mem: Add memswap() function

Add a memswap() function to the mem module, which exchanges two (equal
sized, non-overlapping) memory regions.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/mem/mem.c

@@ -2,6 +2,7 @@
 
 #include "config.h"
 
+#include <assert.h>
 #include <string.h>
 #include <ccan/mem/mem.h>
 
@@ -64,3 +65,26 @@ void *memcchr(void const *data, int c, size_t data_len)
 
 	return NULL;
 }
+
+#define MEMSWAP_TMP_SIZE	256
+
+void memswap(void *a, void *b, size_t n)
+{
+	char *ap = a;
+	char *bp = b;
+	char tmp[MEMSWAP_TMP_SIZE];
+
+	assert(!memoverlaps(a, n, b, n));
+
+	while (n) {
+		size_t m = n > MEMSWAP_TMP_SIZE ? MEMSWAP_TMP_SIZE : n;
+
+		memcpy(tmp, bp, m);
+		memcpy(bp, ap, m);
+		memcpy(ap, tmp, m);
+
+		ap += m;
+		bp += m;
+		n -= m;
+	}
+}

ccan/mem/mem.h

@@ -217,4 +217,14 @@ static inline bool memoverlaps(const void *a_, size_t al,
 	return (a < (b + bl)) && (b < (a + al));
 }
 
+/*
+ * memswap - Exchange two memory regions
+ * @a: first region
+ * @b: second region
+ * @n: length of the regions
+ *
+ * Undefined results if the two memory regions overlap.
+ */
+void memswap(void *a, void *b, size_t n);
+
 #endif /* CCAN_MEM_H */

ccan/mem/test/api.c

+#include "config.h"
+
+#include <assert.h>
+
 #include <ccan/mem/mem.h>
 #include <ccan/tap/tap.h>
 
+#define SWAPSIZE	12
+
 int main(void)
 {
 	char haystack1[] = "abcd\0efgh";
@@ -9,9 +15,10 @@ int main(void)
 	char needle2[] = "d\0e";
 	char scan1[] = "aaaab";
 	char scan2[] = "\0\0\0b";
+	char tmp1[SWAPSIZE], tmp2[SWAPSIZE];
 
 	/* This is how many tests you plan to run */
-	plan_tests(60);
+	plan_tests(62);
 
 	ok1(memmem(haystack1, sizeof(haystack1), needle1, 2) == haystack1);
 	ok1(memmem(haystack1, sizeof(haystack1), needle1, 3) == NULL);
@@ -96,6 +103,16 @@ int main(void)
 	ok1(memoverlaps(haystack1 + 4, 7, haystack1, 5));
 	ok1(!memoverlaps(haystack1 + 5, 6, haystack1, 5));
 
+	assert(sizeof(haystack1) <= SWAPSIZE);
+	assert(sizeof(haystack2) <= SWAPSIZE);
+	memset(tmp1, 0, sizeof(tmp1));
+	memset(tmp2, 0, sizeof(tmp2));
+	memcpy(tmp1, haystack1, sizeof(haystack1));
+	memcpy(tmp2, haystack2, sizeof(haystack2));
+	memswap(tmp1, tmp2, SWAPSIZE);
+	ok1(memcmp(tmp1, haystack2, sizeof(haystack2)) == 0);
+	ok1(memcmp(tmp2, haystack1, sizeof(haystack1)) == 0);
+
 	/* This exits depending on whether all tests passed */
 	return exit_status();
 }