Originální popis anglicky:
pwrite, write - write on a file
Návod, kniha: POSIX Programmer's Manual
#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 this incremented file offset is greater than the length
of the file, the length of the file shall be set to this file offset.
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 process' file size limit 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
st_ctime and
st_mtime fields 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 without changing the file pointer. 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.
Upon successful completion,
write() and
pwrite() 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.
The
write() and
pwrite() functions shall fail if:
- EAGAIN
- 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 process' file size
limit, 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
process is neither ignoring nor blocking 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.
- 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.
- ERANGE
- The transfer request size was outside the range supported
by the STREAMS file associated with fildes.
The
write() function shall fail if:
- EAGAIN or EWOULDBLOCK
-
The file descriptor is for a socket, is marked O_NONBLOCK, and write would
block.
- ECONNRESET
- A write was attempted on a socket that is not
connected.
- 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, the SIGPIPE signal is generated to the calling
process.
The
write() and
pwrite() 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
pwrite() function shall fail and the file pointer remain unchanged
if:
- EINVAL
- The offset argument is invalid. The value is
negative.
- ESPIPE
- fildes is associated with a pipe or FIFO.
The following sections are informative.
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 IEEE Std 1003.1-2001 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
process 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
IEEE Std 1003.1-2001 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
IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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
IEEE Std 1003.1-2001). The error indications in this volume of
IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 is also silent about any effects
of application-level caching (such as that done by
stdio).
This volume of IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 does not specify behavior of
concurrent writes to a file from multiple processes. Applications should use
some form of concurrency control.
None.
chmod() ,
creat() ,
dup() ,
fcntl() ,
getrlimit() ,
lseek() ,
open() ,
pipe() ,
ulimit() ,
writev() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<limits.h>,
<stropts.h>,
<sys/uio.h>,
<unistd.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
