--- a/def.h
+++ b/def.h
@@ -107,7 +107,7 @@
  * Given a file address, determine the block number it represents.
  */
 #define blockof(off)			((int) ((off) / 4096))
-#define offsetof(off)			((int) ((off) % 4096))
+#define offsetofmailx(off)			((int) ((off) % 4096))
 #define positionof(block, offset)	((off_t)(block) * 4096 + (offset))
 
 /*
--- a/edit.c
+++ b/edit.c
@@ -133,7 +133,7 @@
 			(void)fseek(otf, 0L, SEEK_END);
 			size = ftell(otf);
 			mp->m_block = blockof(size);
-			mp->m_offset = offsetof(size);
+			mp->m_offset = offsetofmailx(size);
 			mp->m_size = fsize(fp);
 			mp->m_lines = 0;
 			mp->m_flag |= MODIFY;
--- a/fio.c
+++ b/fio.c
@@ -139,7 +139,7 @@
 			this.m_size = 0;
 			this.m_lines = 0;
 			this.m_block = blockof(offset);
-			this.m_offset = offsetof(offset);
+			this.m_offset = offsetofmailx(offset);
 			inhead = 1;
 		} else if (linebuf[0] == 0) {
 			inhead = 0;