What is `writev'?
Marc Evans Ultrix Q/A
evans at testmax.zk3.dec.com
Sat Apr 14 02:00:41 AEST 1990
write(2)
Name
write, writev - write on a file
Syntax
write (fd, buf, nbytes)
int fd;
char *buf;
int nbytes;
#include <sys/types.h>
#include <sys/uio.h>
writev (fd, iov, ioveclen)
int fd;
struct iovec *iov;
int ioveclen;
Arguments
fd Descriptor returned by a creat, open, dup,
fcntl, pipe, or socket system call.
buf Points to the buffer containing the data to be
written.
nbytes Positive integer defining the number of bytes to
be written from the buffer.
iov Points to a data structure of type iovec, which
defines the starting location of the set of vec-
tors forming the array and the length of each
individual vector in the array to be written.
This structure is defined in <sys/uio.h> as fol-
lows:
struct iovec {
caddr_t iov_base ;
int iov_len ;
} ;
The caddr_t data type is defined in
<sys/types.h> and is the recommended way to
define an address for a character value. In any
case, the address iov_base is the starting
address of the set of vectors. The integer
value iov_len is the length of each individual
vector, in bytes.
ioveclen Defines the number of vectors in the array of
data to be written. Note that the numbering of
the vectors begins with 0 and proceeds through
1
write(2)
ioveclen -1.
Description
The write system call attempts to write a buffer of data to
a file. The writev system call attempts to write an array
of buffers of data to a file.
When a file is opened to a device capable of seeking (such
as a disk or tape), the write starts at the position given
by the file pointer associated with the file descriptor, fd.
This file pointer is the offset, in bytes, from the begin-
ning of the file where the write is to begin. When the file
is first opened, the file pointer is set at 0. It can be
modified by the read(2) lseek(2) and write system calls.
When the write call returns, the file pointer is incremented
by the number of bytes actually written.
When the file is opened to a device not capable of seeking
(such as sockets, pipes, or terminals), the write starts at
the current position. The value of the pointer associated
with such an object is undefined.
By default, write does asynchronous writes. That is, after
the data is written to a buffer cache, control returns to
the program. The actual write to a device takes place after
control returns. However, if you use an open or fcntl call
to open a file for synchronous writes, control does not
return to the program until after the buffer cache has been
written to the device.
If a program is using write to a remote file over NFS, and
an asynchronous write error occurs, then all subsequent
write requests will return -1 and errno will be set to the
asynchronous error code. Also, a subsequent fsync(2) or
close(2) will likewise fail. The return code from close(2)
should be inspected by any program that can write over NFS.
Write requests to a pipe (or FIFO) are handled the same as a
regular file, with the following exceptions:
o A file offset is not associated with a pipe. Therefore,
each write request appends to the end of the pipe.
o Write requests less than or equivalent to {PIPE_BUF}
bytes are not interleaved with data from other
processes doing writes on the same pipe. Write
requests greater than {PIPE_BUF} bytes can interleave
on arbitrary boundaries with writes by other processes.
o If the O_NDELAY and O_NONBLOCK flags are clear, a write
can cause the process to block, but, under normal com-
pletion, it returns nbytes.
2
write(2)
o If the O_NDELAY or O_NONBLOCK flag is set, the write
function does not block the process. Write requests
less than or equal to {PIPE_BUF} bytes either succeed
and return nbytes or -1, and errno is set to [EWOULD-
BLOCK]. Write requests that exceed {PIPE_BUF} bytes
can return complete success, partial write, or no suc-
cess, and errno is to [EWOULDBLOCK].
Environment
SYSTEM V
When your program is compiled using the System V environ-
ment, and the file was opened with the O_NDELAY flag set, a
write to a full pipe (or FIFO) returns a zero (0), rather
than an error, as for the ULTRIX non-System V environment.
Differs from the System V definition in that the value
nbytes is int, rather than unsigned.
When your program is compiled using POSIX environment,
EAGAIN is returned in errno, in place of EWOULDBLOCK.
Return Value
Upon successful completion, the number of bytes actually
written is returned. Otherwise, a -1 is returned, and errno
is set to indicate the error.
Diagnostics
The write system call fails and the file pointer will remain
unchanged, if any of the following is true:
[EACCESS] The file does not permit writing. NFS only.
[EBADF] The fd argument is not a valid descriptor
open for writing.
[EPIPE] An attempt was made to write to a pipe that
is not open for reading by any process.
[EPIPE] An attempt was 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, set by
ulimit(2) or the maximum file size (approxi-
mately 2 Gigabytes).
[EFAULT] Part of the array pointed to by iov or data
to be written to the file points outside the
process's allocated address space.
3
write(2)
[EWOULDBLOCK] The O_NDELAY or O_NONBLOCK flag is set for
the file descriptor and the process would be
delayed in the write operation.
[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] The write operation was interrupted, no data
was transferred.
[EINVAL] The nbytes argument is negative.
[EROFS] The file is on a read-only file system. NFS
only.
[ESTALE] The fd argument is invalid because the file
referred to by that file handle no longer
exists or has been revoked. NFS only.
[ETIMEDOUT] A write operation failed because the server
did not properly respond after a period of
time that is dependent on the mount(8nfs)
options. NFS only.
See Also
close(2), creat(2), dup(2), fcntl(2), fsync(2), lseek(2),
open(2), pipe(2), socket(2)
4
===========================================================================
Marc Evans - WB1GRH - evans at decvax.DEC.COM | Synergytics (603)635-8876
Unix/X-window Software Contractor | 21 Hinds Ln, Pelham, NH 03076
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