DragonFly On-Line Manual Pages
WRITE(2) DragonFly System Calls Manual WRITE(2)
NAME
write, writev, pwrite, pwritev, extpwrite, extpwritev -- write output
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
write(int d, const void *buf, size_t nbytes);
ssize_t
writev(int d, const struct iovec *iov, int iovcnt);
ssize_t
pwrite(int d, const void *buf, size_t nbytes, off_t offset);
ssize_t
pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);
ssize_t
extpwrite(int d, const void *buf, size_t nbytes, int flags,
off_t offset);
ssize_t
extpwritev(int d, const struct iovec *iov, int iovcnt, int flags,
off_t offset);
DESCRIPTION
Write() attempts to write nbytes of data to the object referenced by the
descriptor d from the buffer pointed to by buf. Writev() and pwritev()
perform the same action, but gather the output data from the iovcnt
buffers specified by the members of the iov array: iov[0], iov[1], ...,
iov[iovcnt-1]. Pwrite() and pwritev() perform the same function, 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 {
char *iov_base; /* Base address. */
size_t iov_len; /* Length. */
};
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. Writev() and pwritev() will
always write a complete area before proceeding to the next.
The extpwrite() and extpwritev() allow a 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:
O_FBLOCKING Force the system call to operate in a blocking fashion.
O_FNONBLOCKING Force the system call to operate in a non-blocking
fashion.
O_FAPPEND Force append mode for the operation.
O_FOFFSET Force offset mode for the operation.
O_FSYNCWRITE Force the system call to issue the write synchronously.
O_FASYNCWRITE Force the system call to issue the write asynchronously.
On objects capable of seeking, the write() starts at a position given by
the pointer associated with d, 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
position. The value of the pointer associated with such an object is
undefined.
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
requested; the return value must be noted, and the remainder of the
operation should be retried when possible.
RETURN VALUES
Upon successful completion the number of bytes which were written is
returned. Otherwise a -1 is returned and the global variable errno is
set to indicate the error.
ERRORS
Write(), writev(), pwrite(), pwritev(), extpwrite() and extpwritev() will
fail and the file pointer will remain unchanged if:
[EBADF] D is not a valid descriptor open for writing.
[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 d 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
completed.
[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 disk. Use disklabel(8) -W to
enable writing on the disk label area.
In addition, writev() pwritev(), and extpwritev() may return one of the
following errors:
[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] Iovcnt was less than or equal to 0, or greater than
UIO_MAXIOV.
[EINVAL] One of the iov_len values in the iov array was
negative.
[EINVAL] The sum of the iov_len values in the iov array
overflowed a 32-bit integer.
[ENOBUFS] The mbuf pool has been completely exhausted when
writing to a socket.
The pwrite(), pwritev(), extpwrite() and extpwritev() calls may also
return the following errors:
[EINVAL] The specified file offset is invalid.
[ESPIPE] The file descriptor is associated with a pipe, socket,
or FIFO.
SEE ALSO
fcntl(2), lseek(2), open(2), pipe(2), select(2)
STANDARDS
The write() function call is expected to conform to ISO/IEC 9945-1:1990
(``POSIX.1''). The writev() and pwrite() functions are expected to
conform to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').
The extpwrite() and extpwritev() functions are DragonFly specific
extensions.
HISTORY
The pwritev() function call was added in DragonFly 1.5. The pwrite()
function call appeared in AT&T System V Release 4 UNIX. The writev()
function call appeared in 4.2BSD. A write() function call appeared in
Version 6 AT&T UNIX.
DragonFly 5.1 March 2, 2018 DragonFly 5.1