libc/sys/write.2

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.\"     @(#)write.2	8.5 (Berkeley) 4/2/94
.\" $FreeBSD$
.\"
.Dd April 2, 1994
.Dt WRITE 2
.Os
.Sh NAME
.Nm write ,
.Nm writev ,
.Nm pwrite
.Nd write output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/uio.h>
.Fd #include <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"
.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
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].
.Fn Pwrite
performs the same function, but writes to the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn writev ,
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
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 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 IMPLEMENTATION NOTES
In the non-threaded library
.Fn write
is implemented as the
.Va write
syscall.
.Pp
In the threaded library, the
.Va write
syscall is assembled to
.Fn _thread_sys_write
and
.Fn write
is implemented as a function which locks
.Fa d
for read and write, then calls
.Fn _thread_sys_write .
If the call to
.Fn _thread_sys_write
would block, a context switch is performed.
Before returning,
.Fn write
unlocks
.Fa d .
.Pp
In the non-threaded library
.Fn writev
is implemented as the
.Va writev
syscall.
.Pp
In the threaded library, the
.Va writev
syscall is assembled to
.Fn _thread_sys_writev
and
.Fn writev
is implemented as a function which locks
.Fa d
for read and write, then calls
.Fn _thread_sys_writev .
If the call to
.Fn _thread_sys_writev
would block, a context switch is performed.
Before returning,
.Fn writev
unlocks
.Fa d .
.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 ,
and
.Fn pwrite
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.
.El
.Pp
In addition,
.Fn writev
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
call 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 .
.Sh HISTORY
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 .