libc: Document functions in stdlib.h

Also, add prototypes for calloc() and realloc(), which were already implemented but not in the header.

libs/libc/include/stdlib.h

@@ -9,13 +9,37 @@ extern "C"
 {
 #endif
 
+    /* Aborts the program. */
     noreturn void abort();
-    noreturn void exit(int);
+
+    /* Exits the program with the specified status code. */
+    noreturn void exit(int status);
+
+    /* Not implemented. */
     int atexit(void (*)(void));
+
+    /* Not implemented.*/
     int atoi(const char*);
-    void free(void*);
+
+    /* Not implemented. */
     char* getenv(const char*);
-    void* malloc(size_t);
+
+    /* Allocates n bytes of memory and returns a pointer to it. This memory should be freed by calling free() when it is
+     * not in use anymore. */
+    void* malloc(size_t n);
+
+    /* Allocates enough bytes of memory for an array containing nmemb items of size n and returns a pointer to it. This
+     * memory should be freed by calling free() when it is not in use anymore. */
+    void* calloc(size_t nmemb, size_t n);
+
+    /* Resizes memory allocated by malloc() or calloc() to n bytes. Returns a pointer to the new resized region of
+     * memory, which should be used instead of the old one. This memory should be freed by calling free() when it is not
+     * in use anymore. */
+    void* realloc(void* ptr, size_t n);
+
+    /* Frees a pointer to memory allocated by malloc(), calloc() or realloc(). Accessing the contents of ptr afterwards
+     * is undefined behavior. */
+    void free(void* ptr);
 
 #ifdef __cplusplus
 }

libs/libc/src/stdlib.cpp

@@ -3,11 +3,6 @@
 #include <sys/syscall.h>
 #include <unistd.h>
 
-#define noreturn __attribute__((noreturn))
-
-#define maybe_unused __attribute__((maybe_unused))
-#define unused __attribute__((unused))
-
 extern "C"
 {
     noreturn void abort()
@@ -25,10 +20,12 @@ extern "C"
     {
         NOT_IMPLEMENTED("atexit");
     }
+
     int atoi(const char*)
     {
         NOT_IMPLEMENTED("atoi");
     }
+
     char* getenv(const char*)
     {
         NOT_IMPLEMENTED("getenv");