# write

## NAME



           write - write to a file descriptor
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <unistd.h>**
    
           **ssize_t write(** size_t count;
                         **int** _fd_**, const void** _buf_**[**_count_**], 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 offset 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.  On error, -1
           is returned, and _errno_ is set to indicate the error.
    
           Note that a successful **write**() may transfer fewer than _count_
           bytes.  Such partial writes can occur for various reasons; for
           example, because there was insufficient space on the disk device
           to write all of the requested bytes, or because a blocked **write**()
           to a socket, pipe, or similar was interrupted by a signal handler
           after it had transferred some, but before it had transferred all
           of the requested bytes.  In the event of a partial write, the
           caller can make another **write**() call to transfer the remaining
           bytes.  The subsequent call will either transfer further bytes or
           may result in an error (e.g., if the disk is now full).
    
           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
           is 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 specified.
    


## 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
                  implementation-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
                  written; 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**(), 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 subsequent.  **write**() requests, and _will_
                  be reported by a subsequent fsync(2) (whether or not they
                  were also reported by **write**()).  An alternate cause of **EIO**
                  on networked filesystems is when an advisory lock had been
                  taken out on the file descriptor 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 **SIGPIPE** 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_.
    


## STANDARDS



           POSIX.1-2008.
    


## NOTES



           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**(), 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
           actually transferred.  (This is true on both 32-bit and 64-bit
           systems.)
    
           An error return value while performing **write**() using direct I/O
           does not mean the entire write has failed.  Partial data may be
           written and the data at the file offset on which the **write**() was
           attempted should be considered inconsistent.
    


## 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, before Linux
           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 to 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.
    


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