WRITE(3P) - phpMan

WRITE(3P)                  POSIX Programmer's Manual                 WRITE(3P)
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 detec-
       tion  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  inter-
       vening  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 imple-
       mentation-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 modi-
           fied.
        *  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  pre-
                   viously  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  deter-
       mined  by  the  values  of  the minimum and maximum nbyte range (packet
       size) accepted by the STREAM. These values are determined by  the  top-
       most  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  condi-
           tions), 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_NON-
              BLOCK  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 implementa-
              tion-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 sig-
              nal, 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  condi-
              tions.
       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 opera-
              tion.
       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 inter-
              face 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  characteris-
       tics:
        *  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 block-
           ing) 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  accord-
                     ing 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 per-
                     haps do later writes with a smaller value, on the assump-
                     tion 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 implementa-
       tion.  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 par-
       tial 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  indica-
       tions 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  check-
       ing 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  consid-
       ered,  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 mul-
       tiple 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  net-
       worked  file  systems,  where some caching schemes violate these seman-
       tics.
       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  serializa-
       tion  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 off-
       set after an error is returned; there are too many cases. For  program-
       ming  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 Electri-
       cal 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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            WRITE(3P)