path: root/usr/src/man/man2/write.2

'\" te
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.TH WRITE 2 "Jan 15, 2015"
.SH NAME
write, pwrite, writev, pwritev\- write on a file
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>

\fBssize_t\fR \fBwrite\fR(\fBint\fR \fIfildes\fR, \fBconst void *\fR\fIbuf\fR, \fBsize_t\fR \fInbyte\fR);
.fi

.LP
.nf
\fBssize_t\fR \fBpwrite\fR(\fBint\fR \fIfildes\fR, \fBconst void *\fR\fIbuf\fR, \fBsize_t\fR \fInbyte\fR,
     \fBoff_t\fR \fIoffset\fR);
.fi

.LP
.nf
#include <sys/uio.h>

\fBssize_t\fR \fBwritev\fR(\fBint\fR \fIfildes\fR, \fBconst struct iovec *\fR\fIiov\fR, \fBint\fR \fIiovcnt\fR);
.fi
.LP
.nf
\fBssize_t\fR \fBpwritev\fR(\fBint\fR \fIfildes\fR, \fBconst struct iovec *\fR\fIiov\fR, \fBint\fR \fIiovcnt\fR, \fBoff_t\fR \fIoffset\fR);
.fi

.SH DESCRIPTION
.LP
The \fBwrite()\fR function attempts to write \fInbyte\fR bytes from the buffer
pointed to by \fIbuf\fR to the file associated with the open file descriptor,
\fIfildes\fR.
.sp
.LP
If \fInbyte\fR is 0, \fBwrite()\fR will return 0 and have no other results if
the file is a regular file; otherwise, the results are unspecified.
.sp
.LP
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 \fIfildes\fR. Before successful return from \fBwrite()\fR, 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.
.sp
.LP
If the \fBO_SYNC\fR bit has been set, write I/O operations on the file
descriptor complete as defined by synchronized I/O file integrity completion.
.sp
.LP
If \fIfildes\fR refers to a socket, \fBwrite()\fR is equivalent to
\fBsend\fR(3SOCKET) with no flags set.
.sp
.LP
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.
.sp
.LP
If the \fBO_APPEND\fR 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.
.sp
.LP
For regular files, no data transfer will occur past the offset maximum
established in the open file description with \fIfildes\fR.
.sp
.LP
A \fBwrite()\fR to a regular file is blocked if mandatory file/record locking
is set (see \fBchmod\fR(2)), and there is a record lock owned by another
process  on the segment of the file to be written:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR or \fBO_NONBLOCK\fR is set, \fBwrite()\fR returns \fB\(mi1\fR
and sets \fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are clear, \fBwrite()\fR sleeps until
all blocking locks are removed or the \fBwrite()\fR is terminated by a signal.
.RE
.sp
.LP
If a  \fBwrite()\fR requests that more bytes be written than there is room
for\(emfor example, if the write would exceed the process file size limit (see
\fBgetrlimit\fR(2) and \fBulimit\fR(2)), the system file size limit, or the
free space on the device\(emonly 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 \fBwrite()\fR of 512-bytes returns 20. The next
\fBwrite()\fR of a non-zero number of bytes gives a failure return (except as
noted for pipes and FIFO below).
.sp
.LP
If \fBwrite()\fR is interrupted by a signal before it writes any data, it will
return \(mi1 with \fBerrno\fR set to \fBEINTR\fR.
.sp
.LP
If \fBwrite()\fR is interrupted by a signal after it successfully writes some
data, it will return the number of bytes written.
.sp
.LP
If \fBwrite()\fR exceeds the process file size limit, the application generates
a \fBSIGXFSZ\fR signal, whose default behavior is to dump core.
.sp
.LP
After a \fBwrite()\fR to a regular file has successfully returned:
.RS +4
.TP
.ie t \(bu
.el o
Any successful \fBread\fR(2) from each byte position in the file that was
modified by that write will return the data specified by the \fBwrite()\fR for
that position until such byte positions are again modified.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Any subsequent successful \fBwrite()\fR to the same byte position in the file
will overwrite that file data.
.RE
.sp
.LP
Write requests to a pipe or FIFO are handled the same as a regular file with
the following exceptions:
.RS +4
.TP
.ie t \(bu
.el o
There is no file offset associated with a pipe, hence each write request
appends to the end of the pipe.
.RE
.RS +4
.TP
.ie t \(bu
.el o
Write requests of \fB{PIPE_BUF}\fR bytes or less are guaranteed not to be
interleaved with data from other processes doing writes on the same pipe.
Writes of greater than \fB{PIPE_BUF}\fR bytes may have data interleaved, on
arbitrary boundaries, with writes by other processes, whether or not the
\fBO_NONBLOCK\fR or \fBO_NDELAY\fR flags are set.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NONBLOCK\fR and \fBO_NDELAY\fR are clear, a write request may cause the
process to block, but on normal completion it returns \fInbyte\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NONBLOCK\fR and \fBO_NDELAY\fR are set, \fBwrite()\fR does not block
