lib/libsys/write.2

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.Dd February 11, 2021
.Dt WRITE 2
.Os
.Sh NAME
.Nm write ,
.Nm writev ,
.Nm pwrite ,
.Nm pwritev
.Nd write output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft ssize_t
.Fn write "int fd" "const void *buf" "size_t nbytes"
.Ft ssize_t
.Fn pwrite "int fd" "const void *buf" "size_t nbytes" "off_t offset"
.In sys/uio.h
.Ft ssize_t
.Fn writev "int fd" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn pwritev "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset"
.Sh DESCRIPTION
The
.Fn write
system call
attempts to write
.Fa nbytes
of data to the object referenced by the descriptor
.Fa fd
from the buffer pointed to by
.Fa buf .
The
.Fn writev
system call
performs the same action, but gathers the output data
from the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
The
.Fn pwrite
and
.Fn pwritev
system calls
perform the same functions, 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 {
	void   *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.
The
.Fn writev
system call
will always write a complete area before proceeding
to the next.
.Pp
On objects capable of seeking, the
.Fn write
starts at a position
given by the pointer associated with
.Fa fd ,
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 ATOMICITY OF WRITES
When operating on regular files on local file systems, the effects of
.Fn write
are atomic.
As required by the POSIX standard,
the
.Fn read ,
.Fn write ,
and
.Fn ftruncate
functions and their variations are atomic with respect to
each other on the file data and metadata for regular files.
See for instance
.St -p1003.1-2024
Volume 2, Section 2.9.7 for more information.
.Pp
.Fx
implements the requirement by taking
a read/write range lock on the file byte range
affected by the corresponding function.
.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
The
.Fn write ,
.Fn writev ,
.Fn pwrite
and
.Fn pwritev
system calls
will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument
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 fd
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 EINVAL
The value
.Fa nbytes
is greater than
.Dv SSIZE_MAX
(or greater than
.Dv INT_MAX ,
if the sysctl
.Va debug.iosize_max_clamp
is non-zero).
.It Bq Er EINVAL
The file descriptor refers to a raw device, and the write
offset or size is not a multiple of the device's block size.
.It Bq Er EINTEGRITY
The backing store for
.Fa 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.)
.El
.Pp
In addition,
.Fn writev
and
.Fn pwritev
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
The
.Fa iovcnt
argument
was less than or equal to 0, or greater than
.Dv IOV_MAX .
.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 is greater than
.Dv SSIZE_MAX
(or greater than
.Dv INT_MAX ,
if the sysctl
.Va debug.iosize_max_clamp
is non-zero).
.It Bq Er ENOBUFS
The mbuf pool has been completely exhausted when writing to a socket.
.El
.Pp
The
.Fn pwrite
and
.Fn pwritev
system calls may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa offset
value was negative.
.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
system call is expected to conform to
.St -p1003.1-90 .
The
.Fn writev
and
.Fn pwrite
system calls are expected to conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn pwritev
system call appeared in
.Fx 6.0 .
The
.Fn pwrite
function appeared in
.At V.4 .
The
.Fn writev
system call appeared in
.Bx 4.2 .
The
.Fn write
function appeared in
.At v1 .
.Sh BUGS
The
.Fn pwrite
system call appends the file without changing the file offset if
.Dv O_APPEND
is set, contrary to
.St -p1003.1-2008
where
.Fn pwrite
writes into
.Fa offset
regardless of whether
.Dv O_APPEND
is set.