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WRITE(2)		  FreeBSD System Calls Manual		      WRITE(2)

     write, writev, pwrite, pwritev -- write output

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     write(int fd, const void *buf, size_t nbytes);

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

     #include <sys/uio.h>

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

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

     The write() system	call attempts to write nbytes of data to the object
     referenced	by the descriptor fd from the buffer pointed to	by buf.	 The
     writev() system call performs the same action, but	gathers	the output
     data from the iovcnt buffers specified by the members of the iov array:
     iov[0], iov[1], ..., iov[iovcnt-1].  The pwrite() and pwritev() system
     calls perform the same functions, but write to the	specified position in
     the file without modifying	the file pointer.

     For writev() and pwritev(), 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 from which data should	be written.  The writev() system call will al-
     ways write	a complete area	before proceeding to the next.

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

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

     If	the real user is not the super-user, then write() clears the set-user-
     id	bit on a file.	This prevents penetration of system security by	a user
     who "captures" a writable set-user-id file	owned by the super-user.

     When using	non-blocking I/O on objects such as sockets that are subject
     to	flow control, write() and writev() may write fewer bytes than re-
     quested; the return value must be noted, and the remainder	of the opera-
     tion should be retried when possible.

     Upon successful completion	the number of bytes which were written is re-
     turned.  Otherwise	a -1 is	returned and the global	variable errno is set
     to	indicate the error.

     The write(), writev(), pwrite() and pwritev() system calls	will fail and
     the file pointer will remain unchanged if:

     [EBADF]		The fd argument	is not a valid descriptor open for

     [EPIPE]		An attempt is made to write to a pipe that is not open
			for reading by any process.

     [EPIPE]		An attempt is made to write to a socket	of type
			SOCK_STREAM that is not	connected to a peer socket.

     [EFBIG]		An attempt was made to write a file that exceeds the
			process's file size limit or the maximum file size.

     [EFAULT]		Part of	iov or data to be written to the file points
			outside	the process's allocated	address	space.

     [EINVAL]		The pointer associated with fd was negative.

     [ENOSPC]		There is no free space remaining on the	file system
			containing the file.

     [EDQUOT]		The user's quota of disk blocks	on the file system
			containing the file has	been exhausted.

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

     [EINTR]		A signal interrupted the write before it could be com-

     [EAGAIN]		The file was marked for	non-blocking I/O, and no data
			could be written immediately.

     [EROFS]		An attempt was made to write over a disk label area at
			the beginning of a slice.  Use disklabel(8) -W to en-
			able writing on	the disk label area.

     [EINVAL]		The value nbytes is greater than SSIZE_MAX (or greater
			than INT_MAX, if the sysctl debug.iosize_max_clamp is

     [EINTEGRITY]	The backing store for fd detected corrupted data while
			reading.  (For example,	writing	a partial filesystem
			block may require first	reading	the existing block
			which may trigger this error.)

     In	addition, writev() and pwritev() may return one	of the following er-

     [EDESTADDRREQ]	The destination	is no longer available when writing to
			a UNIX domain datagram socket on which connect(2) had
			been used to set a destination address.

     [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 is greater than
			SSIZE_MAX (or greater than INT_MAX, if the sysctl
			debug.iosize_max_clamp is non-zero).

     [ENOBUFS]		The mbuf pool has been completely exhausted when writ-
			ing to a socket.

     The pwrite() and pwritev()	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.

     fcntl(2), lseek(2), open(2), pipe(2), select(2)

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

     The pwritev() system call appeared	in FreeBSD 6.0.	 The pwrite() function
     appeared in AT&T System V Release 4 UNIX.	The writev() system call ap-
     peared in 4.2BSD.	The write() function appeared in Version 1 AT&T	UNIX.

     The pwrite() system call appends the file without changing	the file off-
     set if O_APPEND is	set, contrary to IEEE Std 1003.1-2008 ("POSIX.1")
     where pwrite() writes into	offset regardless of whether O_APPEND is set.

FreeBSD	13.0		       February	11, 2021		  FreeBSD 13.0


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