WRITE(2) BSD Programmer's Manual WRITE(2)NAME
write, writev, pwrite, pwritev - write output
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
ssize_t
write(int d, const void *buf, size_t nbytes);
ssize_t
pwrite(int d, const void *buf, size_t nbytes, off_t offset);
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
writev(int d, const struct iovec *iov, int iovcnt);
ssize_t
pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);
DESCRIPTIONwrite() attempts to write nbytes of data to the object referenced by the
descriptor d from the buffer pointed to by buf. writev() performs the
same action, but gathers the output data from the iovcnt buffers speci-
fied by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
pwrite() and pwritev() perform the same functions, 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 {
void *iov_base;
size_t iov_len;
};
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. writev() and pwritev() will al-
ways write a complete area before proceeding to the next.
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 po-
sition. The value of the pointer associated with such an object is unde-
fined.
If the real user is not the superuser, 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 superuser.
If write() succeeds it will update the st_ctime and st_mtime fields of
the file's meta-data (see stat(2)).
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 request-
ed; the return value must be noted, and the remainder of the operation
should be retried when possible.
Note that writev() and pwritev() will fail if the value of iovcnt exceeds
the constant IOV_MAX.
RETURN VALUES
Upon successful completion the number of bytes which were written is re-
turned. Otherwise, a -1 is returned and the global variable errno is set
to indicate the error.
ERRORSwrite(), pwrite(), writev(), and pwritev() 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.
[EINVAL] The pointer associated with d was negative.
[ENOSPC] There is no free space remaining on the file system con-
taining the file.
[EDQUOT] The user's quota of disk blocks on the file system contain-
ing the file has been exhausted.
[EIO] An I/O error occurred while reading from or writing to the
file system.
[EAGAIN] The file was marked for non-blocking I/O, and no data could
be written immediately.
In addition, write() and pwrite() may return the following error:
[EFAULT] Part of iov or data to be written to the file points out-
side the process's allocated address space.
[EINVAL] nbytes was larger than SSIZE_MAX.
Also, writev() and pwritev() 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
IOV_MAX.
[EINVAL] The sum of the iov_len values in the iov array overflowed
an ssize_t.
SEE ALSOfcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2)STANDARDS
The write() function conforms to IEEE Std 1003.1-1990 ("POSIX"). The wri-
tev() and pwrite() functions conform to X/Open Portability Guide Issue
4.2 ("XPG4.2").
HISTORY
The pwritev() function call appeared in OpenBSD 2.7. The pwrite() func-
tion call appeared in AT&T System V.4 UNIX. The writev() function call
appeared in 4.2BSD. The write() function call appeared in Version 2 AT&T
UNIX.
CAVEATS
Error checks should explicitly test for -1. Code such as
while ((nr = write(fd, buf, sizeof(buf))) > 0)
is not maximally portable, as some platforms allow for nbytes to range
between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
error-free write() may appear as a negative number distinct from -1.
Proper loops should use
while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0)
MirOS BSD #10-current July 28, 1998 2
