implement reallocarray

reallocarray is an extension introduced by OpenBSD, which introduces
calloc overflow checking to realloc.

glibc 2.28 introduced support for this function behind _GNU_SOURCE,
while glibc 2.29 allows its usage in _DEFAULT_SOURCE.

include/stdlib.h

@@ -145,6 +145,7 @@ int getloadavg(double *, int);
 int clearenv(void);
 #define WCOREDUMP(s) ((s) & 0x80)
 #define WIFCONTINUED(s) ((s) == 0xffff)
+void *reallocarray (void *, size_t, size_t);
 #endif
 
 #ifdef _GNU_SOURCE

src/malloc/reallocarray.c

@@ -0,0 +1,13 @@
+#define _BSD_SOURCE
+#include <errno.h>
+#include <stdlib.h>
+
+void *reallocarray(void *ptr, size_t m, size_t n)
+{
+	if (n && m > -1 / n) {
+		errno = ENOMEM;
+		return 0;
+	}
+
+	return realloc(ptr, m * n);
+}