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OpenSolaris 2009.06 - man page for read (opensolaris section 2)

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read(2)  				   System Calls 				    read(2) 

NAME
       read, readv, pread - read from file

SYNOPSIS
       #include <unistd.h>

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

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

       #include <sys/uio.h>

       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() returns 0 and has no other results.

       On files that support seeking (for example, a regular file), the read() starts at a  posi-
       tion  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.

       If fildes refers to a socket, read() is equivalent to   recv(3SOCKET)  with no flags set.

       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.

       When  attempting  to  read from a regular file with mandatory file/record locking set (see
         chmod(2) ), and there is a write lock owned by another process on the segment of	the  file
       to be read:

	   o	  If O_NDELAY or O_NONBLOCK is set, read() returns -1 and sets errno to EAGAIN.

	   o	  If  O_NDELAY	and O_NONBLOCK are clear, read() sleeps until the blocking record
		  lock is removed.

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

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

	   o	  If  some  process  has  the  pipe  open for writing and O_NDELAY is set, read()
		  returns 0.

	   o	  If some process has the pipe open for writing and  O_NONBLOCK  is  set,  read()
		  returns -1 and sets errno to EAGAIN.

	   o	  If  O_NDELAY	and  O_NONBLOCK are clear, read() blocks until data is written to
		  the pipe or the pipe is closed by all processes that had opened  the	pipe  for
		  writing.

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

	   o	  If O_NDELAY is set, read() returns 0.

	   o	  If O_NONBLOCK is set, read() returns -1 and sets errno to EAGAIN.

	   o	  If O_NDELAY and O_NONBLOCK are clear, read() blocks until  data  become  avail-
		  able.

       When  attempting to read a file associated with a socket or a stream that is not a pipe, a
       FIFO, or a terminal,  and the file has no data currently available:

	   o	  If O_NDELAY or O_NONBLOCK is set, read() returns -1 and sets errno to EAGAIN.

	   o	  If O_NDELAY and O_NONBLOCK are clear, read() blocks until data  becomes  avail-
		  able.

       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(2)  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.

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

       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-nondiscard mode, and message-discard mode.  The default is byte-stream mode.  This
       can  be	changed  using the I_SRDOPT   ioctl(2)  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  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(2)  call.

       How read() handles zero-byte STREAMS messages is determined by the current read mode  set-
       ting.   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(2) .  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() con-
       verts 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.

   readv()
       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}.

       The iovec structure contains the following members:

	 caddr_t   iov_base;
	 int	   iov_len;

       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.

   pread()
       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 to
       pread() are the same as read() with the addition of  a  fourth  argument  offset  for  the
       desired	position  inside  the file. pread() will read up to the maximum offset value that
       can be represented in an off_t for regular files. An attempt to perform	a  pread()  on	a
       file that is incapable of seeking results in an error.

RETURN VALUES
       Upon  successful  completion,  read() 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(), readv(), and pread() functions will fail if:

       EAGAIN	  Mandatory  file/record  locking  was	set,  O_NDELAY or O_NONBLOCK was set, and
		  there was a blocking record lock; total amount of system memory available  when
		  reading  using  raw  I/O  is temporarily insufficient; no data is waiting to be
		  read on a file associated with a tty device and O_NONBLOCK was set; or no  mes-
		  sage is waiting to be read on a stream and O_NDELAY or O_NONBLOCK was set.

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

       EBADMSG	  Message waiting to be read on a stream is not a data message.

       EDEADLK	  The read was going to go to sleep and cause a deadlock to occur.

       EINTR	  A signal was caught during the read operation and no data was transferred.

       EINVAL	  An attempt was made to read from a stream linked to a multiplexor.

       EIO	  A  physical  I/O  error has occurred, or the process is in a background process
		  group and is attempting to read from its controlling terminal, and  either  the
		  process  is ignoring or blocking the SIGTTIN signal or the process group of the
		  process is orphaned.

       EISDIR	  The fildes argument refers to a directory on a file system type that	does  not
		  support read operations on directories.

       ENOLCK	  The system record lock table was full, so the read() or readv() could not go to
		  sleep until the blocking record lock was removed.

       ENOLINK	  The fildes argument is on a remote machine and the link to that machine  is  no
		  longer active.

       ENXIO	  The  device associated with fildes is a block special or character special file
		  and the value of the file pointer is out of range.

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

       EFAULT	 The buf argument points to an illegal address.

       EINVAL	 The nbyte argument overflowed an ssize_t.

       The read() and readv() functions will fail if:

       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 readv() function may fail if:

       EFAULT	 The iov argument points outside the allocated address space.

       EINVAL	 The  iovcnt  argument was less than or equal to 0 or greater than {IOV_MAX}. See
		   Intro(2)  for a definition of {IOV_MAX}).

		 One of the iov_len values in the iov array was  negative,  or	the  sum  of  the
		 iov_len values in the iov array overflowed an ssize_t.

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

       ESPIPE	 The fildes argument is associated with a pipe or FIFO.

USAGE
       The pread() function has a transitional interface for 64-bit file offsets.  See   lf64(5) .

ATTRIBUTES
       See   attributes(5)  for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Committed 		   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |read() is Async-Signal-Safe  |
       +-----------------------------+-----------------------------+
       |Standard		     |See   standards(5) . 	   |
       +-----------------------------+-----------------------------+

SEE ALSO
         Intro(2) ,    chmod(2) ,    creat(2) ,	  dup(2) ,   fcntl(2) ,   getmsg(2) ,   ioctl(2) ,   lseek(2) ,   open(2) ,
         pipe(2) ,   recv(3SOCKET) ,   attributes(5) ,   lf64(5) ,   standards(5) ,   streamio(7I) ,   termio(7I) 

SunOS 5.11				   13 Sep 2007					    read(2)
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