Tribblix: manual page: write.2

WRITE(2) System Calls WRITE(2)

NAME

write, pwrite, writev, pwritev- write on a file

SYNOPSIS

#include <unistd.h>

ssize_t write(int fildes, const void *buf, size_t nbyte);

ssize_t pwrite(int fildes, const void *buf, size_t nbyte,
off_t offset);

#include <sys/uio.h>

ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);

ssize_t pwritev(int fildes, const struct iovec *iov, int iovcnt, off_t offset);

DESCRIPTION

The write() function attempts to write nbyte bytes from the buffer
pointed to by buf to the file associated with the open file
descriptor, fildes.

If nbyte is 0, write() will return 0 and have no other results if the
file is a regular file; otherwise, the results are unspecified.

On a regular file or other file capable of seeking, the actual
writing of data proceeds from the position in the file indicated by
the file offset associated with fildes. Before successful return from
write(), the file offset is incremented by the number of bytes
actually written. On a regular file, if this incremented file offset
is greater than the length of the file, the length of the file will
be set to this file offset.

If the O_SYNC bit has been set, write I/O operations on the file
descriptor complete as defined by synchronized I/O file integrity
completion.

If fildes refers to a socket, write() is equivalent to send(3SOCKET)
with no flags set.

On a file not capable of seeking, writing always takes place starting
at the current position. The value of a file offset associated with
such a device is undefined.

If the O_APPEND flag of the file status flags is set, the file offset
will be set to the end of the file prior to each write and no
intervening file modification operation will occur between changing
the file offset and the write operation.

For regular files, no data transfer will occur past the offset
maximum established in the open file description with fildes.

A write() to a regular file is blocked if mandatory file/record
locking is set (see chmod(2)), and there is a record lock owned by
another process on the segment of the file to be written:

o If O_NDELAY or O_NONBLOCK is set, write() returns -1 and
sets errno to EAGAIN.

o If O_NDELAY and O_NONBLOCK are clear, write() sleeps until
all blocking locks are removed or the write() is
terminated by a signal.

If a write() requests that more bytes be written than there is room
for--for example, if the write would exceed the process file size
limit (see getrlimit(2) and ulimit(2)), the system file size limit,
or the free space on the device--only as many bytes as there is room
for will be written. For example, suppose there is space for 20 bytes
more in a file before reaching a limit. A write() of 512-bytes
returns 20. The next write() of a non-zero number of bytes gives a
failure return (except as noted for pipes and FIFO below).

If write() is interrupted by a signal before it writes any data, it
will return -1 with errno set to EINTR.

If write() is interrupted by a signal after it successfully writes
some data, it will return the number of bytes written.

If write() exceeds the process file size limit, the application
generates a SIGXFSZ signal, whose default behavior is to dump core.

After a write() to a regular file has successfully returned:

o Any successful read(2) from each byte position in the file
that was modified by that write will return the data
specified by the write() for that position until such byte
positions are again modified.

o Any subsequent successful write() to the same byte
position in the file will overwrite that file data.

Write requests to a pipe or FIFO are handled the same as a regular
file with the following exceptions:

o There is no file offset associated with a pipe, hence each
write request appends to the end of the pipe.

o Write requests of {PIPE_BUF} bytes or less are guaranteed
not to be interleaved with data from other processes doing
writes on the same pipe. Writes of greater than
{PIPE_BUF} bytes may have data interleaved, on arbitrary
boundaries, with writes by other processes, whether or not
the O_NONBLOCK or O_NDELAY flags are set.

o If O_NONBLOCK and O_NDELAY are clear, a write request may
cause the process to block, but on normal completion it
returns nbyte.

o If O_NONBLOCK and O_NDELAY are set, write() does not block
the process. If a write() request for PIPE_BUF or fewer
bytes succeeds completely write() returns nbyte.
Otherwise, if O_NONBLOCK is set, it returns -1 and sets
errno to EAGAIN or if O_NDELAY is set, it returns 0. A
write() request for greater than {PIPE_BUF} bytes
transfers what it can and returns the number of bytes
written or it transfers no data and, if O_NONBLOCK is set,
returns -1 with errno set to EAGAIN or if O_NDELAY is set,
it returns 0. Finally, if a request is greater than
PIPE_BUF bytes and all data previously written to the pipe
has been read, write() transfers at least PIPE_BUF bytes.