the process. If a \fBwrite()\fR request for \fBPIPE_BUF\fR or fewer bytes
succeeds completely  \fBwrite()\fR returns \fInbyte\fR. Otherwise, if
\fBO_NONBLOCK\fR is set, it returns \fB\(mi1\fR and sets \fBerrno\fR to
\fBEAGAIN\fR or if \fBO_NDELAY\fR is set, it returns \fB0\fR.  A \fBwrite()\fR
request for greater than \fB{PIPE_BUF}\fR bytes  transfers what it can and
returns the number of bytes written or it transfers no data and, if
\fBO_NONBLOCK\fR is set, returns \fB\(mi1\fR with \fBerrno\fR set to
\fBEAGAIN\fR or if \fBO_NDELAY\fR is set, it returns \fB0\fR.  Finally, if a
request is greater than \fBPIPE_BUF\fR bytes and all data previously written to
the pipe has been read, \fBwrite()\fR transfers at least \fBPIPE_BUF\fR bytes.
.RE
.sp
.LP
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:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NONBLOCK\fR and \fBO_NDELAY\fR are clear, \fBwrite()\fR blocks until
the data can be accepted.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NONBLOCK\fR or \fBO_NDELAY\fR is set, \fBwrite()\fR does not block the
process. If some data can be written without blocking the process,
\fBwrite()\fR writes what it can and returns the number of bytes written.
Otherwise, if \fBO_NONBLOCK\fR is set, it returns \fB\(mi1\fR  and sets
\fBerrno\fR to \fBEAGAIN\fR or if \fBO_NDELAY\fR is set,  it returns \fB0\fR.
.RE
.sp
.LP
Upon successful completion, where \fInbyte\fR is greater than 0, \fBwrite()\fR
will mark for update the \fBst_ctime\fR and \fBst_mtime\fR fields of the file,
and if the file is a regular file, the \fBS_ISUID\fR and \fBS_ISGID\fR bits of
the file mode may be cleared.
.sp
.LP
For streams files (see \fBIntro\fR(2) and \fBstreamio\fR(7I)), the operation of
\fBwrite()\fR is determined by the values of the minimum and maximum
\fInbyte\fR 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 \fInbyte\fR falls within the packet size range, \fInbyte\fR bytes are
written. If \fInbyte\fR does not fall within the range and the minimum packet
size value  is zero, \fBwrite()\fR 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 \fInbyte\fR does not fall within the range
and the minimum value is non-zero, \fBwrite()\fR fails and sets \fBerrno\fR to
\fBERANGE\fR. Writing a zero-length buffer (\fInbyte\fR 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 \fBI_SWROPT\fR \fBioctl\fR(2) to enable
zero-length messages to be sent across the pipe or FIFO (see
\fBstreamio\fR(7I)).
.sp
.LP
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:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are not set, and the stream cannot
accept data (the stream write queue is full due to internal flow control
conditions), \fBwrite()\fR blocks until data can be accepted.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR or \fBO_NONBLOCK\fR is set and the stream cannot accept data,
\fBwrite()\fR returns \fB-1\fR and sets \fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR or \fBO_NONBLOCK\fR is set and part of the buffer has already
been written when a condition occurs in which the stream cannot accept
additional data, \fBwrite()\fR terminates and returns the number of bytes
written.
.RE
.sp
.LP
The \fBwrite()\fR and \fBwritev()\fR functions will fail if the stream head had
processed an asynchronous error before the call.  In this case, the value of
\fBerrno\fR does not reflect the result of \fBwrite()\fR or \fBwritev()\fR but
reflects the prior error.
.SS "\fBpwrite()\fR"
.LP
The \fBpwrite()\fR function is equivalent to \fBwrite()\fR, except that it
writes into a given position and does not change the file offset (regardless of
whether \fBO_APPEND\fR is set). The first three arguments to \fBpwrite()\fR are
the same as \fBwrite()\fR, with the addition of a fourth argument \fIoffset\fR
for the desired position inside the file.
.SS "\fBwritev()\fR"
.LP
The \fBwritev()\fR function performs the same action as \fBwrite()\fR, but
gathers the output data from the \fIiovcnt\fR buffers specified by the members
of the \fIiov\fR array: \fIiov\fR[0], \fIiov\fR[1], \&.\|.\|.,
\fIiov\fR[\fIiovcnt\fR\|\(mi\|1]. The \fIiovcnt\fR buffer is valid if greater
than 0 and less than or equal to \fB{IOV_MAX}\fR. See \fBIntro\fR(2) for a
definition of \fB{IOV_MAX}\fR.
.sp
.SS "\fBpwritev()\fR"
.LP
The \fBpwritev()\fR function is equivalent to \fBwritev()\fR, except that it
writes into a given position and does not change the file offset (regardless of
whether \fBO_APPEND\fR is set). The first three arguments to \fBpwritev()\fR are
the same as \fBwritev()\fR, with the addition of a fourth argument \fIoffset\fR
for the desired position inside the file.
.LP
The \fBiovec\fR structure contains the following members:
.sp
.in +2
.nf
caddr_t  iov_base;
int      iov_len;
.fi
.in -2

