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man:write

WRITE(2) Linux Programmer's Manual WRITE(2)

NAME

     write - write to a file descriptor

SYNOPSIS

     #include <unistd.h>
     ssize_t write(int fd, const void *buf, size_t count);

DESCRIPTION

     write() writes up to count bytes from the buffer starting at buf to the
     file referred to by the file descriptor fd.
     The number of bytes written may be less than  count  if,  for  example,
     there  is  insufficient space on the underlying physical medium, or the
     RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)),  or  the
     call was interrupted by a signal handler after having written less than
     count bytes.  (See also pipe(7).)
     For a seekable file (i.e., one to which lseek(2) may  be  applied,  for
     example,  a  regular  file) writing takes place at the file offset, and
     the file offset is incremented by the number of bytes actually written.
     If  the  file was open(2)ed with O_APPEND, the file offset is first set
     to the end of the file before writing.  The adjustment of the file off-
     set and the write operation are performed as an atomic step.
     POSIX  requires  that  a  read(2)  that  can be proved to occur after a
     write() has returned will return the  new  data.   Note  that  not  all
     filesystems are POSIX conforming.
     According to POSIX.1, if count is greater than SSIZE_MAX, the result is
     implementation-defined; see NOTES for the upper limit on Linux.

RETURN VALUE

     On success, the number of bytes written  is  returned  (zero  indicates
     nothing  was  written).   It  is not an error if this number is smaller
     than the number of bytes requested; this may happen for example because
     the disk device was filled.  See also NOTES.
     On error, -1 is returned, and errno is set appropriately.
     If  count  is  zero  and  fd refers to a regular file, then write() may
     return a failure status if one of the errors below is detected.  If  no
     errors  are  detected,  or  error detection is not performed, 0 will be
     returned without causing any other effect.  If count  is  zero  and  fd
     refers  to a file other than a regular file, the results are not speci-
     fied.

ERRORS

     EAGAIN The file descriptor fd refers to a file other than a socket  and
            has  been  marked  nonblocking (O_NONBLOCK), and the write would
            block.  See open(2) for further details on the O_NONBLOCK  flag.
     EAGAIN or EWOULDBLOCK
            The  file  descriptor  fd refers to a socket and has been marked
            nonblocking   (O_NONBLOCK),   and   the   write   would   block.
            POSIX.1-2001  allows  either error to be returned for this case,
            and does not require these constants to have the same value,  so
            a portable application should check for both possibilities.
     EBADF  fd is not a valid file descriptor or is not open for writing.
     EDESTADDRREQ
            fd  refers to a datagram socket for which a peer address has not
            been set using connect(2).
     EDQUOT The user's quota of disk blocks on the filesystem containing the
            file referred to by fd has been exhausted.
     EFAULT buf is outside your accessible address space.
     EFBIG  An attempt was made to write a file that exceeds the implementa-
            tion-defined maximum file size or the process's file size limit,
            or to write at a position past the maximum allowed offset.
     EINTR  The  call  was interrupted by a signal before any data was writ-
            ten; see signal(7).
     EINVAL fd is attached to an object which is unsuitable for writing;  or
            the  file  was  opened  with  the  O_DIRECT flag, and either the
            address specified in buf, the value specified in count,  or  the
            file offset is not suitably aligned.
     EIO    A  low-level I/O error occurred while modifying the inode.  This
            error may relate to the write-back of data written by an earlier
            write(2),  which  may  have  been  issued  to  a  different file
            descriptor on the same file.   Since  Linux  4.13,  errors  from
            write-back come with a promise that they may be reported by sub-
            sequent.  write(2) requests, and will be reported  by  a  subse-
            quent  fsync(2)  (whether  or  not  they  were  also reported by
            write(2)).  An alternate cause of EIO on  networked  filesystems
            is when an advisory lock had been taken out on the file descrip-
            tor and this lock has been lost.  See the Lost locks section  of
            fcntl(2) for further details.
     ENOSPC The device containing the file referred to by fd has no room for
            the data.
     EPERM  The operation was prevented by a file seal; see fcntl(2).
     EPIPE  fd is connected to a pipe or socket whose reading end is closed.
            When  this  happens the writing process will also receive a SIG-
            PIPE signal.  (Thus, the write return value is seen only if  the
            program catches, blocks or ignores this signal.)
     Other errors may occur, depending on the object connected to fd.

CONFORMING TO

     SVr4, 4.3BSD, POSIX.1-2001.
     Under  SVr4  a  write may be interrupted and return EINTR at any point,
     not just before any data is written.

NOTES

     The types size_t and ssize_t are,  respectively,  unsigned  and  signed
     integer data types specified by POSIX.1.
     A  successful return from write() does not make any guarantee that data
     has been committed to disk.  On some  filesystems,  including  NFS,  it
     does  not  even guarantee that space has successfully been reserved for
     the data.  In this case, some errors might be delayed  until  a  future
     write(2),  fsync(2),  or  even close(2).  The only way to be sure is to
     call fsync(2) after you are done writing all your data.
     If a write() is interrupted by a signal handler before  any  bytes  are
     written, then the call fails with the error EINTR; if it is interrupted
     after at least one byte  has  been  written,  the  call  succeeds,  and
     returns the number of bytes written.
     On  Linux,  write()  (and  similar  system calls) will transfer at most
     0x7ffff000 (2,147,479,552) bytes, returning the number of  bytes  actu-
     ally transferred.  (This is true on both 32-bit and 64-bit systems.)

BUGS

     According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
     with Regular File Operations"):
         All of the following functions shall be atomic with respect to each
         other in the effects specified in POSIX.1-2008 when they operate on
         regular files or symbolic links: ...
     Among the APIs subsequently listed  are  write()  and  writev(2).   And
     among  the effects that should be atomic across threads (and processes)
     are updates of the file offset.  However, on Linux before version 3.14,
     this  was  not  the  case:  if  two  processes  that share an open file
     description (see open(2)) perform a write() (or writev(2)) at the  same
     time, then the I/O operations were not atomic with respect updating the
     file offset, with the result that the blocks of data output by the  two
     processes might (incorrectly) overlap.  This problem was fixed in Linux
     3.14.

SEE ALSO

     close(2), fcntl(2), fsync(2), ioctl(2), lseek(2),  open(2),  pwrite(2),
     read(2), select(2), writev(2), fwrite(3)

COLOPHON

     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at
     https://www.kernel.org/doc/man-pages/.

Linux 2018-02-02 WRITE(2)

/data/webs/external/dokuwiki/data/pages/man/write.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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