When attempting to write to a file descriptor (other than a pipe, a
FIFO, a socket, or a stream) that supports nonblocking writes and
cannot accept the data immediately:

o If O_NONBLOCK and O_NDELAY are clear, write() blocks until
the data can be accepted.

o If O_NONBLOCK or O_NDELAY is set, write() does not block
the process. If some data can be written without blocking
the process, write() writes what it can and returns the
number of bytes written. Otherwise, if O_NONBLOCK is set,
it returns -1 and sets errno to EAGAIN or if O_NDELAY is
set, it returns 0.

Upon successful completion, where nbyte is greater than 0, write()
will mark for update the st_ctime and st_mtime fields of the file,
and if the file is a regular file, the S_ISUID and S_ISGID bits of
the file mode may be cleared.

For streams files (see Intro(2) and streamio(4I)), the operation of
write() is determined by the values of the minimum and maximum nbyte
range ("packet size") accepted by the stream. These values are
contained in the topmost stream module, and can not be set or tested
from user level. If nbyte falls within the packet size range, nbyte
bytes are written. If nbyte does not fall within the range and the
minimum packet size value is zero, write() breaks the buffer into
maximum packet size segments prior to sending the data downstream
(the last segment may be smaller than the maximum packet size). If
nbyte does not fall within the range and the minimum value is non-
zero, write() fails and sets errno to ERANGE. Writing a zero-length
buffer (nbyte is zero) to a streams device sends a zero length
message with zero returned. However, writing a zero-length buffer to
a pipe or FIFO sends no message and zero is returned. The user
program may issue the I_SWROPT ioctl(2) to enable zero-length
messages to be sent across the pipe or FIFO (see streamio(4I)).

When writing to a stream, data messages are created with a priority
band of zero. When writing to a socket or to a stream that is not a
pipe or a FIFO:

o If O_NDELAY and O_NONBLOCK are not set, and the stream
cannot accept data (the stream write queue is full due to
internal flow control conditions), write() blocks until
data can be accepted.

o If O_NDELAY or O_NONBLOCK is set and the stream cannot
accept data, write() returns -1 and sets errno to EAGAIN.

o If O_NDELAY or O_NONBLOCK is set and part of the buffer
has already been written when a condition occurs in which
the stream cannot accept additional data, write()
terminates and returns the number of bytes written.

The write() and writev() functions will fail if the stream head had
processed an asynchronous error before the call. In this case, the
value of errno does not reflect the result of write() or writev() but
reflects the prior error.

If an asynchronous error occurs on a socket, it is possible for the
write() and writev() to return an asynchronous error, just as in the
STREAMS case described above. This might occur, for example, if a TCP
socket that is using TCP keep-alive is closed due to failing the
keep-alive check.

pwrite()
The pwrite() function is equivalent to write(), except that it writes
into a given position and does not change the file offset (regardless
of whether O_APPEND is set). The first three arguments to pwrite()
are the same as write(), with the addition of a fourth argument
offset for the desired position inside the file.

writev()
The writev() function performs the same action as write(), but
gathers the output data from the iovcnt buffers specified by the
members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1]. The
iovcnt buffer is valid if greater than 0 and less than or equal to
{IOV_MAX}. See Intro(2) for a definition of {IOV_MAX}.

pwritev()
The pwritev() function is equivalent to writev(), except that it
writes into a given position and does not change the file offset
(regardless of whether O_APPEND is set). The first three arguments to
pwritev() are the same as writev(), with the addition of a fourth
argument offset for the desired position inside the file.

The iovec structure contains the following members:

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. The writev() function
always writes all data from an area before proceeding to the next.

If fildes refers to a regular file and all of the iov_len members in
the array pointed to by iov are 0, writev() will return 0 and have no
other effect. For other file types, the behavior is unspecified.

