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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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 offset shall remain
unchanged.
ESPIPE The file is incapable of seeking.
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.1‐2017 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.1‐2017 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.1‐2017 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.1‐2017 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.1‐2017).
The error indications in this volume of POSIX.1‐2017 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.1‐2017 is also silent about any
effects of application-level caching (such as that done by stdio).
This volume of POSIX.1‐2017 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.1‐2017 does not specify the behavior of
concurrent writes to a regular file from multiple threads, except
that each write is atomic (see Section 2.9.7, Thread Interactions
with Regular File Operations). Applications should use some form
of concurrency control.
This volume of POSIX.1‐2017 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.1‐2017, 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 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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 2017 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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