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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 pointer shall remain
              unchanged.

       ESPIPE The file is a pipe, FIFO, or socket.

       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.1‐2008 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.1‐2008 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.1‐2008 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         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.1‐2008, 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, 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 .

IEEE/The Open Group                 2013                           WRITE(3P)