write(3posix)
NAME
pwrite, write --- write on a file
SYNOPSIS
#include <unistd.h>
ssize_t pwrite(int fildes, const void *buf, size_t nbyte,
off_t offset);
ssize_t write(int fildes, const void *buf, size_t nbyte);
DESCRIPTION
The write() function shall attempt to write nbyte bytes from the buffer
pointed to by buf to the file associated with the open file descriptor,
fildes.
Before any action described below is taken, and if nbyte is zero and
the file is a regular file, the write() function may detect and return
errors as described below. In the absence of errors, or if error
detection is not performed, the write() function shall return zero and
have no other results. If nbyte is zero and the file is not a regular
file, the results are unspecified.
On a regular file or other file capable of seeking, the actual writing
of data shall proceed from the position in the file indicated by the
file offset associated with fildes. Before successful return from
write(), the file offset shall be incremented by the number of bytes
actually written. On a regular file, if the position of the last byte
written is greater than or equal to the length of the file, the length
of the file shall be set to this position plus one.
On a file not capable of seeking, writing shall always take 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
shall be set to the end of the file prior to each write and no
intervening file modification operation shall occur between changing
the file offset and the write operation.
If a write() requests that more bytes be written than there is room for
(for example, the file size limit of the process or the physical end of
a medium), only as many bytes as there is room for shall be written.
For example, suppose there is space for 20 bytes more in a file before
reaching a limit. A write of 512 bytes will return 20. The next write
of a non-zero number of bytes would give a failure return (except as
noted below).
If the request would cause the file size to exceed the soft file size
limit for the process and there is no room for any bytes to be written,
the request shall fail and the implementation shall generate the
SIGXFSZ signal for the thread.
If write() is interrupted by a signal before it writes any data, it
shall return 1 with errno set to [EINTR].
If write() is interrupted by a signal after it successfully writes some
data, it shall return the number of bytes written.
If the value of nbyte is greater than {SSIZE_MAX}, the result is
implementation-defined.
After a write() to a regular file has successfully returned:
* Any successful read() from each byte position in the file that was
modified by that write shall return the data specified by the
write() for that position until such byte positions are again
modified.
* Any subsequent successful write() to the same byte position in the
file shall overwrite that file data.
Write requests to a pipe or FIFO shall be handled in the same way as a
regular file with the following exceptions:
* There is no file offset associated with a pipe, hence each write
request shall append to the end of the pipe.
* Write requests of {PIPE_BUF} bytes or less shall not 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 flag of the file status flags is set.
* If the O_NONBLOCK flag is clear, a write request may cause the
thread to block, but on normal completion it shall return nbyte.
* If the O_NONBLOCK flag is set, write() requests shall be handled
differently, in the following ways:
-- The write() function shall not block the thread.
-- A write request for {PIPE_BUF} or fewer bytes shall have the
following effect: if there is sufficient space available in the
pipe, write() shall transfer all the data and return the number
of bytes requested. Otherwise, write() shall transfer no data
and return 1 with errno set to [EAGAIN].
-- A write request for more than {PIPE_BUF} bytes shall cause one
of the following:
-- When at least one byte can be written, transfer what it can
and return the number of bytes written. When all data
previously written to the pipe is read, it shall transfer
at least {PIPE_BUF} bytes.
-- When no data can be written, transfer no data, and return
1 with errno set to [EAGAIN].
When attempting to write to a file descriptor (other than a pipe or
FIFO) that supports non-blocking writes and cannot accept the data
immediately:
* If the O_NONBLOCK flag is clear, write() shall block the calling
thread until the data can be accepted.
* If the O_NONBLOCK flag is set, write() shall not block the thread.
If some data can be written without blocking the thread, write()
shall write what it can and return the number of bytes written.
Otherwise, it shall return 1 and set errno to [EAGAIN].
Upon successful completion, where nbyte is greater than 0, write()
shall mark for update the last data modification and last file status
change timestamps 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 regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with fildes.
If fildes refers to a socket, write() shall be equivalent to send()
with no flags set.
If the O_DSYNC bit has been set, write I/O operations on the file
descriptor shall complete as defined by synchronized I/O data integrity
completion.
If the O_SYNC bit has been set, write I/O operations on the file
descriptor shall complete as defined by synchronized I/O file integrity
completion.
If fildes refers to a shared memory object, the result of the write()
function is unspecified.
If fildes refers to a typed memory object, the result of the write()
function is unspecified.
If fildes refers to a STREAM, the operation of write() shall be
determined by the values of the minimum and maximum nbyte range (packet
size) accepted by the STREAM. These values are determined by the
topmost STREAM module. If nbyte falls within the packet size range,
nbyte bytes shall be written. If nbyte does not fall within the range
and the minimum packet size value is 0, write() shall break the buffer
into maximum packet size segments prior to sending the data downstream
(the last segment may contain less than the maximum packet size). If
nbyte does not fall within the range and the minimum value is non-zero,
write() shall fail with errno set to [ERANGE]. Writing a zero-length
buffer (nbyte is 0) to a STREAMS device sends 0 bytes with 0 returned.
