[PATCH 2/2] 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 <[email protected]>
---
 ccan/mem/mem.c      | 21 +++++++++++++++++++++
 ccan/mem/mem.h      | 10 ++++++++++
 ccan/mem/test/api.c | 19 ++++++++++++++++++-
 3 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/ccan/mem/mem.c b/ccan/mem/mem.c
index 853f975..bab1a86 100644
--- a/ccan/mem/mem.c
+++ b/ccan/mem/mem.c
@@ -40,3 +40,24 @@ void *memrchr(const void *s, int c, size_t n)
 	return NULL;
 }
 #endif
+
+#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];
+
+	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;
+	}
+}
diff --git a/ccan/mem/mem.h b/ccan/mem/mem.h
index dcb44b8..41120a4 100644
--- a/ccan/mem/mem.h
+++ b/ccan/mem/mem.h
@@ -15,4 +15,14 @@ void *memmem(const void *haystack, size_t haystacklen,
 void *memrchr(const void *s, int c, size_t n);
 #endif
 
+/**
+ * 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 */
diff --git a/ccan/mem/test/api.c b/ccan/mem/test/api.c
index d97ab28..8885d48 100644
--- a/ccan/mem/test/api.c
+++ b/ccan/mem/test/api.c
@@ -1,15 +1,22 @@
+#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";
 	char haystack2[] = "ab\0ab\0ab\0ab";
 	char needle1[] = "ab";
 	char needle2[] = "d\0e";
+	char tmp1[SWAPSIZE], tmp2[SWAPSIZE];
 
 	/* This is how many tests you plan to run */
-	plan_tests(19);
+	plan_tests(21);
 
 	ok1(memmem(haystack1, sizeof(haystack1), needle1, 2) == haystack1);
 	ok1(memmem(haystack1, sizeof(haystack1), needle1, 3) == NULL);
@@ -36,6 +43,16 @@ int main(void)
 
 	ok1(memrchr(needle1, '\0', 2) == NULL);
 
+	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();
 }
-- 
2.4.3

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