<!-- Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009, 2013 The President and Fellows of Harvard College. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. --> <html> <head> <title>free</title> <body bgcolor=#ffffff> <h2 align=center>free</h2> <h4 align=center>OS/161 Reference Manual</h4> <h3>Name</h3> <p> free - release/deallocate memory </p> <h3>Library</h3> <p> Standard C Library (libc, -lc) </p> <h3>Synopsis</h3> <p> <tt>#include <stdlib.h></tt><br> <br> <tt>void</tt><br> <tt>free(void *</tt><em>ptr</em><tt>);</tt> </p> <h3>Description</h3> <p> <tt>free</tt> releases a block of memory previously allocated with <A HREF=malloc.html>malloc</A>, <A HREF=calloc.html>calloc</A>, or <A HREF=realloc.html>realloc</A>. </p> <p> Once free has been called, <em>ptr</em> is no longer valid and attempts to dereference it result in undefined behavior. (Pedantically, in fact, even using the <i>value</i> of <em>ptr</em> may produce undefined behavior.) Passing <em>ptr</em> to free a second or subsequent time (unless of course the same pointer value is again returned from malloc) is also undefined and particularly likely to provoke adverse behavior in most implementations. </p> <p> <tt>free(NULL)</tt> has no effect. </p> <p> In practice it is desirable for implementations of <tt>free</tt> to detect, to the extent practically possible, pointers that were not previously allocated by one of the above functions or that are passed to <tt>free</tt> multiple times. However, this can be difficult and there is no useful standard mechanism for error reporting. </p> <p> <tt>free</tt> does not necessarily unmap free memory or return it to the operating system, but may do so if it chooses. </p> <h3>Return Values</h3> <p> <tt>free</tt> returns no value. <p> <h3>See Also</h3> <p> <A HREF=calloc.html>calloc</A>, <A HREF=malloc.html>malloc</A>, <A HREF=realloc.html>realloc</A> </p> </body> </html>