576 lines
11 KiB
HTML
576 lines
11 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of MALLOC</TITLE>
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</HEAD><BODY>
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<H1>MALLOC</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2020-02-09<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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malloc, free, calloc, realloc - allocate and free dynamic memory
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></B>
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<B>void *malloc(size_t </B><I>size</I><B>);</B>
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<B>void free(void </B><I>*ptr</I><B>);</B>
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<B>void *calloc(size_t </B><I>nmemb</I><B>, size_t </B><I>size</I><B>);</B>
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<B>void *realloc(void </B><I>*ptr</I><B>, size_t </B><I>size</I><B>);</B>
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<B>void *reallocarray(void </B><I>*ptr</I><B>, size_t </B><I>nmemb</I><B>, size_t </B><I>size</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>reallocarray</B>():
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<BR> Since glibc 2.29:
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<BR> _DEFAULT_SOURCE
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<BR> Glibc 2.28 and earlier:
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<BR> _GNU_SOURCE
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<P>
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The
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<B>malloc</B>()
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function allocates
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<I>size</I>
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bytes and returns a pointer to the allocated memory.
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<I>The memory is not initialized</I>.
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If
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<I>size</I>
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is 0, then
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<B>malloc</B>()
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returns either NULL,
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or a unique pointer value that can later be successfully passed to
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<B>free</B>().
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<P>
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The
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<B>free</B>()
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function frees the memory space pointed to by
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<I>ptr</I>,
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which must have been returned by a previous call to
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<B>malloc</B>(),
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<B>calloc</B>(),
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or
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<B>realloc</B>().
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Otherwise, or if
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<I>free(ptr)</I>
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has already been called before, undefined behavior occurs.
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If
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<I>ptr</I>
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is NULL, no operation is performed.
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<P>
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The
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<B>calloc</B>()
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function allocates memory for an array of
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<I>nmemb</I>
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elements of
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<I>size</I>
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bytes each and returns a pointer to the allocated memory.
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The memory is set to zero.
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If
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<I>nmemb</I>
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or
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<I>size</I>
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is 0, then
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<B>calloc</B>()
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returns either NULL,
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or a unique pointer value that can later be successfully passed to
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<B>free</B>().
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If the multiplication of
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<I>nmemb</I>
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and
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<I>size</I>
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would result in integer overflow, then
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<B>calloc</B>()
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returns an error.
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By contrast,
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an integer overflow would not be detected in the following call to
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<B>malloc</B>(),
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with the result that an incorrectly sized block of memory would be allocated:
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<P>
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malloc(nmemb * size);
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<P>
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The
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<B>realloc</B>()
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function changes the size of the memory block pointed to by
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<I>ptr</I>
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to
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<I>size</I>
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bytes.
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The contents will be unchanged in the range from the start of the region
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up to the minimum of the old and new sizes.
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If the new size is larger than the old size, the added memory will
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<I>not</I>
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be initialized.
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If
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<I>ptr</I>
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is NULL, then the call is equivalent to
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<I>malloc(size)</I>,
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for all values of
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<I>size</I>;
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if
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<I>size</I>
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is equal to zero,
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and
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<I>ptr</I>
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is not NULL, then the call is equivalent to
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<I>free(ptr)</I>.
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Unless
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<I>ptr</I>
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is NULL, it must have been returned by an earlier call to
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<B>malloc</B>(),
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<B>calloc</B>(),
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or
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<B>realloc</B>().
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If the area pointed to was moved, a
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<I>free(ptr)</I>
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is done.
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<P>
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The
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<B>reallocarray</B>()
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function changes the size of the memory block pointed to by
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<I>ptr</I>
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to be large enough for an array of
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<I>nmemb</I>
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elements, each of which is
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<I>size</I>
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bytes.
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It is equivalent to the call
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<P>
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<BR> realloc(ptr, nmemb * size);
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<P>
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However, unlike that
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<B>realloc</B>()
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call,
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<B>reallocarray</B>()
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fails safely in the case where the multiplication would overflow.
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If such an overflow occurs,
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<B>reallocarray</B>()
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returns NULL, sets
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<I>errno</I>
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to
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<B>ENOMEM</B>,
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and leaves the original block of memory unchanged.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>malloc</B>()
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and
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<B>calloc</B>()
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functions return a pointer to the allocated memory,
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which is suitably aligned for any built-in type.
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On error, these functions return NULL.
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NULL may also be returned by a successful call to
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<B>malloc</B>()
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with a
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<I>size</I>
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of zero,
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or by a successful call to
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<B>calloc</B>()
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with
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<I>nmemb</I>
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or
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<I>size</I>
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equal to zero.
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<P>
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The
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<B>free</B>()
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function returns no value.
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<P>
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The
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<B>realloc</B>()
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function returns a pointer to the newly allocated memory, which is suitably
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aligned for any built-in type, or NULL if the request failed.
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The returned pointer may be the same as
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<I>ptr</I>
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if the allocation was not moved
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(e.g., there was room to expand the allocation in-place), or different from
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<I>ptr</I>
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if the allocation was moved to a new address.
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If
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<I>size</I>
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was equal to 0, either NULL or a pointer suitable to be passed to
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<B>free</B>()
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is returned.
