'\" et
.TH WRITE "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
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.

.SH NAME
pwrite,
write
\(em write on a file
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
ssize_t pwrite(int \fIfildes\fP, const void *\fIbuf\fP, size_t \fInbyte\fP,
    off_t \fIoffset\fP);
ssize_t write(int \fIfildes\fP, const void *\fIbuf\fP, size_t \fInbyte\fP);
.fi
.SH DESCRIPTION
The
\fIwrite\fR()
function shall attempt to write
.IR nbyte
bytes from the buffer pointed to by
.IR buf
to the file associated with the open file descriptor,
.IR fildes .
.P
Before any action described below is taken, and if
.IR nbyte
is zero and the file is a regular file, the
\fIwrite\fR()
function may detect and return errors as described below. In the
absence of errors, or if error detection is not performed, the
\fIwrite\fR()
function shall return zero and have no other results. If
.IR nbyte
is zero and the file is not a regular file, the results are
unspecified.
.P
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
.IR fildes .
Before successful return from
\fIwrite\fR(),
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.
.P
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.
.P
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.
.P
If a
\fIwrite\fR()
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).
.P
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.
.P
If
\fIwrite\fR()
is interrupted by a signal before it writes any data, it shall
return \(mi1 with
.IR errno
set to
.BR [EINTR] .
.P
If
\fIwrite\fR()
is interrupted by a signal after it successfully writes some data, it
shall return the number of bytes written.
.P
If the value of
.IR nbyte
is greater than
{SSIZE_MAX},
the result is implementation-defined.
.P
After a
\fIwrite\fR()
to a regular file has successfully returned:
.IP " *" 4
Any successful
\fIread\fR()
from each byte position in the file that was modified by that write
shall return the data specified by the
\fIwrite\fR()
for that position until such byte positions are again modified.
.IP " *" 4
Any subsequent successful
\fIwrite\fR()
to the same byte position in the file shall overwrite that file data.
.br
.P
Write requests to a pipe or FIFO shall be handled in the same way
as a regular file with the following exceptions:
.IP " *" 4
There is no file offset associated with a pipe, hence each write
request shall append to the end of the pipe.
.IP " *" 4
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.
.IP " *" 4
If the O_NONBLOCK flag is clear, a write request may cause the thread
to block, but on normal completion it shall return
.IR nbyte .
.IP " *" 4
If the O_NONBLOCK flag is set,
\fIwrite\fR()
requests shall be handled differently, in the following ways:
.RS 4 
.IP -- 4
The
\fIwrite\fR()
function shall not block the thread.
.IP -- 4
A write request for
{PIPE_BUF}
or fewer bytes shall have the following effect: if there is sufficient
space available in the pipe,
\fIwrite\fR()
shall transfer all the data and return the number of bytes requested.
Otherwise,
\fIwrite\fR()
shall transfer no data and return \(mi1 with
.IR errno
set to
.BR [EAGAIN] .
.IP -- 4
A write request for more than
{PIPE_BUF}
bytes shall cause one of the following:
.RS 4 
.IP -- 4
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.
.IP -- 4
When no data can be written, transfer no data, and return \(mi1 with
.IR errno
set to
.BR [EAGAIN] .
.RE
.RE
.P
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:
.IP " *" 4
If the O_NONBLOCK flag is clear,
\fIwrite\fR()
shall block the calling thread until the data can be accepted.
.IP " *" 4
If the O_NONBLOCK flag is set,
\fIwrite\fR()
shall not block the thread. If some data can be written without
blocking the thread,
\fIwrite\fR()
shall write what it can and return the number of bytes written.
Otherwise, it shall return \(mi1 and set
.IR errno
to
.BR [EAGAIN] .
.P
Upon successful completion, where
.IR nbyte
is greater than 0,
\fIwrite\fR()
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.
.P
For regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with
.IR fildes .
.P
If
.IR fildes
refers to a socket,
\fIwrite\fR()
shall be equivalent to
\fIsend\fR()
with no flags set.
.P
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.
.P
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.
.P
If
.IR fildes
refers to a shared memory object, the result of the
\fIwrite\fR()
function is unspecified.
.P
If
.IR fildes
refers to a typed memory object, the result of the
\fIwrite\fR()
function is unspecified.
.P
If
.IR fildes
refers to a STREAM, the operation of
\fIwrite\fR()
shall be determined by the values of the minimum and maximum
.IR nbyte
range (packet size) accepted by the STREAM. These values are determined
by the topmost STREAM module. If
.IR nbyte
falls within the packet size range,
.IR nbyte
bytes shall be written. If
.IR nbyte
does not fall within the range and the minimum packet size value is 0,
\fIwrite\fR()
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
.IR nbyte
does not fall within the range and the minimum value is non-zero,
\fIwrite\fR()
shall fail with
.IR errno
set to
.BR [ERANGE] .