.sp
.LP
Each \fBiovec\fR entry specifies the base address and length of an area in
memory from which data should be written.  The \fBwritev()\fR function always
writes all data from an area before proceeding to the next.
.sp
.LP
If \fIfildes\fR refers to a regular file and all of the \fBiov_len\fR members
in the array pointed to by \fIiov\fR are 0, \fBwritev()\fR will return 0 and
have no other effect.  For other file types, the behavior is unspecified.
.sp
.LP
If the sum of the \fBiov_len\fR values is greater than \fBSSIZE_MAX\fR, the
operation fails and no data is transferred.
.SH RETURN VALUES
.LP
Upon successful completion, \fBwrite()\fR returns the number of bytes actually
written to the file associated with \fIfildes\fR. This number is never greater
than \fInbyte\fR. Otherwise, \fB\(mi1\fR is returned, the file-pointer remains
unchanged, and \fBerrno\fR is set to indicate the error.
.sp
.LP
Upon successful completion, \fBwritev()\fR returns the number of bytes actually
written.  Otherwise, it returns  \fB\(mi1\fR, the file-pointer remains
unchanged, and \fBerrno\fR is set to indicate an error.
.SH ERRORS
.LP
The \fBwrite()\fR, \fBpwrite()\fR, \fBwritev()\fR, and \fBpwritev()\fR
functions will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 11n
Mandatory file/record locking is set, \fBO_NDELAY\fR or \fBO_NONBLOCK\fR 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 \fBO_NDELAY\fR or \fBO_NONBLOCK\fR
flag set; or a write to a pipe or FIFO of \fBPIPE_BUF\fR bytes or less is
requested and less than \fInbytes\fR of free space is available.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument is not a valid file descriptor open for writing.
.RE

.sp
.ne 2
.na
\fB\fBECONNRESET\fR\fR
.ad
.RS 11n
The \fIfiledes\fR 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 \fIfiledes\fR.
.RE

.sp
.ne 2
.na
\fB\fBEDEADLK\fR\fR
.ad
.RS 11n
The write was going to go to sleep  and cause a deadlock situation to occur.
.RE

.sp
.ne 2
.na
\fB\fBEDQUOT\fR\fR
.ad
.RS 11n
The user's quota of disk blocks on the file system containing the file has been
exhausted.
.RE

.sp
.ne 2
.na
\fB\fBEFBIG\fR\fR
.ad
.RS 11n
An attempt is made to write a file that exceeds the process's file size limit
or the maximum file size (see \fBgetrlimit\fR(2) and \fBulimit\fR(2)).
.RE

.sp
.ne 2
.na
\fB\fBEFBIG\fR\fR
.ad
.RS 11n
The file is a regular file, \fInbyte\fR is greater than 0, and the starting
position is greater than or equal to the offset maximum established in the file
description associated with \fIfildes\fR.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 11n
A signal was caught during the write  operation and no data was transferred.
.RE

