write (system call)

<h2 id="posix-usage">POSIX usage</h2>
<p>The write call interface<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a><a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> is standardized by the POSIX specification. Data is written to a file by calling the write function. The function prototype is:
</p>
 ssize_t write(int fildes, const void *buf, size_t nbyte);

<table><tbody><tr><th>Argument</th><th>Description</th></tr><tr><td>fildes</td><td>The file descriptor obtained from a call to <a href="/facts/Open_(system_call)/TP39FgXh">open()</a>. It is an integer value. The values 0, 1, 2 can also be given, for standard input, standard output & standard error, respectively .</td></tr><tr><td>buf</td><td>Points to a character array, with content to be written to the file pointed to by filedes.</td></tr><tr><td>nbyte</td><td>Specifies the number of bytes to be written from the character array, buf, into the file pointed to by filedes.</td></tr></tbody></table>
<p>In above syntax, <a href="/facts/Ssize_t/49S2hMa1">ssize_t</a> is a <a href="/facts/Typedef/kqSR4VXn">typedef</a>. It is a signed data type defined in <a href="/facts/Stddef.h/49S2hMa1">stddef.h</a>. Note that <a href="/facts/Write()/R4NsXaBc">write()</a> does not return an unsigned value; it returns -1 if an error occurs so it must return a signed value.
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.
</p>
<h2 id="linux">Linux</h2>
<p>On Linux, write is system call number 1.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Fwrite/m5n82Fqz">fwrite</a></li>
<li><a href="/facts/Getchar/m5n82Fqz">getchar</a></li>
<li><a href="/facts/Fprintf/m5n82Fqz">fprintf</a></li>
<li><a href="/facts/Read_(system_call)/m4oKv1Fc">read (system call)</a></li>
<li><a href="/facts/Sync_(Unix)/adtIxczx">sync (Unix)</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://pubs.opengroup.org/onlinepubs/009695399/functions/write.html">POSIX write</a></li>
<li> C_Programming/C_Reference/stdio.h/fwrite at Wikibooks</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>http://www.unix.com/man-page/FreeBSD/2/write/ Manual page for Write <a href="http://www.unix.com/man-page/FreeBSD/2/write/" target="_blank">http://www.unix.com/man-page/FreeBSD/2/write/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives I/O Primitives <a href="https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives" target="_blank">https://www.gnu.org/s/hello/manual/libc/I_002fO-Primitives.html#I_002fO-Primitives</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Write". <a href="http://pubs.opengroup.org/onlinepubs/007904875/functions/write.html" target="_blank">http://pubs.opengroup.org/onlinepubs/007904875/functions/write.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Linux/Arch/X86/Entry/Syscalls/Syscall_64.TBL at master · torvalds/Linux". GitHub. <a href="https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl" target="_blank">https://github.com/torvalds/linux/blob/master/arch/x86/entry/syscalls/syscall_64.tbl</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
</ol>

write (system call) open-in-new

write (system call)