the allocated
			  address space.  The reliable detection of this
			  error is implementation-dependent.




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 read(2)							     read(2)




 EXAMPLES
      Assuming a process opened a file for reading, the following call to
      read(2) reads BUFSIZ bytes from the file into the buffer pointed to by
      mybuf:

	   #include <&lt&lt&lt;stdio.h>&gt&gt&gt;	/* include this for BUFSIZ definition */

	   char mybuf[BUFSIZ];
	   int nbytes, fildes;

	   nbytes = read (fildes, mybuf, BUFSIZ);

 WARNINGS
      Record locking might not be enforced by the system, depending on the
      setting of the file's mode bits (see lockf(2)).

      Character-special devices, and raw disks in particular, apply
      constraints on how read() can be used.  See the specific Section (7)
      entries for details on particular devices.

      Check all references to signal(5) for appropriateness on systems that
      support sigvector(2).  sigvector() can affect the behavior described
      on this page.

      In general, avoid using read() to get the contents of a directory; use
      the readdir() library routine (see directory(3C)).

 DEPENDENCIES
    NFS
      When obtaining the contents of a directory on an NFS file system, the
      readdir() library routine must be used (see directory(3C)).  read()
      returns with an error if used to read a directory using NFS.

 AUTHOR
      read() was developed by HP, AT&T, and the University of California,
      Berkeley.

 SEE ALSO
      creat(2), dup(2), fcntl(2), ioctl(2), lockf(2), open(2), pipe(2),
      select(2), ustat(2), directory(3C), tty(7).

 STANDARDS CONFORMANCE
      read(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,
      POSIX.4










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