1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" the American National Standards Committee X3, on Information 6.\" Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 21.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.Dd October 2, 2025 30.Dt REALLOCARRAY 3 31.Os 32.Sh NAME 33.Nm reallocarray 34.Nd memory reallocation function 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In stdlib.h 39.Ft void * 40.Fn reallocarray "void *ptr" "size_t nmemb" "size_t size" 41.Ft void * 42.Fn recallocarray "void *ptr" "size_t oldnmeb" "size_t nmemb" size_t size" 43.Sh DESCRIPTION 44The 45.Fn reallocarray 46function is similar to the 47.Fn realloc 48function 49except it operates on 50.Fa nmemb 51members of size 52.Fa size 53and checks for integer overflow in the calculation 54.Fa nmemb 55* 56.Fa size . 57.Pp 58The 59.Fn recallocarray 60function is similar to the 61.Fn reallocarray 62function 63except it ensures newly allocated memory is cleared similar to 64.Fn calloc . 65If 66.Fa ptr 67is 68.Dv NULL , 69.Fa oldnmemb 70is ignored and the call is equivalent to 71.Fn calloc . 72If 73.Fa ptr 74is not 75.Dv NULL , 76.Fa oldnmemb 77must be a value such that 78.Fa oldnmemb 79* 80.Fa size 81is the size of the earlier allocation that returned 82.Fa ptr , 83otherwise the behaviour is undefined. 84.Sh RETURN VALUES 85The 86.Fn reallocarray 87function returns a pointer to the allocated space; otherwise, a 88.Dv NULL 89pointer is returned and 90.Va errno 91is set to 92.Er ENOMEM . 93.Sh EXAMPLES 94Consider 95.Fn reallocarray 96when there is multiplication in the 97.Fa size 98argument of 99.Fn malloc 100or 101.Fn realloc . 102For example, avoid this common idiom as it may lead to integer overflow: 103.Bd -literal -offset indent 104if ((p = malloc(num * size)) == NULL) 105 err(1, "malloc"); 106.Ed 107.Pp 108A drop-in replacement is the 109.Ox 110extension 111.Fn reallocarray : 112.Bd -literal -offset indent 113if ((p = reallocarray(NULL, num, size)) == NULL) 114 err(1, "reallocarray"); 115.Ed 116.Pp 117When using 118.Fn realloc , 119be careful to avoid the following idiom: 120.Bd -literal -offset indent 121size += 50; 122if ((p = realloc(p, size)) == NULL) 123 return (NULL); 124.Ed 125.Pp 126Do not adjust the variable describing how much memory has been allocated 127until the allocation has been successful. 128This can cause aberrant program behavior if the incorrect size value is used. 129In most cases, the above sample will also result in a leak of memory. 130As stated earlier, a return value of 131.Dv NULL 132indicates that the old object still remains allocated. 133Better code looks like this: 134.Bd -literal -offset indent 135newsize = size + 50; 136if ((newp = realloc(p, newsize)) == NULL) { 137 free(p); 138 p = NULL; 139 size = 0; 140 return (NULL); 141} 142p = newp; 143size = newsize; 144.Ed 145.Pp 146As with 147.Fn malloc , 148it is important to ensure the new size value will not overflow; 149i.e. avoid allocations like the following: 150.Bd -literal -offset indent 151if ((newp = realloc(p, num * size)) == NULL) { 152 ... 153.Ed 154.Pp 155Instead, use 156.Fn reallocarray : 157.Bd -literal -offset indent 158if ((newp = reallocarray(p, num, size)) == NULL) { 159 ... 160.Ed 161.Sh SEE ALSO 162.Xr realloc 3 163.Sh STANDARDS 164.Fn reallocarray 165conforms to 166.St -p1003.1-2024 . 167.Sh HISTORY 168The 169.Fn reallocarray 170function first appeared in 171.Ox 5.6 172and 173.Fx 11.0 . 174The 175.Fn recallocarray 176function first appeared in 177.Ox 6.1 178and 179.Fx 16.0 . 180