.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 11n
The process is in the background and is attempting to write to its controlling
terminal whose \fBTOSTOP\fR flag is set, or the process is  neither ignoring
nor blocking \fBSIGTTOU\fR signals and the process  group of the process is
orphaned.
.RE

.sp
.ne 2
.na
\fB\fBENOLCK\fR\fR
.ad
.RS 11n
Enforced record locking was enabled and \fB{LOCK_MAX}\fR 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.
.RE

.sp
.ne 2
.na
\fB\fBENOLINK\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument is on a remote machine and the link to that machine
is no longer active.
.RE

.sp
.ne 2
.na
\fB\fBENOSPC\fR\fR
.ad
.RS 11n
During a write to an ordinary file, there is no  free space left on the device.
.RE

.sp
.ne 2
.na
\fB\fBENOSR\fR\fR
.ad
.RS 11n
An attempt is made to write to a streams with insufficient streams memory
resources  available in the system.
.RE

.sp
.ne 2
.na
\fB\fBENXIO\fR\fR
.ad
.RS 11n
A hangup occurred on the stream being written to.
.RE

.sp
.ne 2
.na
\fB\fBEPIPE\fR\fR
.ad
.RS 11n
An attempt is made to write to a pipe or a \fBFIFO\fR that is not open for
reading by any process, or that has only one end open (or to a file descriptor
created by \fBsocket\fR(3SOCKET), using type \fBSOCK_STREAM\fR that is no
longer connected to a peer endpoint). A \fBSIGPIPE\fR signal will also be sent
to the thread. The process dies unless special provisions were taken to catch
or ignore the signal.
.RE

.sp
.ne 2
.na
\fB\fBERANGE\fR\fR
.ad
.RS 11n
The transfer request size was outside the range supported by the streams file
associated with \fIfildes\fR.
.RE

.sp
.LP
The \fBwrite()\fR and \fBpwrite()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
The \fIbuf\fR argument points to an illegal address.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fInbyte\fR argument overflowed an \fBssize_t\fR.
.RE

.sp
.LP
The \fBpwrite()\fR and \fBpwritev()\fR functions fail and the file pointer
remains unchanged if:
.sp
.ne 2
.na
\fB\fBESPIPE\fR\fR
.ad
.RS 10n
The \fIfildes\fR argument is associated with a pipe or FIFO.
.RE

.sp
.LP
The \fBwrite()\fR and \fBwritev()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The stream or multiplexer referenced by \fIfildes\fR is linked (directly or
indirectly) downstream from a multiplexer.
.RE

.sp
.ne 2
.na
\fB\fBENXIO\fR\fR
.ad
.RS 10n
A request was made of a non-existent device, or the request was outside the
capabilities of the device.
.RE

.sp
.ne 2
.na
\fB\fBENXIO\fR\fR
.ad
.RS 10n
A hangup occurred on the stream being written to.
.RE

.sp
.LP
A write to a streams file may fail if an error message has been received at the
stream head.  In this case, \fBerrno\fR is set to the value included in the
error message.
.sp
.LP
The \fBwritev()\fR and \fBpwritev()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIiovcnt\fR argument was less than or equal to 0 or greater than
{\fBIOV_MAX\fR}; one of the \fBiov_len\fR values in the \fIiov\fR array was
negative; or the sum of the \fBiov_len\fR values in the \fIiov\fR array
overflowed an \fBssize_t\fR.
.RE

.SH USAGE
.LP
The \fBpwrite()\fR function has a transitional interface for 64-bit file
offsets.  See \fBlf64\fR(5).
.SH ATTRIBUTES
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	\fBwrite()\fR is Async-Signal-Safe
_
Standard	See \fBstandards\fR(5).
.TE

.SH SEE ALSO
.LP
\fBIntro\fR(2), \fBchmod\fR(2), \fBcreat\fR(2), \fBdup\fR(2), \fBfcntl\fR(2),
\fBgetrlimit\fR(2), \fBioctl\fR(2), \fBlseek\fR(2), \fBopen\fR(2),
\fBpipe\fR(2), \fBulimit\fR(2), \fBsend\fR(3SOCKET), \fBsocket\fR(3SOCKET),
\fBattributes\fR(5), \fBlf64\fR(5), \fBstandards\fR(5), \fBstreamio\fR(7I)