Writing a zero-length buffer (\c
.IR 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
\fIioctl\fR()
to enable zero-length messages to be sent across the pipe or FIFO.
.P
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:
.IP " *" 4
If O_NONBLOCK is clear, and the STREAM cannot accept data (the STREAM
write queue is full due to internal flow control conditions),
\fIwrite\fR()
shall block until data can be accepted.
.IP " *" 4
If O_NONBLOCK is set and the STREAM cannot accept data,
\fIwrite\fR()
shall return \(mi1 and set
.IR errno
to
.BR [EAGAIN] .
.IP " *" 4
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,
\fIwrite\fR()
shall terminate and return the number of bytes written.
.P
In addition,
\fIwrite\fR()
shall fail if the STREAM head has processed an asynchronous error
before the call. In this case, the value of
.IR errno
does not reflect the result of
\fIwrite\fR(),
but reflects the prior error.
.P
The
\fIpwrite\fR()
function shall be equivalent to
\fIwrite\fR(),
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
\fIpwrite\fR()
are the same as
\fIwrite\fR()
with the addition of a fourth argument
.IR offset
for the desired position inside the file. An attempt to perform a
\fIpwrite\fR()
on a file that is incapable of seeking shall result in an error.
.SH "RETURN VALUE"
Upon successful completion, these functions shall return the number of
bytes actually written to the file associated with
.IR fildes .
This number shall never be greater than
.IR nbyte .
Otherwise, \(mi1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR 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
\fIwrite\fR()
operation.
.TP
.BR EBADF
The
.IR fildes
argument is not a valid file descriptor open for writing.
.TP
.BR 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.
.TP
.BR EFBIG
The file is a regular file,
.IR 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
.IR fildes .
.TP
.BR EINTR
The write operation was terminated due to the receipt of a signal, and
no data was transferred.
.TP
.BR 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.
.TP
.BR ENOSPC
There was no free space remaining on the device containing the file.
.TP
.BR ERANGE
The transfer request size was outside the range supported by the
STREAMS file associated with
.IR fildes .
.P
The
\fIpwrite\fR()
function shall fail if:
.TP
.BR EINVAL
The file is a regular file or block special file, and the
.IR offset
argument is negative. The file pointer shall remain unchanged.
.TP
.BR ESPIPE
The file is a pipe, FIFO, or socket.
.P
The
\fIwrite\fR()
function shall fail if:
.TP
.BR 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.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
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.
.TP
.BR ECONNRESET
A write was attempted on a socket that is not connected.
.TP
.BR 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.
.TP
.BR 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.
.P
These functions may fail if:
.TP
.BR EINVAL
The STREAM or multiplexer referenced by
.IR fildes
is linked (directly or indirectly) downstream from a multiplexer.
.TP
.BR EIO
A physical I/O error has occurred.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP
.BR ENXIO
A request was made of a nonexistent device, or the request was outside
the capabilities of the device.
.TP
.BR ENXIO
A hangup occurred on the STREAM being written to.
.P
A write to a STREAMS file may fail if an error message has been
received at the STREAM head. In this case,
.IR errno
is set to the value included in the error message.
.br
.P
The
\fIwrite\fR()
function may fail if:
.TP
.BR EACCES
A write was attempted on a socket and the calling
process does not have appropriate privileges.
.TP
.BR ENETDOWN
A write was attempted on a socket and the local network interface used
to reach the destination is down.
.TP
.BR ENETUNREACH
.br
A write was attempted on a socket and no route to the network is
present.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Writing from a Buffer"
.P
The following example writes data from the buffer pointed to by
.IR buf
to the file associated with the file descriptor
.IR fd .
.sp
.RS 4
.nf
\fB
#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\en");
nbytes = strlen(buf);
.P
bytes_written = write(fd, buf, nbytes);
\&...
.fi \fR
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
See also the RATIONALE section in
\fIread\fR().
.P
An attempt to write to a pipe or FIFO has several major
characteristics:
.IP " *" 4
\fIAtomic/non-atomic\fP: 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\(hy2008 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.
.IP " *" 4
\fIBlocking/immediate\fP: 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.
.IP " *" 4
\fIComplete/partial/deferred\fP: A write request:
.RS 4 
.sp
.RS 4
.nf
\fB
int fildes;
size_t nbyte;
ssize_t ret;
char *buf;
.P
ret = write(fildes, buf, nbyte);
.fi \fR
.P
.RE
.P
may return:
.IP Complete 10
\fIret\fP=\fInbyte\fP
.IP Partial 10
\fIret\fP<\fInbyte\fP
.RS 10 
.P
This shall never happen if
.IR nbyte \(<=\c
{PIPE_BUF}.
If it does happen (with
.IR nbyte >\c
{PIPE_BUF}),
\&this volume of POSIX.1\(hy2008 does not guarantee atomicity, even if
.IR ret \(<=\c
{PIPE_BUF},
because atomicity is guaranteed according to the amount
.IR requested ,
not the amount
.IR written .
