libc/sys/write.2

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.\"     @(#)write.2	8.5 (Berkeley) 4/2/94
.\" $FreeBSD: src/lib/libc/sys/write.2,v 1.12.2.8 2002/10/13 17:42:14 schweikh Exp $
.\"
.Dd March 2, 2018
.Dt WRITE 2
.Os
.Sh NAME
.Nm write ,
.Nm writev ,
.Nm pwrite ,
.Nm pwritev ,
.Nm extpwrite ,
.Nm extpwritev
.Nd write output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/uio.h
.In unistd.h
.Ft ssize_t
.Fn write "int d" "const void *buf" "size_t nbytes"
.Ft ssize_t
.Fn writev "int d" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
.Ft ssize_t
.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
.Ft ssize_t
.Fn extpwrite "int d" "const void *buf" "size_t nbytes" "int flags" "off_t offset"
.Ft ssize_t
.Fn extpwritev "int d" "const struct iovec *iov" "int iovcnt" "int flags" "off_t offset"
.Sh DESCRIPTION
.Fn Write
attempts to write
.Fa nbytes
of data to the object referenced by the descriptor
.Fa d
from the buffer pointed to by
.Fa buf .
.Fn Writev
and
.Fn pwritev
perform the same action, but gather the output data
from the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
.Fn Pwrite
and
.Fn pwritev
perform the same function, but write to the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn writev
and
.Fn pwritev ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
	char   *iov_base;  /* Base address. */
	size_t iov_len;    /* Length. */
};
.Ed
.Pp
Each
.Fa iovec
entry specifies the base address and length of an area
in memory from which data should be written.
.Fn Writev
and
.Fn pwritev
will always write a complete area before proceeding
to the next.
.Pp
The
.Fn extpwrite
and
.Fn extpwritev
allow a
.Ar flags
argument
to also be passed in, controlling blocking/non-blocking and other features
on a call-by-call basis, ignoring the related default for the descriptor.
Allowed flags are:
.Bl -tag -width ".Dv O_FNONBLOCKING"
.It Dv O_FBLOCKING
Force the system call to operate in a blocking fashion.
.It Dv O_FNONBLOCKING
Force the system call to operate in a non-blocking fashion.
.It Dv O_FAPPEND
Force append mode for the operation.
.It Dv O_FOFFSET
Force offset mode for the operation.
.It Dv O_FSYNCWRITE
Force the system call to issue the write synchronously.
.It Dv O_FASYNCWRITE
Force the system call to issue the write asynchronously.
.El
.Pp
On objects capable of seeking, the
.Fn write
starts at a position
given by the pointer associated with
.Fa d ,
see
.Xr lseek 2 .
Upon return from
.Fn write ,
the pointer is incremented by the number of bytes which were written.
.Pp
Objects that are not capable of seeking always write from the current
position.  The value of the pointer associated with such an object
is undefined.
.Pp
If the real user is not the super-user, then
.Fn write
clears the set-user-id bit on a file.
This prevents penetration of system security
by a user who
.Dq captures
a writable set-user-id file
owned by the super-user.
.Pp
When using non-blocking I/O on objects such as sockets that are subject
to flow control,
.Fn write
and
.Fn writev
may write fewer bytes than requested;
the return value must be noted,
and the remainder of the operation should be retried when possible.
.Sh RETURN VALUES
Upon successful completion the number of bytes which were written
is returned.  Otherwise a -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Write ,
.Fn writev ,
.Fn pwrite ,
.Fn pwritev ,
.Fn extpwrite
and
.Fn extpwritev
will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa D
is not a valid descriptor open for writing.
.It Bq Er EPIPE
An attempt is made to write to a pipe that is not open
for reading by any process.
.It Bq Er EPIPE
An attempt is made to write to a socket of type
.Dv SOCK_STREAM
that is not connected to a peer socket.
.It Bq Er EFBIG
An attempt was made to write a file that exceeds the process's
file size limit or the maximum file size.
.It Bq Er EFAULT
Part of
.Fa iov
or data to be written to the file
points outside the process's allocated address space.
.It Bq Er EINVAL
The pointer associated with
.Fa d
was negative.
.It Bq Er ENOSPC
There is no free space remaining on the file system
containing the file.
.It Bq Er EDQUOT
The user's quota of disk blocks on the file system
containing the file has been exhausted.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTR
A signal interrupted the write before it could be completed.
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data could be written immediately.
.It Bq Er EROFS
An attempt was made to write over a disk label area at the beginning
of a disk.
Use
.Xr disklabel 8
.Fl W
to enable writing on the disk label area.
.El
.Pp
In addition,
.Fn writev
.Fn pwritev ,
and
.Fn extpwritev
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EDESTADDRREQ
The destination is no longer available when writing to a
.Ux
domain datagram socket on which
.Xr connect 2
had been used to set a destination address.
.It Bq Er EINVAL
.Fa Iovcnt
was less than or equal to 0, or greater than
.Dv UIO_MAXIOV .
.It Bq Er EINVAL
One of the
.Fa iov_len
values in the
.Fa iov
array was negative.
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.It Bq Er ENOBUFS
The mbuf pool has been completely exhausted when writing to a socket.
.El
.Pp
The
.Fn pwrite ,
.Fn pwritev ,
.Fn extpwrite
and
.Fn extpwritev
calls may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The specified file offset is invalid.
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr lseek 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr select 2
.Sh STANDARDS
The
.Fn write
function call is expected to conform to
.St -p1003.1-90 .
The
.Fn writev
and
.Fn pwrite
functions are expected to conform to
.St -xpg4.2 .
.Pp
The
.Fn extpwrite
and
.Fn extpwritev
functions are
.Dx
specific extensions.
.Sh HISTORY
The
.Fn pwritev
function call was added in
.Dx 1.5 .
The
.Fn pwrite
function call
appeared in
.At V.4 .
The
.Fn writev
function call
appeared in
.Bx 4.2 .
A
.Fn write
function call appeared in
.At v6 .