No <string.h> functions used at all if MSPACK_NO_DEFAULT_SYSTEM set

Author: kyz

libmspack/ChangeLog

@@ -3,6 +3,11 @@
 	* macros.h: new header for the D(), LD/LU and EndGet???() macros.
 	Use this instead of system.h.
 
+        * system.h: if MSPACK_NO_DEFAULT_SYSTEM is defined, define
+        inline versions of the only standard C functions used in
+        mspack (strlen, memcmp, memset), so that no standard C library
+        functions are needed at all.
+
 2020-01-08  Stuart Caie <[email protected]>
 
 	* lzxd_decompress(): do not apply the E8 transformation on the

libmspack/Makefile.am

@@ -5,7 +5,10 @@ TESTS =                 $(check_PROGRAMS)
 
 ACLOCAL_AMFLAGS =       -I m4
 AM_CFLAGS =
-# add "-DMSPACK_NO_DEFAULT_SYSTEM" to remove default mspack_system
+# add "-DMSPACK_NO_DEFAULT_SYSTEM" to remove default mspack_system.
+# however, note that many of the tests and examples provided DO rely on the
+# default mspack_system and will fail without it -- any program with a call
+# like "mspack_create_...(NULL)" expects a default mspack_system.
 if GCC
 AM_CFLAGS +=            -Wall -Wextra -Wno-unused-parameter -Wno-unused-result
 endif

libmspack/mspack/system.h

@@ -22,7 +22,25 @@ extern "C" {
 #include <macros.h>
 
 /* assume <string.h> exists */
-#include <string.h>
+#ifndef MSPACK_NO_DEFAULT_SYSTEM
+# include <string.h>
+#else
+ /* but if no default system wanted, avoid using <string.h> entirely,
+  * to avoid linking to even these standard C library functions */
+static inline int memcmp(const void *s1, const void *s2, size_t n) {
+    const unsigned char *a = s1, *b = s2;
+    while (n--) if (*a++ != *b++) return a[-1] - b[-1];
+    return 0;
+}
+static inline void *memset(void *s, int c, size_t n) {
+    unsigned char *s2 = s, c2 = (unsigned char) c;
+    while (n--) *s2++ = c2;
+    return s;
+}
+static inline size_t strlen(const char *s) {
+    size_t c = 0; while (*s++) c++; return c;
+}
+#endif
 
 /* fix for problem with GCC 4 and glibc (thanks to Ville Skytta)
  * http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=150429