However, writing a zero-length buffer to a STREAMS-based pipe or FIFO
sends no message and 0 is returned. The process may issue I_SWROPT
ioctl() to enable zero-length messages to be sent across the pipe or
FIFO.
When writing to a STREAM, data messages are created with a priority
band of 0. When writing to a STREAM that is not a pipe or FIFO:
* If O_NONBLOCK is clear, and the STREAM cannot accept data (the
STREAM write queue is full due to internal flow control
conditions), write() shall block until data can be accepted.
* If O_NONBLOCK is set and the STREAM cannot accept data, write()
shall return 1 and set errno to [EAGAIN].
* If O_NONBLOCK is set and part of the buffer has been written while
a condition in which the STREAM cannot accept additional data
occurs, write() shall terminate and return the number of bytes
written.
In addition, write() shall fail if the STREAM head has processed an
asynchronous error before the call. In this case, the value of errno
does not reflect the result of write(), but reflects the prior error.
The pwrite() function shall be 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. An attempt to perform
a pwrite() on a file that is incapable of seeking shall result in an
error.
RETURN VALUE
Upon successful completion, these functions shall return the number of bytes actually written to the file associated with fildes. This number shall never be greater than nbyte. Otherwise, 1 shall be returned and errno set to indicate the error.
ERRORS
These functions shall fail if:
EAGAIN The file is neither a pipe, nor a FIFO, nor a socket, the
O_NONBLOCK flag is set for the file descriptor, and the thread
would be delayed in the write() operation.
EBADF The fildes argument is not a valid file descriptor open for
writing.
EFBIG An attempt was made to write a file that exceeds the
implementation-defined maximum file size or the file size limit
of the process, and there was no room for any bytes to be
written.
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 open file description associated with fildes.
EINTR The write operation was terminated due to the receipt of a
signal, and no data was transferred.
EIO The process is a member of a background process group attempting
to write to its controlling terminal, TOSTOP is set, the calling
thread is not blocking SIGTTOU, the process is not ignoring
SIGTTOU, and the process group of the process is orphaned. This
error may also be returned under implementation-defined
conditions.
ENOSPC There was no free space remaining on the device containing the
file.
ERANGE The transfer request size was outside the range supported by the
STREAMS file associated with fildes.
The pwrite() function shall fail if:
EINVAL The file is a regular file or block special file, and the offset
argument is negative. The file pointer shall remain unchanged.
ESPIPE The file is a pipe, FIFO, or socket.
The write() function shall fail if:
EAGAIN The file is a pipe or FIFO, the O_NONBLOCK flag is set for the
file descriptor, and the thread would be delayed in the write
operation.
EAGAIN or EWOULDBLOCK
The file is a socket, the O_NONBLOCK flag is set for the file
descriptor, and the thread would be delayed in the write
operation.
ECONNRESET
A write was attempted on a socket that is not connected.
EPIPE An attempt is made to write to a pipe or FIFO that is not open
for reading by any process, or that only has one end open. A
SIGPIPE signal shall also be sent to the thread.
EPIPE A write was attempted on a socket that is shut down for writing,
or is no longer connected. In the latter case, if the socket is
of type SOCK_STREAM, a SIGPIPE signal shall also be sent to the
thread.
These functions may fail if:
EINVAL The STREAM or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer.
EIO A physical I/O error has occurred.
ENOBUFS
Insufficient resources were available in the system to perform
the operation.
ENXIO A request was made of a nonexistent 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 write() function may fail if:
EACCES A write was attempted on a socket and the calling process does
not have appropriate privileges.
ENETDOWN
A write was attempted on a socket and the local network
interface used to reach the destination is down.
ENETUNREACH
A write was attempted on a socket and no route to the network is
present.
The following sections are informative.
EXAMPLES
Writing from a Buffer
The following example writes data from the buffer pointed to by buf to
the file associated with the file descriptor fd.
#include <sys/types.h>
#include <string.h>
...
char buf[20];
size_t nbytes;
ssize_t bytes_written;
int fd;
...
strcpy(buf, "This is a test\n");
nbytes = strlen(buf);
bytes_written = write(fd, buf, nbytes);
...
APPLICATION USAGE
None.
RATIONALE
See also the RATIONALE section in read().
An attempt to write to a pipe or FIFO has several major
characteristics:
* Atomic/non-atomic: A write is atomic if the whole amount written in
one operation is not interleaved with data from any other process.
This is useful when there are multiple writers sending data to a
single reader. Applications need to know how large a write request
can be expected to be performed atomically. This maximum is called
{PIPE_BUF}. This volume of POSIX.12008 does not say whether write
requests for more than {PIPE_BUF} bytes are atomic, but requires
that writes of {PIPE_BUF} or fewer bytes shall be atomic.
* Blocking/immediate: Blocking is only possible with O_NONBLOCK
clear. If there is enough space for all the data requested to be
written immediately, the implementation should do so. Otherwise,
the calling thread may block; that is, pause until enough space is
available for writing. The effective size of a pipe or FIFO (the
maximum amount that can be written in one operation without
blocking) may vary dynamically, depending on the implementation, so
it is not possible to specify a fixed value for it.
