ALIGNOF() helper

alignof/_info.c

+#include <stdio.h>
+#include <string.h>
+#include "config.h"
+
+/**
+ * alignof - ALIGNOF() macro to determine alignment of a type.
+ *
+ * Many platforms have requirements that certain types must be aligned
+ * to certain address boundaries, such as ints needing to be on 4-byte
+ * boundaries.  Attempting to access variables with incorrect
+ * alignment may cause performance loss or even program failure (eg. a
+ * bus signal).
+ *
+ * There are times which it's useful to be able to programatically
+ * access these requirements, such as for dynamic allocators.
+ *
+ * Example:
+ *	#include <stdio.h>
+ *	#include "alignof/alignoff.h"
+ *
+ *	int main(int argc, char *argv[])
+ *	{
+ *		char arr[sizeof(int)];
+ *
+ *		if ((unsigned long)arr % ALIGNOF(int)) {
+ *			printf("arr %p CANNOT hold an int\n", arr);
+ *			exit(1);
+ *		} else {
+ *			printf("arr %p CAN hold an int\n", arr);
+ *			exit(0);
+ *		}
+ *	}
+ */
+int main(int argc, char *argv[])
+{
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		printf("build_assert\n");
+		return 0;
+	}
+
+	return 1;
+}

alignof/alignof.h

+#ifndef CCAN_ALIGNOF_H
+#define CCAN_ALIGNOF_H
+#include "config.h"
+
+/**
+ * ALIGNOF - get the alignment of a type
+ * @t: the type to test
+ *
+ * This returns a safe alignment for the given type.
+ */
+#if HAVE_ALIGNOF
+/* A GCC extension. */
+#define ALIGNOF(t) __alignof__(t)
+#else
+/* Alignment by measuring structure padding. */
+#define ALIGNOF(t) ((char *)(&((struct { char c; t _h; } *)0)->_h) - (char *)0)
+#endif
+
+#endif /* CCAN_ALIGNOF_H */

alignof/test/run.c

+#include <stdlib.h>
+#include <stddef.h>
+#include <tap/tap.h>
+#include "alignof/alignof.h"
+
+/* Alignment is remarkably difficult to test.  The rules may be more
+ * complex than ALIGNOF() can know: eg. on i386 __alignof__(double) == 8, but
+ * __alignof__(struct containing double) == 4.
+ *
+ * Technically, we can only test that we give *at least* the alignment which
+ * naturally occurs, and that accesses work.
+ *
+ * For the moment, we work around double. */
+struct lots_of_types
+{
+	char c;
+	short s;
+	char c2;
+	int i;
+	char c3;
+	float f;
+	char c4;
+	double d;
+	char c5;
+};
+
+int main(int argc, char *argv[])
+{
+	struct lots_of_types lots_of_types, *lp = malloc(sizeof(*lp));
+	char c;
+	short s;
+	char c2;
+	int i;
+	char c3;
+	float f;
+	char c4;
+	double d;
+
+	/* Make sure we use all the variables. */
+	c = 0;
+	c2 = c3 = c4 = c;
+
+	plan_tests(15);
+	ok1((unsigned long)&c % ALIGNOF(char) == 0);
+	ok1((unsigned long)&s % ALIGNOF(short) == 0);
+	ok1((unsigned long)&i % ALIGNOF(int) == 0);
+	ok1((unsigned long)&f % ALIGNOF(float) == 0);
+	ok1((unsigned long)&d % ALIGNOF(double) == 0);
+
+	ok1((unsigned long)&lots_of_types.c % ALIGNOF(char) == 0);
+	ok1((unsigned long)&lots_of_types.s % ALIGNOF(short) == 0);
+	ok1((unsigned long)&lots_of_types.i % ALIGNOF(int) == 0);
+	ok1((unsigned long)&lots_of_types.f % ALIGNOF(float) == 0);
+	ok1(offsetof(struct lots_of_types, d) % ALIGNOF(double) == 0);
+
+	ok1((unsigned long)&lp->c % ALIGNOF(char) == 0);
+	ok1((unsigned long)&lp->s % ALIGNOF(short) == 0);
+	ok1((unsigned long)&lp->i % ALIGNOF(int) == 0);
+	ok1((unsigned long)&lp->f % ALIGNOF(float) == 0);
+	ok1((unsigned long)&lp->d % ALIGNOF(double) == 0);
+	exit(exit_status());
+}