write, pwrite, writev, pwritev- write on a file
#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);
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.
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.
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}.
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.
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.
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.
The pwrite() function has a transitional interface for 64-bit file
offsets. See lf64(7).
See attributes(7) for descriptions of the following attributes:
ATTRIBUTE
TYPE |
ATTRIBUTE VALUE |
Interface Stability |
Committed |
MT-Level |
write() is Async-Signal-Safe |
Standard |
See standards(7). |
Intro(2), chmod(2), creat(2), dup(2),
fcntl(2), getrlimit(2), ioctl(2), lseek(2),
open(2), pipe(2), ulimit(2), send(3SOCKET),
socket(3SOCKET), streamio(4I), tcp(4P),
attributes(7), lf64(7), standards(7)