Manpages

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.

NAME

pwrite, write — write on a file

SYNOPSIS

#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);

DESCRIPTION

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:

Write requests to a pipe or FIFO shall be handled in the same way as a regular file with the following exceptions:

--

--

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:

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:

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.

RETURN VALUE

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.

ERRORS

These functions shall fail if:

The pwrite() function shall fail if:

The write() function shall fail if:

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.

These functions may fail if:

ENOBUFS

Insufficient resources were available in the system to perform the operation.

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:

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.

EXAMPLES

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);
...

APPLICATION USAGE

None.

RATIONALE

See also the RATIONALE section in read().

An attempt to write to a pipe or FIFO has several major characteristics:

int fildes;
size_t nbyte;
ssize_t ret;
char *buf;

ret = write(fildes, buf, nbyte);

may return:

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.

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:

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.

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].

FUTURE DIRECTIONS

None.

SEE ALSO

chmod(), creat(), dup(), fcntl(), getrlimit(), lseek(), open(), pipe(), read(), ulimit(), writev()

The Base Definitions volume of POSIX.1-2008, <limits.h>, <stropts.h>, <sys_uio.h>, <unistd.h>

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 .