libc: permit users to supply their own malloc.

Severely memory constrained systems with known allocation patterns can
benefit from providing their own implementation of malloc with
specifically tuned bucket sizes. Provide a switch to allow users to
replace the default malloc implementation with their own.

Signed-off-by: Josh Gao <josh@jmgao.dev>

lib/libc/Kconfig

@@ -80,6 +80,15 @@ endif # NEWLIB_LIBC
 
 if MINIMAL_LIBC
 
+config MINIMAL_LIBC_MALLOC
+	bool "Enable minimal libc malloc implementation"
+	default y
+	help
+	  Enable the minimal libc's implementation of malloc, free, and realloc.
+	  Disable if you wish to provide your own implementations of these functions.
+
+if MINIMAL_LIBC_MALLOC
+
 config MINIMAL_LIBC_MALLOC_ARENA_SIZE
 	int "Size of the minimal libc malloc arena"
 	default 0
@@ -88,6 +97,22 @@ config MINIMAL_LIBC_MALLOC_ARENA_SIZE
 	  malloc() implementation. This size value must be compatible with
 	  a sys_mem_pool definition with nmax of 1 and minsz of 16.
 
+endif
+
+config MINIMAL_LIBC_CALLOC
+	bool "Enable minimal libc trivial calloc implementation"
+	default y
+	help
+	  Enable the minimal libc's trivial implementation of calloc, which
+	  forwards to malloc and memset.
+
+config MINIMAL_LIBC_REALLOCARRAY
+	bool "Enable minimal libc trivial reallocarray implementation"
+	default y
+	help
+	  Enable the minimal libc's trivial implementation of reallocarray, which
+	  forwards to realloc.
+
 config MINIMAL_LIBC_LL_PRINTF
 	bool "Build with minimal libc long long printf" if !64BIT
 	default y if 64BIT

lib/libc/minimal/source/stdlib/malloc.c

@@ -17,6 +17,8 @@
 #include <logging/log.h>
 LOG_MODULE_DECLARE(os);
 
+#ifdef CONFIG_MINIMAL_LIBC_MALLOC
+
 #if (CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE > 0)
 #ifdef CONFIG_USERSPACE
 K_APPMEM_PARTITION_DEFINE(z_malloc_partition);
@@ -62,29 +64,6 @@ void *malloc(size_t size)
 }
 #endif
 
-void free(void *ptr)
-{
-	sys_mem_pool_free(ptr);
-}
-
-void *calloc(size_t nmemb, size_t size)
-{
-	void *ret;
-
-	if (size_mul_overflow(nmemb, size, &size)) {
-		errno = ENOMEM;
-		return NULL;
-	}
-
-	ret = malloc(size);
-
-	if (ret != NULL) {
-		(void)memset(ret, 0, size);
-	}
-
-	return ret;
-}
-
 void *realloc(void *ptr, size_t requested_size)
 {
 	void *new_ptr;
@@ -116,7 +95,33 @@ void *realloc(void *ptr, size_t requested_size)
 	return new_ptr;
 }
 
+void free(void *ptr)
+{
+	sys_mem_pool_free(ptr);
+}
+#endif /* CONFIG_MINIMAL_LIBC_MALLOC */
 
+#ifdef CONFIG_MINIMAL_LIBC_CALLOC
+void *calloc(size_t nmemb, size_t size)
+{
+	void *ret;
+
+	if (size_mul_overflow(nmemb, size, &size)) {
+		errno = ENOMEM;
+		return NULL;
+	}
+
+	ret = malloc(size);
+
+	if (ret != NULL) {
+		(void)memset(ret, 0, size);
+	}
+
+	return ret;
+}
+#endif /* CONFIG_MINIMAL_LIBC_CALLOC */
+
+#ifdef CONFIG_MINIMAL_LIBC_REALLOCARRAY
 void *reallocarray(void *ptr, size_t nmemb, size_t size)
 {
 	if (size_mul_overflow(nmemb, size, &size)) {
@@ -125,3 +130,4 @@ void *reallocarray(void *ptr, size_t nmemb, size_t size)
 	}
 	return realloc(ptr, size);
 }
+#endif /* CONFIG_MINIMAL_LIBC_REALLOCARRAY */