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READ(2)			      System Calls Manual		       READ(2)

NAME
       read, readv, pread, preadv -- read input

LIBRARY
       Standard	C Library (libc, -lc)

SYNOPSIS
       #include	<unistd.h>

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

       ssize_t
       pread(int fd, void *buf,	size_t nbytes, off_t offset);

       #include	<sys/uio.h>

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

       ssize_t
       preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);

DESCRIPTION
       The  read() system call attempts	to read	nbytes of data from the	object
       referenced by the descriptor fd into the	buffer pointed to by buf.  The
       readv() system call performs the	same action, but  scatters  the	 input
       data into the iovcnt buffers specified by the members of	the iov	array:
       iov[0],	iov[1],	 ...,  iov[iovcnt-1].  The pread() and preadv()	system
       calls perform the same functions, but read from the specified  position
       in the file without modifying the file pointer.

       For readv() and preadv(), the iovec structure is	defined	as:

	     struct iovec {
		     void   *iov_base;	/* Base	address. */
		     size_t iov_len;	/* Length. */
	     };

       Each  iovec  entry  specifies the base address and length of an area in
       memory where data should	be placed.  The	readv()	system call  will  al-
       ways fill an area completely before proceeding to the next.

       On objects capable of seeking, the read() starts	at a position given by
       the  pointer  associated	 with  fd  (see	 lseek(2)).   Upon return from
       read(), the pointer is incremented by  the  number  of  bytes  actually
       read.

       Objects	that  are  not capable of seeking always read from the current
       position.  The value of the pointer associated with such	an  object  is
       undefined.

       Upon  successful	 completion, read(), readv(), pread() and preadv() re-
       turn the	number of bytes	actually read and placed in the	 buffer.   The
       system guarantees to read the number of bytes requested if the descrip-
       tor  references	a normal file that has that many bytes left before the
       end-of-file, but	in no other case.

       In accordance with IEEE Std 1003.1-2004 ("POSIX.1"), both  read(2)  and
       write(2)	 syscalls are atomic with respect to each other	in the effects
       on file content,	when they operate on regular files.   If  two  threads
       each  call one of the read(2) or	write(2), syscalls, each call will see
       either all of the changes of the	other call,  or	 none  of  them.   The
       FreeBSD kernel implements this guarantee	by locking the file ranges af-
       fected by the calls.

RETURN VALUES
       If  successful,	the  number  of	bytes actually read is returned.  Upon
       reading end-of-file, zero is returned.  Otherwise, a -1 is returned and
       the global variable errno is set	to indicate the	error.

ERRORS
       The read(), readv(), pread() and	preadv() system	calls will succeed un-
       less:

       [EBADF]		  The fd argument is not a valid file  or  socket  de-
			  scriptor open	for reading.

       [ECONNRESET]	  The  fd  argument refers to a	socket,	and the	remote
			  socket end is	forcibly closed.

       [EFAULT]		  The buf argument points outside  the	allocated  ad-
			  dress	space.

       [EIO]		  An  I/O  error  occurred while reading from the file
			  system.

       [EINTEGRITY]	  Corrupted data was detected while reading  from  the
			  file system.

       [EBUSY]		  Failed  to  read  from a file, e.g. /proc/<pid>/regs
			  while	<pid> is not stopped

       [EINTR]		  A read from a	slow device (i.e. one that might block
			  for an arbitrary amount of time) was interrupted  by
			  the delivery of a signal before any data arrived.

       [EINVAL]		  The pointer associated with fd was negative.

       [EAGAIN]		  The  file  was  marked  for non-blocking I/O,	and no
			  data were ready to be	read.

       [EISDIR]		  The file descriptor is associated with a  directory.
			  Directories may only be read directly	by root	if the
			  filesystem	   supports	  it	  and	   the
			  security.bsd.allow_read_dir sysctl MIB is set	 to  a
			  non-zero  value.  For	most scenarios,	the readdir(3)
			  function should be used instead.

       [EOPNOTSUPP]	  The file descriptor is associated with a file	system
			  and file type	that do	not allow regular read	opera-
			  tions	on it.

       [EOVERFLOW]	  The  file  descriptor	 is  associated	with a regular
			  file,	nbytes is greater than 0, offset is before the
			  end-of-file, and offset is greater than or equal  to
			  the offset maximum established for this file system.

       [EINVAL]		  The value nbytes is greater than INT_MAX.

       In  addition,  readv() and preadv() may return one of the following er-
       rors:

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

       [EINVAL]		  One of the iov_len values in the iov array was nega-
			  tive.

       [EINVAL]		  The sum of the iov_len values	in the iov array over-
			  flowed a 32-bit integer.

       [EFAULT]		  Part	of  the	iov array points outside the process's
			  allocated address space.

       The pread() and preadv()	system calls may also return the following er-
       rors:

       [EINVAL]		  The offset value was negative.

       [ESPIPE]		  The file  descriptor	is  associated	with  a	 pipe,
			  socket, or FIFO.

SEE ALSO
       dup(2),	 fcntl(2),   getdirentries(2),	open(2),  pipe(2),  select(2),
       socket(2), socketpair(2), fread(3), readdir(3)

STANDARDS
       The read() system call is expected to conform  to  ISO/IEC  9945-1:1990
       ("POSIX.1").  The readv() and pread() system calls are expected to con-
       form to X/Open Portability Guide	Issue 4, Version 2 ("XPG4.2").

HISTORY
       The preadv() system call	appeared in FreeBSD 6.0.  The pread() function
       appeared	 in AT&T System	V Release 4 UNIX.  The readv() system call ap-
       peared in 4.2BSD.  The read() function appeared in Version 1 AT&T UNIX.

FreeBSD	13.2			 June 4, 2020			       READ(2)

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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