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WRITE(3P) POSIX Programmer's Manual WRITE(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
pwrite, write — write on a file
#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);
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.
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.
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.
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);
...
None.
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.1‐2008 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.1‐2008 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.1‐2008 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.1‐2008 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.1‐2008). The
error indications in this volume of POSIX.1‐2008 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.1‐2008 is also silent about any effects of
application-level caching (such as that done by stdio).
This volume of POSIX.1‐2008 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.1‐2008 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.1‐2008 intentionally does not specify any
pwrite() errors related to pipes, FIFOs, and sockets other than
[ESPIPE].
None.
chmod(3p), creat(3p), dup(3p), fcntl(3p), getrlimit(3p), lseek(3p),
open(3p), pipe(3p), read(3p), ulimit(3p), writev(3p)
The Base Definitions volume of POSIX.1‐2008, limits.h(0p),
stropts.h(0p), sys_uio.h(0p), unistd.h(0p)
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 .
IEEE/The Open Group 2013 WRITE(3P)
Pages that refer to this page: aio.h(0p), stropts.h(0p), sys_uio.h(0p), unistd.h(0p), pax(1p), aio_fsync(3p), aio_write(3p), fchmod(3p), fdatasync(3p), fseek(3p), fsetpos(3p), fstatvfs(3p), fwrite(3p), getmsg(3p), ioctl(3p), lockf(3p), open(3p), pipe(3p), poll(3p), pselect(3p), putmsg(3p), pwrite(3p), recv(3p), recvfrom(3p), send(3p), shutdown(3p), ulimit(3p), writev(3p)