View source for Write (system call)

{{Short description|System call in a Unix-like operating system kernel}}
{{Lowercase}}

The '''write''' is one of the most basic [[Subroutine|routines]] provided by a [[Unix-like]] [[Operating system|operating system]] [[Software:Kernel (operating system)|kernel]]. It writes data from a buffer declared by the user to a given device, such as a file. This is the primary way to output data from a program by directly using a system call. The destination is identified by a [[File descriptor|numeric code]]. The [[String (computer science)|data]] to be written, for instance a piece of text, is defined by a [[Pointer (computer programming)|pointer]] and a size, given in number of bytes.

<code>write</code> thus takes three arguments:
# The file code ([[File descriptor|file descriptor]] or '''fd''').
# The pointer to a [[Data buffer|buffer]] where the data is stored ('''buf''').
# The number of bytes to write from the buffer ('''nbytes''').

== POSIX usage ==

The write call interface<ref>http://www.unix.com/man-page/FreeBSD/2/write/ Manual page for Write</ref><ref>https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives I/O Primitives</ref><ref>{{Cite web|url=http://pubs.opengroup.org/onlinepubs/007904875/functions/write.html|title = Write}}</ref> is standardized by the POSIX specification. Data is written to a file by calling the write function. The function prototype is:

<source lang="c"> ssize_t write(int fildes, const void *buf, size_t nbyte);</source>

{| class = 'wikitable' style="width: 65%; height: 175px"
|-
!   Argument  !! Description
|-
|| <div class="center" style="width:auto; margin-left:auto; margin-right:auto;">'''fildes'''</div> || The file descriptor obtained from a call to [[Open (system call)|open()]]. It is an integer value. The values 0, 1, 2 can also be given, for standard input, standard output & standard error, respectively .
|-
|| <div class="center" style="width:auto; margin-left:auto; margin-right:auto;">'''buf'''</div> || Points to a character array, with content to be written to the file pointed to by filedes.
|-
|| <div class="center" style="width:auto; margin-left:auto; margin-right:auto;">'''nbyte'''</div> || Specifies the number of bytes to be written from the character array, buf, into the file pointed to by filedes.
|}

In above syntax, <code>ssize t</code> is a <code>[[Typedef|typedef]]</code>. It is a signed data type defined in <code>stddef.h</code>. Note that <code>write()</code> does not return an unsigned value; it returns -1 if an error occurs so it must return a signed value.<br>
The write function returns the number of bytes successfully written into the file, which may at times be less than the specified '''nbytes'''. It returns -1 if an exceptional condition is encountered, see section on errors below.

== See also ==
* fwrite
* getchar
* fprintf
* [[Read (system call)|read (system call)]]
* [[Sync (Unix)|sync (Unix)]]

== References ==
{{Reflist}}

== External links ==
* [http://pubs.opengroup.org/onlinepubs/009695399/functions/write.html POSIX write]

{{Computer files}}

[[Category:C POSIX library]]
[[Category:System calls]]

{{Sourceattribution|Write (system call)}}