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141 lines
3.7 KiB
Groff
141 lines
3.7 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the American National Standards Committee X3, on Information
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.\" Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd May 1, 2015
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.Dt REALLOCARRAY 3
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.Os
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.Sh NAME
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.Nm reallocarray
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.Nd memory reallocation function
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft void *
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.Fn reallocarray "void *ptr" "size_t nmemb" "size_t size"
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.Sh DESCRIPTION
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The
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.Fn reallocarray
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function is similar to the
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.Fn realloc
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function
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except it operates on
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.Fa nmemb
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members of size
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.Fa size
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and checks for integer overflow in the calculation
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.Fa nmemb
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*
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.Fa size .
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.Sh RETURN VALUES
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The
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.Fn reallocarray
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function returns a pointer to the allocated space; otherwise, a
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.Dv NULL
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pointer is returned and
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.Va errno
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is set to
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.Er ENOMEM .
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.Sh EXAMPLES
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Consider
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.Fn reallocarray
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when there is multiplication in the
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.Fa size
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argument of
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.Fn malloc
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or
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.Fn realloc .
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For example, avoid this common idiom as it may lead to integer overflow:
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.Bd -literal -offset indent
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if ((p = malloc(num * size)) == NULL)
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err(1, "malloc");
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.Ed
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.Pp
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A drop-in replacement is the
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.Ox
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extension
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.Fn reallocarray :
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.Bd -literal -offset indent
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if ((p = reallocarray(NULL, num, size)) == NULL)
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err(1, "reallocarray");
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.Ed
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.Pp
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When using
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.Fn realloc ,
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be careful to avoid the following idiom:
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.Bd -literal -offset indent
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size += 50;
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if ((p = realloc(p, size)) == NULL)
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return (NULL);
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.Ed
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.Pp
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Do not adjust the variable describing how much memory has been allocated
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until the allocation has been successful.
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This can cause aberrant program behavior if the incorrect size value is used.
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In most cases, the above sample will also result in a leak of memory.
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As stated earlier, a return value of
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.Dv NULL
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indicates that the old object still remains allocated.
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Better code looks like this:
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.Bd -literal -offset indent
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newsize = size + 50;
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if ((newp = realloc(p, newsize)) == NULL) {
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free(p);
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p = NULL;
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size = 0;
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return (NULL);
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}
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p = newp;
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size = newsize;
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.Ed
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.Pp
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As with
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.Fn malloc ,
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it is important to ensure the new size value will not overflow;
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i.e. avoid allocations like the following:
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.Bd -literal -offset indent
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if ((newp = realloc(p, num * size)) == NULL) {
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...
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.Ed
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.Pp
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Instead, use
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.Fn reallocarray :
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.Bd -literal -offset indent
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if ((newp = reallocarray(p, num, size)) == NULL) {
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...
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.Ed
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.Sh SEE ALSO
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.Xr realloc 3
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.Sh HISTORY
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The
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.Fn reallocarray
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function first appeared in
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.Ox 5.6
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and
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.Fx 11.0 .
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