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READ(2)			  FreeBSD 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 d, void *buf, size_t nbytes);

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

     #include <sys/uio.h>

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

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

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

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

	   struct iovec	{
		   void	*iov_base;
		   size_t iov_len;
	   };

     Each iovec	entry specifies	the base address and length of an area in mem-
     ory where data should be placed.  readv() will always fill	an area	com-
     pletely before proceeding to the next.

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

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

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

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

ERRORS
     read(), readv(), pread(), and preadv() will succeed unless:

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

     [EBADF]		d is not a valid file or socket	descriptor open	for
			reading.

     [EFAULT]		buf points outside the allocated address space.

     [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.  See
			sigaction(2) for more information on the interaction
			between	signals	and system calls.

     [EINVAL]		The file pointer associated with d was negative; or
			the total length of the	I/O is more than can be	ex-
			pressed	by the ssize_t return value.

     [EIO]		An I/O error occurred while reading from the file sys-
			tem.

     [EISDIR]		d 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	in-
			stead.

     In	addition, readv() and preadv() may return one of the following errors:

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

     [EINVAL]		iovcnt was less	than or	equal to 0, or greater than
			{IOV_MAX}; or one of the iov_len values	in the iov ar-
			ray was	negative; or the sum of	the iov_len values in
			the iov	array overflowed a 32-bit integer.

     The pread() and preadv() calls may	also return the	following errors:

     [EINVAL]		The specified file offset is invalid.

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

SEE ALSO
     dup(2), fcntl(2), open(2),	pipe(2), poll(2), select(2), sigaction(2),
     socket(2),	socketpair(2)

STANDARDS
     The read()	function conforms to ISO/IEC 9945-1:1990 ("POSIX.1").  The
     readv() and pread() functions conform to X/Open Portability Guide
     Issue 4, Version 2	("XPG4.2").

HISTORY
     The preadv() function call	appeared in NetBSD 1.4.	 The pread() function
     call appeared in AT&T System V Release 4 UNIX.  The readv() function call
     appeared in 4.2BSD.  The read() function call appeared in Version 1 AT&T
     UNIX.

CAVEATS
     Error checks should explicitly test for -1.  Code such as

	     while ((nr	= read(fd, buf,	sizeof(buf))) >	0)

     is	not maximally portable,	as some	platforms allow	for nbytes to range
     between SSIZE_MAX and SIZE_MAX - 2, in which case the return value	of an
     error-free	read() may appear as a negative	number distinct	from -1.
     Proper loops should use

	     while ((nr	= read(fd, buf,	sizeof(buf))) != -1 && nr != 0)

FreeBSD	13.0		       September 2, 2019		  FreeBSD 13.0

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

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