.RE
.IP Deferred: 10
\fIret\fP=\(mi1, \fIerrno\fP=[EAGAIN]
.RS 10 
.P
This error indicates that a later request may succeed. It does not
indicate that it
.IR shall
succeed, even if
.IR nbyte \(<=\c
{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
.BR [EAGAIN] 
is caused by a particular value of
.IR nbyte >\c
{PIPE_BUF}
and perhaps do later writes with a smaller value, on the assumption
that the effective size of the pipe may have decreased.
.RE
.P
Partial and deferred writes are only possible with O_NONBLOCK set.
.RE
.P
The relations of these properties are shown in the following tables:
.TS
center box tab(!);
cB s s s
cB | cB cB c
l1 | lw(1.25i)1 lw(1.25i)1 lw(1.25i).
Write to a Pipe or FIFO with O_NONBLOCK \fIclear\fP
_
Immediately Writable:!None!Some!\fInbyte\fP
_
\fInbyte\fP\(<={PIPE_BUF}!Atomic blocking!Atomic blocking!Atomic immediate
!\fInbyte\fP!\fInbyte\fP!\fInbyte\fP
_
\fInbyte\fP>{PIPE_BUF}!Blocking \fInbyte\fP!Blocking \fInbyte\fP!Blocking \fInbyte\fP
.TE
.P
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
\fIfcntl\fR()),
a write request shall never block.
.TS
center box tab(!);
cB s s s
cB | cB cB c
l1 | lw(1.25i)1 lw(1.25i)1 lw(1.25i).
Write to a Pipe or FIFO with O_NONBLOCK \fIset\fP
_
Immediately Writable:!None!Some!\fInbyte\fP
_
\fInbyte\fP\(<={PIPE_BUF}!\(mi1, [EAGAIN]!\(mi1, [EAGAIN]!Atomic \fInbyte\fP
_
\fInbyte\fP>{PIPE_BUF}!\(mi1, [EAGAIN]!<\fInbyte\fP or \(mi1,!\(<=\fInbyte\fP or \(mi1,
!![EAGAIN]![EAGAIN]
.TE
.P
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\(hy2008 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.
.P
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.
.P
Where this volume of POSIX.1\(hy2008 requires \(mi1 to be returned and
.IR errno
set to
.BR [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\(hy2008). The error indications in this volume of POSIX.1\(hy2008 were chosen so that an
application can distinguish these cases from end-of-file. While
\fIwrite\fR()
cannot receive an indication of end-of-file,
\fIread\fR()
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
\fIwrite\fR()
indicates a successful write of an end-of-file indication.
.P
Implementations are allowed, but not required, to perform error
checking for
\fIwrite\fR()
requests of zero bytes.
.P
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.
.P
See also the discussion of O_NONBLOCK in
\fIread\fR().
.P
Writes can be serialized with respect to other reads and writes. If a
\fIread\fR()
of file data can be proven (by any means) to occur after a
\fIwrite\fR()
of the data, it must reflect that
\fIwrite\fR(),
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
\fIwrite\fR()
calls to subsequent
\fIread\fR()
calls. This requirement is particularly significant for networked file
systems, where some caching schemes violate these semantics.
.P
Note that this is specified in terms of
\fIread\fR()
and
\fIwrite\fR().
The XSI extensions
\fIreadv\fR()
and
\fIwritev\fR()
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\(hy2008 is also silent about any effects of
application-level caching (such as that done by
.IR stdio ).
.P
This volume of POSIX.1\(hy2008 does not specify the value of the file offset after an error is
returned; there are too many cases. For programming errors, such as
.BR [EBADF] ,
the concept is meaningless since no file is involved. For errors that
are detected immediately, such as
.BR [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.
.P
This volume of POSIX.1\(hy2008 does not specify behavior of concurrent writes to a file from
multiple processes. Applications should use some form of concurrency
control.
.P
This volume of POSIX.1\(hy2008 intentionally does not specify any
\fIpwrite\fR()
errors related to pipes, FIFOs, and sockets other than
.BR [ESPIPE] .
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIchmod\fR\^(\|)",
.IR "\fIcreat\fR\^(\|)",
.IR "\fIdup\fR\^(\|)",
.IR "\fIfcntl\fR\^(\|)",
.IR "\fIgetrlimit\fR\^(\|)",
.IR "\fIlseek\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIpipe\fR\^(\|)",
.IR "\fIread\fR\^(\|)",
.IR "\fIulimit\fR\^(\|)",
.IR "\fIwritev\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "\fB<limits.h>\fP",
.IR "\fB<stropts.h>\fP",
.IR "\fB<sys_uio.h>\fP",
.IR "\fB<unistd.h>\fP"
.SH 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 .