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READ(2)			  FreeBSD System Calls Manual		       READ(2)

     read, readv, pread, preadv	-- read	input

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

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

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

     #include <sys/uio.h>

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

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

     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 mem-
     ory where data should be placed.  The readv() system call will always
     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 po-
     sition.  The value	of the pointer associated with such an object is unde-

     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.

     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 ker-
     nel implements this guarantee by locking the file ranges affected by the

     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.

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

     [EBADF]		The fd argument	is not a valid file or socket descrip-
			tor 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 address

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

     [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 off-
			set 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 errors:

     [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-

     [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 al-
			located	address	space.

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

     [EINVAL]		The offset value was negative.

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

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

     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").

     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	13.0			 June 4, 2020			  FreeBSD 13.0


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