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WRITE(3P)               POSIX Programmer's Manual              WRITE(3P)

PROLOG         top

       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         top

       pwrite, write — write on a file

SYNOPSIS         top

       #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         top

       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.

RETURN VALUE         top

       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         top

       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.

EXAMPLES         top

   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         top

       None.

RATIONALE         top

       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].

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       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)

COPYRIGHT         top

       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)

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