diff --git a/as/assemble.c b/as/assemble.c
index 5d575ec..35812a3 100644
--- a/as/assemble.c
+++ b/as/assemble.c
@@ -28,7 +28,7 @@
 
 static int assemble_section(struct hashnode *hashnode, void *data)
 {
-    struct section *section = (struct section*)hashnode;	/*XXX*/
+    struct section *section = container_of(hashnode, struct section, hashnode);
     struct tunit *tunit = data;
     unsigned long dot;
     struct stmt *stmt;
diff --git a/as/output.c b/as/output.c
index 2eb6c0b..26880a2 100644
--- a/as/output.c
+++ b/as/output.c
@@ -67,7 +67,7 @@ static int append_section(struct context *context, struct section *section)
 
 static int process_section(struct hashnode *hashnode, void *data)
 {
-    struct section *section = (struct section*)hashnode;
+    struct section *section = container_of(hashnode, struct section, hashnode);
     struct context *context = data;
 
     return append_section(context, section);
@@ -95,7 +95,7 @@ static void output_section_epilogue(struct context *context, struct section *sec
 
 static int output_section(struct hashnode *hashnode, void *data)
 {
-    struct section *section = (struct section*)hashnode;
+    struct section *section = container_of(hashnode, struct section, hashnode);
     struct context *context = data;
 
     if (section->dot == 0 || (section->image == NULL && section->output == NULL))
@@ -140,7 +140,7 @@ static int output_section_header(struct context *context, const struct section *
 
 static int output_shdr(struct hashnode *hashnode, void *data)
 {
-    struct section *section = (struct section*)hashnode;
+    struct section *section = container_of(hashnode, struct section, hashnode);
     struct context *context = data;
 
     if (section->dot == 0)
@@ -156,7 +156,7 @@ static int output_strtab(struct context *context, struct strtab *strtab)
 
 static int process_symbol(struct hashnode *hashnode, void *data)
 {
-    struct symbol *symbol = (struct symbol*)hashnode;
+    struct symbol *symbol = container_of(hashnode, struct symbol, hashnode);
     struct context *context = data;
 
     ++context->symnum;
@@ -175,7 +175,7 @@ struct finalize_symbol_context {
 
 static int finalize_symbol(struct hashnode *hashnode, void *data)
 {
-    struct symbol *symbol = (struct symbol*)hashnode;
+    struct symbol *symbol = container_of(hashnode, struct symbol, hashnode);
     struct finalize_symbol_context *fsctx = data;
     Elf36_Word st_shndx;
 
diff --git a/as/tunit.c b/as/tunit.c
index a150801..9e24f0a 100644
--- a/as/tunit.c
+++ b/as/tunit.c
@@ -48,8 +48,7 @@ static uintptr_t string_hash(const char *string)
  */
 static struct section *section_from_hashnode(const struct hashnode *hashnode)
 {
-    /* hashnode is first in section, so no need to mess with offsetof */
-    return (struct section*)hashnode;
+    return container_of(hashnode, struct section, hashnode);
 }
 
 static int section_eq(const struct hashnode *hashnode, const void *data)
@@ -167,8 +166,7 @@ pdp10_uint36_t strtab_enter(struct tunit *tunit, struct strtab *strtab, const ch
 
 static int strtab_section_output(PDP10_FILE *pdp10fp, const struct section *section)
 {
-    /* section is first in strtab, so no need to mess with offsetof */
-    const struct strtab *strtab = (const struct strtab*)section;
+    const struct strtab *strtab = container_of(section, const struct strtab, section);
     struct strtab_entry *here;
     unsigned int i;
 
@@ -197,8 +195,7 @@ void strtab_init(struct strtab *strtab, const char *name)
  */
 static struct symbol *symbol_from_hashnode(const struct hashnode *hashnode)
 {
-    /* hashnode is first in symbol, so no need to mess with offsetof */
-    return (struct symbol*)hashnode;
+    return container_of(hashnode, struct symbol, hashnode);
 }
 
 static int symbol_eq(const struct hashnode *hashnode, const void *data)
diff --git a/as/tunit.h b/as/tunit.h
index 01c7995..ad5737b 100644
--- a/as/tunit.h
+++ b/as/tunit.h
@@ -20,6 +20,7 @@
 #ifndef TUNIT_H
 #define TUNIT_H
 
+#include <stddef.h>
 #include "pdp10-elf36.h"
 #include "pdp10-stdio.h"
 #include "hashtab.h"
@@ -88,7 +89,7 @@ struct strtab_entry {
 };
 
 struct strtab {
-    struct section section;	/* first to simplify mapping &section to &strtab */
+    struct section section;
     struct strtab_entry *head;
 };
 
@@ -125,4 +126,14 @@ struct symbol *tunit_symbol_enter(struct tunit *tunit, const char *name);
 pdp10_uint36_t strtab_enter(struct tunit *tunit, struct strtab *strtab, const char *name);
 void strtab_init(struct strtab *strtab, const char *name);
 
+/**
+ * container_of - cast a member of a structure out to the containing structure
+ * @ptr:	the pointer to the member.
+ * @type:	the type of the container struct this is embedded in.
+ * @member:	the name of the member within the struct.
+ */
+#define container_of(ptr, type, member) ({			\
+	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
+	(type *)( (char *)__mptr - offsetof(type,member) );})
+
 #endif /* TUNIT_H */