Introduce a new error return code:

#define EDOFUS          88              /* Programming error */
This can be used to signal error situations which indicate that the
program logic or assumptions is deficient.

lib/libc/gen/errlst.c

@@ -146,6 +146,9 @@ const char *const sys_errlist[] = {
 	"Operation canceled",			/* 85 - ECANCELED */
 	"Illegal byte sequence",		/* 86 - EILSEQ */
 	"Attribute not found",			/* 87 - ENOATTR */
+
+/* General */
+	"Programming error",			/* 88 - EDOFUS */
 };
 int errno;
 const int sys_nerr = sizeof(sys_errlist) / sizeof(sys_errlist[0]);

sys/sys/errno.h

@@ -170,6 +170,8 @@ __END_DECLS
 #define	ENOATTR		87		/* Attribute not found */
 #define	ELAST		87		/* Must be equal largest errno */
 
+#define EDOFUS		88		/* Programming error */
+
 #endif /* _POSIX_SOURCE */
 
 #ifdef _KERNEL