WRITE(2) BSD WRITE(2)
NAME
write, writev - write output
SYNOPSIS
cc = write(d, buf, nbytes)
int cc, d;
char *buf;
int nbytes;
#include <sys/types.h>
#include <sys/uio.h>
cc = writev(d, iov, iovcnt)
int cc, d;
struct iovec *iov;
int iovcnt;
DESCRIPTION
write 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 specified by
the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
For writev, the iovec structure is defined as
struct iovec {
caddr_t iov_base;
int iov_len;
};
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. writev will always 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 actually 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.
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.
Assuming a per-process maximum file size is in effect (RLIMIT_FSIZE), in
an AES, POSIX or XPG3 environment, the following behavior is true:
The write function writes as much data as it can prior to bumping into
this limit and return success. As always, an error [EFBIG] is returned if
the starting offset is at or beyond the limit.
ERRORS
write and writev fails and the file pointer remains unchanged if one or
more of the following are true:
[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' 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
containing the file.
[EIO] An I/O error occurred while reading from or writing to
the file system.
[EWOULDBLOCK] The file was marked for non-blocking I/O, and no data
could be written immediately.
[EDQUOT] The user's quota of disk blocks on the file system
containing the file has been exhausted.
In addition, writev may return one of the following errors:
[EINVAL] iovcnt was less than or equal to 0, or greater than 16.
[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.
SEE ALSO
fcntl(2), lseek(2), open(2), pipe(2), select(2)
DIAGNOSTICS
Upon successful completion the number of bytes actually written is
returned. Otherwise a -1 is returned and the global variable errno is
set to indicate the error.
If the nbytes argument is 0 (zero), the write function returns 0 (zero)
and has no other results if the file is a regular file, a pipe, or raw-
disk file.
If the nbytes argument is greater than {INT_MAX}, the write function
returns 0 (zero) and has no other results if the file is a regular file,
or pipe.
NOTES
In other implementations, if the real user is not the super-user, then
write clears the set-user-id bit on a file.