* Complete/partial/deferred: A write request:
int fildes;
size_t nbyte;
ssize_t ret;
char *buf;
ret = write(fildes, buf, nbyte);
may return:
Complete ret=nbyte
Partial ret<nbyte
This shall never happen if nbyte{PIPE_BUF}. If it does
happen (with nbyte>{PIPE_BUF}), this volume of
POSIX.12008 does not guarantee atomicity, even if
ret{PIPE_BUF}, because atomicity is guaranteed according
to the amount requested, not the amount written.
Deferred: ret=1, errno=[EAGAIN]
This error indicates that a later request may succeed. It
does not indicate that it shall succeed, even if
nbyte{PIPE_BUF}, because if no process reads from the
pipe or FIFO, the write never succeeds. An application
could usefully count the number of times [EAGAIN] is
caused by a particular value of nbyte>{PIPE_BUF} and
perhaps do later writes with a smaller value, on the
assumption that the effective size of the pipe may have
decreased.
Partial and deferred writes are only possible with O_NONBLOCK set.
The relations of these properties are shown in the following tables:
Write to a Pipe or FIFO with O_NONBLOCK clear
Immediately Writable: None Some nbyte
nbyte{PIPE_BUF} Atomic blocking Atomic blocking Atomic immediate
nbyte nbyte nbyte
nbyte>{PIPE_BUF} Blocking nbyte Blocking nbyte Blocking nbyte
If the O_NONBLOCK flag is clear, a write request shall block if the
amount writable immediately is less than that requested. If the flag is
set (by fcntl()), a write request shall never block.
Write to a Pipe or FIFO with O_NONBLOCK set
Immediately Writable: None Some nbyte
nbyte{PIPE_BUF} 1, [EAGAIN] 1, [EAGAIN] Atomic nbyte
nbyte>{PIPE_BUF} 1, [EAGAIN] <nbyte or 1, nbyte or 1,
[EAGAIN] [EAGAIN]
There is no exception regarding partial writes when O_NONBLOCK is set.
With the exception of writing to an empty pipe, this volume of
POSIX.12008 does not specify exactly when a partial write is performed
since that would require specifying internal details of the
implementation. Every application should be prepared to handle partial
writes when O_NONBLOCK is set and the requested amount is greater than
{PIPE_BUF}, just as every application should be prepared to handle
partial writes on other kinds of file descriptors.
The intent of forcing writing at least one byte if any can be written
is to assure that each write makes progress if there is any room in the
pipe. If the pipe is empty, {PIPE_BUF} bytes must be written; if not,
at least some progress must have been made.
Where this volume of POSIX.12008 requires 1 to be returned and errno
set to [EAGAIN], most historical implementations return zero (with the
O_NDELAY flag set, which is the historical predecessor of O_NONBLOCK,
but is not itself in this volume of POSIX.12008). The error
indications in this volume of POSIX.12008 were chosen so that an
application can distinguish these cases from end-of-file. While write()
cannot receive an indication of end-of-file, read() can, and the two
functions have similar return values. Also, some existing systems (for
example, Eighth Edition) permit a write of zero bytes to mean that the
reader should get an end-of-file indication; for those systems, a
return value of zero from write() indicates a successful write of an
end-of-file indication.
Implementations are allowed, but not required, to perform error
checking for write() requests of zero bytes.
The concept of a {PIPE_MAX} limit (indicating the maximum number of
bytes that can be written to a pipe in a single operation) was
considered, but rejected, because this concept would unnecessarily
limit application writing.
See also the discussion of O_NONBLOCK in read().
Writes can be serialized with respect to other reads and writes. If a
read() of file data can be proven (by any means) to occur after a
write() of the data, it must reflect that write(), even if the calls
are made by different processes. A similar requirement applies to
multiple write operations to the same file position. This is needed to
guarantee the propagation of data from write() calls to subsequent
read() calls. This requirement is particularly significant for
networked file systems, where some caching schemes violate these
semantics.
Note that this is specified in terms of read() and write(). The XSI
extensions readv() and writev() also obey these semantics. A new
``high-performance'' write analog that did not follow these
serialization requirements would also be permitted by this wording.
This volume of POSIX.12008 is also silent about any effects of
application-level caching (such as that done by stdio).
This volume of POSIX.12008 does not specify the value of the file
offset after an error is returned; there are too many cases. For
programming errors, such as [EBADF], the concept is meaningless since
no file is involved. For errors that are detected immediately, such as
[EAGAIN], clearly the pointer should not change. After an interrupt or
hardware error, however, an updated value would be very useful and is
the behavior of many implementations.
This volume of POSIX.12008 does not specify behavior of concurrent
writes to a file from multiple processes. Applications should use some
form of concurrency control.
This volume of POSIX.12008 intentionally does not specify any pwrite()
errors related to pipes, FIFOs, and sockets other than [ESPIPE].
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), creat(), dup(), fcntl(), getrlimit(), lseek(), open(), pipe(), read(), ulimit(), writev() The Base Definitions volume of POSIX.12008, <limits.h>, <stropts.h>, <sys_uio.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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