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If
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<B>realloc</B>()
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fails, the original block is left untouched; it is not freed or moved.
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<P>
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On success, the
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<B>reallocarray</B>()
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function returns a pointer to the newly allocated memory.
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On failure,
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it returns NULL and the original block of memory is left untouched.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<B>calloc</B>(),
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<B>malloc</B>(),
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<B>realloc</B>(),
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and
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<B>reallocarray</B>()
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can fail with the following error:
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<DL COMPACT>
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<DT id="1"><B>ENOMEM</B>
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<DD>
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Out of memory.
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Possibly, the application hit the
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<B>RLIMIT_AS</B>
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or
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<B>RLIMIT_DATA</B>
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limit described in
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<B><A HREF="/cgi-bin/man/man2html?2+getrlimit">getrlimit</A></B>(2).
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</DL>
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<A NAME="lbAG"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>malloc</B>(),
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<B>free</B>(),
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<BR>
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<B>calloc</B>(),
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<B>realloc</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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<B>malloc</B>(),
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<B>free</B>(),
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<B>calloc</B>(),
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<B>realloc</B>():
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POSIX.1-2001, POSIX.1-2008, C89, C99.
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<P>
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<B>reallocarray</B>()
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is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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By default, Linux follows an optimistic memory allocation strategy.
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This means that when
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<B>malloc</B>()
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returns non-NULL there is no guarantee that the memory really
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is available.
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In case it turns out that the system is out of memory,
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one or more processes will be killed by the OOM killer.
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For more information, see the description of
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<I>/proc/sys/vm/overcommit_memory</I>
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and
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<I>/proc/sys/vm/oom_adj</I>
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in
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<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5),
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and the Linux kernel source file
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<I>Documentation/vm/overcommit-accounting.rst</I>.
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<P>
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Normally,
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<B>malloc</B>()
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allocates memory from the heap, and adjusts the size of the heap
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as required, using
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<B><A HREF="/cgi-bin/man/man2html?2+sbrk">sbrk</A></B>(2).
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When allocating blocks of memory larger than
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<B>MMAP_THRESHOLD</B>
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bytes, the glibc
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<B>malloc</B>()
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implementation allocates the memory as a private anonymous mapping using
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2).
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<B>MMAP_THRESHOLD</B>
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is 128 kB by default, but is adjustable using
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<B><A HREF="/cgi-bin/man/man2html?3+mallopt">mallopt</A></B>(3).
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Prior to Linux 4.7
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allocations performed using
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)
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were unaffected by the
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<B>RLIMIT_DATA</B>
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resource limit;
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since Linux 4.7, this limit is also enforced for allocations performed using
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2).
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<P>
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To avoid corruption in multithreaded applications,
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mutexes are used internally to protect the memory-management
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data structures employed by these functions.
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In a multithreaded application in which threads simultaneously
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allocate and free memory,
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there could be contention for these mutexes.
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To scalably handle memory allocation in multithreaded applications,
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glibc creates additional
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<I>memory allocation arenas</I>
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if mutex contention is detected.
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Each arena is a large region of memory that is internally allocated
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by the system
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(using
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<B><A HREF="/cgi-bin/man/man2html?2+brk">brk</A></B>(2)
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or
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)),
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and managed with its own mutexes.
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<P>
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SUSv2 requires
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<B>malloc</B>(),
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<B>calloc</B>(),
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and
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<B>realloc</B>()
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to set
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<I>errno</I>
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to
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<B>ENOMEM</B>
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upon failure.
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Glibc assumes that this is done
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(and the glibc versions of these routines do this); if you
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use a private malloc implementation that does not set
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<I>errno</I>,
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then certain library routines may fail without having
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a reason in
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<I>errno</I>.
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<P>
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Crashes in
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<B>malloc</B>(),
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<B>calloc</B>(),
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<B>realloc</B>(),
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or
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<B>free</B>()
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are almost always related to heap corruption, such as overflowing
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an allocated chunk or freeing the same pointer twice.
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<P>
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The
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<B>malloc</B>()
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implementation is tunable via environment variables; see
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<B><A HREF="/cgi-bin/man/man2html?3+mallopt">mallopt</A></B>(3)
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for details.
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<A NAME="lbAJ"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?1+valgrind">valgrind</A></B>(1),
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<B><A HREF="/cgi-bin/man/man2html?2+brk">brk</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+alloca">alloca</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc_get_state">malloc_get_state</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc_info">malloc_info</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc_trim">malloc_trim</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc_usable_size">malloc_usable_size</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+mallopt">mallopt</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+mcheck">mcheck</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+mtrace">mtrace</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+posix_memalign">posix_memalign</A></B>(3)
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<P>
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For details of the GNU C library implementation, see
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<A NAME="lbAK"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="2"><A HREF="#lbAB">NAME</A><DD>
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<DT id="3"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="4"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="5"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="6"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="7"><A HREF="#lbAG">ATTRIBUTES</A><DD>
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<DT id="8"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="9"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="10"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="11"><A HREF="#lbAK">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:47 GMT, March 31, 2021
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</BODY>
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</HTML>
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