alloc() never returns Null

Mark alloc()--also dupstr() and re_alloc()--for gcc and clang as
always returning non-Null.  This should silence some of the static
analysis complaints.

Almost all the monster and object naming functions (anything that
returns an mbuf or an obuf) should be marked this way too but I'll
leave that for somebody else to deal with.

I didn't attempt to mark alloc() with the 'malloc' attribute because
macro definitions could end up causing trouble.  Specifying its
deallocator would probably be useful but is at even bigger risk of
macro interference.

I'm not sure whether gcc 3 is really the right test for whether the
returns_nonnull attribute setting is available.

include/extern.h

@@ -10,7 +10,7 @@
 #if 0
 extern long *alloc(unsigned int);
 #endif
-extern char *fmt_ptr(const void *);
+extern char *fmt_ptr(const void *) NONNULL;
 
 /* This next pre-processor directive covers almost the entire file,
  * interrupted only occasionally to pick up specific functions as needed. */

include/global.h

@@ -303,9 +303,9 @@ typedef uchar nhsym;
    if nethack is built with MONITOR_HEAP enabled and they aren't; this
    declaration has been moved out of the '#else' below to avoid getting
    a complaint from -Wmissing-prototypes when building with MONITOR_HEAP */
-extern char *dupstr(const char *);
+extern char *dupstr(const char *) NONNULL;
 /* same, but return strlen(string) */
-extern char *dupstr_n(const char *string, unsigned int *lenout);
+extern char *dupstr_n(const char *string, unsigned int *lenout) NONNULL;
 
 /*
  * MONITOR_HEAP is conditionally used for primitive memory leak debugging.
@@ -317,10 +317,10 @@ extern char *dupstr_n(const char *string, unsigned int *lenout);
  */
 #ifdef MONITOR_HEAP
 /* plain alloc() is not declared except in alloc.c */
-extern long *nhalloc(unsigned int, const char *, int);
-extern long *nhrealloc(long *, unsigned int, const char *, int);
+extern long *nhalloc(unsigned int, const char *, int) NONNULL;
+extern long *nhrealloc(long *, unsigned int, const char *, int) NONNULL;
 extern void nhfree(genericptr_t, const char *, int);
-extern char *nhdupstr(const char *, const char *, int);
+extern char *nhdupstr(const char *, const char *, int) NONNULL;
 /* this predates C99's __func__; that is trickier to use conditionally
    because it is not implemented as a preprocessor macro; MONITOR_HEAP
    wouldn't gain much benefit from it anyway so continue to live without it;
@@ -337,8 +337,8 @@ extern char *nhdupstr(const char *, const char *, int);
 #define dupstr(s) nhdupstr(s, __FILE__, (int) __LINE__)
 #else /* !MONITOR_HEAP */
 /* declare alloc.c's alloc(); allocations made with it use ordinary free() */
-extern long *alloc(unsigned int);  /* alloc.c */
-extern long *re_alloc(long *, unsigned int);
+extern long *alloc(unsigned int) NONNULL;  /* alloc.c */
+extern long *re_alloc(long *, unsigned int) NONNULL;
 #endif /* ?MONITOR_HEAP */
 
 /* Used for consistency checks of various data files; declare it here so

include/tradstdc.h

@@ -404,6 +404,7 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #if __GNUC__ >= 3
 #define UNUSED __attribute__((unused))
 #define NORETURN __attribute__((noreturn))
+#define NONNULL __attribute__((returns_nonnull))
 #if !defined(__linux__) || defined(GCC_URWARN)
 /* disable gcc's __attribute__((__warn_unused_result__)) since explicitly
    discarding the result by casting to (void) is not accepted as a 'use' */
@@ -422,5 +423,8 @@ typedef genericptr genericptr_t; /* (void *) or (char *) */
 #ifndef NORETURN
 #define NORETURN
 #endif
+#ifndef NONNULL
+#define NONNULL
+#endif
 
 #endif /* TRADSTDC_H */

src/alloc.c

@@ -19,7 +19,7 @@ extern int FITSint_(LUA_INTEGER, const char *, int);
 #define FITSuint(x) FITSuint_(x, __func__, (int) __LINE__)
 extern unsigned FITSuint_(unsigned long long, const char *, int);
 
-char *fmt_ptr(const genericptr);
+char *fmt_ptr(const genericptr) NONNULL;
 
 #ifdef MONITOR_HEAP
 #undef alloc
@@ -32,8 +32,8 @@ static FILE *heaplog = 0;
 static boolean tried_heaplog = FALSE;
 #endif
 
-long *alloc(unsigned int);
-long *re_alloc(long *, unsigned int);
+long *alloc(unsigned int) NONNULL;
+long *re_alloc(long *, unsigned int) NONNULL;
 extern void panic(const char *, ...);
 
 long *
@@ -221,7 +221,8 @@ char *
 dupstr_n(const char *string, unsigned int *lenout)
 {
     size_t len = strlen(string);
-    if(len >= LARGEST_INT)
+
+    if (len >= LARGEST_INT)
         panic("string too long");
     *lenout = (unsigned int) len;
     return strcpy((char *) alloc(len + 1), string);