If the sum of the iov_len values is greater than SSIZE_MAX, the
operation fails and no data is transferred.

RETURN VALUES

Upon successful completion, write() returns the number of bytes
actually written to the file associated with fildes. This number is
never greater than nbyte. Otherwise, -1 is returned, the file-pointer
remains unchanged, and errno is set to indicate the error.

Upon successful completion, writev() returns the number of bytes
actually written. Otherwise, it returns -1, the file-pointer
remains unchanged, and errno is set to indicate an error.

ERRORS

In addition to the errors documented below, if the filedes argument
refers to a socket, then an asynchronous error generated by the
underlying socket protocol may be returned. For the full list of
errors, please see the corresponding socket protocol manual page. For
example, for a list of TCP errors, please see tcp(4P).

The write(), pwrite(), writev(), and pwritev() functions will fail
if:

EAGAIN
Mandatory file/record locking is set, O_NDELAY or
O_NONBLOCK is set, and there is a blocking record lock; an
attempt is made to write to a stream that can not accept
data with the O_NDELAY or O_NONBLOCK flag set; or a write
to a pipe or FIFO of PIPE_BUF bytes or less is requested
and less than nbytes of free space is available.

EBADF
The fildes argument is not a valid file descriptor open
for writing.

ECONNRESET
The filedes argument refers to a connection oriented
socket and the connection was forcibly closed by the peer
and is no longer valid. I/O can no longer be performed to
filedes.

EDEADLK
The write was going to go to sleep and cause a deadlock
situation to occur.

EDQUOT
The user's quota of disk blocks on the file system
containing the file has been exhausted.

EFBIG
An attempt is made to write a file that exceeds the
process's file size limit or the maximum file size (see
getrlimit(2) and ulimit(2)).

EFBIG
The file is a regular file, nbyte is greater than 0, and
the starting position is greater than or equal to the
offset maximum established in the file description
associated with fildes.

EINTR
A signal was caught during the write operation and no
data was transferred.

EIO
The process is in the background and is attempting to
write to its controlling terminal whose TOSTOP flag is
set, or the process is neither ignoring nor blocking
SIGTTOU signals and the process group of the process is
orphaned.

ENOLCK
Enforced record locking was enabled and {LOCK_MAX} regions
are already locked in the system, or the system record
lock table was full and the write could not go to sleep
until the blocking record lock was removed.

ENOLINK
The fildes argument is on a remote machine and the link to
that machine is no longer active.

ENOSPC
During a write to an ordinary file, there is no free
space left on the device.

ENOSR
An attempt is made to write to a streams with insufficient
streams memory resources available in the system.

ENXIO
A hangup occurred on the stream being written to.

EPIPE
An attempt is made to write to a pipe or a FIFO that is
not open for reading by any process, or that has only one
end open (or to a file descriptor created by
socket(3SOCKET), using type SOCK_STREAM that is no longer
connected to a peer endpoint). A SIGPIPE signal will also
be sent to the thread. The process dies unless special
provisions were taken to catch or ignore the signal.

ERANGE
The transfer request size was outside the range supported
by the streams file associated with fildes.

The write() and pwrite() functions will fail if:

EFAULT
The buf argument points to an illegal address.

EINVAL
The nbyte argument overflowed an ssize_t.

The pwrite() and pwritev() functions fail and the file pointer
remains unchanged if:

ESPIPE
The fildes argument is associated with a pipe or FIFO.

The write() and writev() functions may fail if:

EINVAL
The stream or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer.

ENXIO
A request was made of a non-existent device, or the request
was outside the capabilities of the device.

ENXIO
A hangup occurred on the stream being written to.

A write to a streams file may fail if an error message has been
received at the stream head. In this case, errno is set to the value
included in the error message.

The writev() and pwritev() functions may fail if:

EINVAL
The iovcnt argument was less than or equal to 0 or greater
than {IOV_MAX}; one of the iov_len values in the iov array
was negative; or the sum of the iov_len values in the iov
array overflowed an ssize_t.

USAGE

The pwrite() function has a transitional interface for 64-bit file
offsets. See lf64(7).

ATTRIBUTES