# write

## 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:
    
            *  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



           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:
    
           **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



       **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:
    
            *  _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



           None.
    


* * *