[libc][syscall] add detailed function comments for memory management

Author: ChengyangMa

components/libc/compilers/armlibc/syscall_mem.c

@@ -27,6 +27,21 @@
     #pragma import(__use_no_heap)
 #endif /* __CC_ARM */
 
+/**
+ * @brief Allocate memory block
+ *
+ * Allocates a block of size bytes of memory, returning a pointer to the
+ * beginning of the block. The content of the newly allocated block of
+ * memory is not initialized, remaining with indeterminate values.
+ *
+ * @param[in] n the size of the memory block, in bytes.
+ *
+ * @return On success, a pointer to the memory block allocated by the function.
+ *         If the system is configured without heap (RT_USING_HEAP is not defined),
+ *         the function will assert and return RT_NULL.
+ *
+ * @note The returned pointer is always suitably aligned for any built-in type.
+ */
 void *malloc(size_t n)
 {
 #ifdef RT_USING_HEAP
@@ -38,6 +53,28 @@ void *malloc(size_t n)
 }
 RTM_EXPORT(malloc);
 
+/**
+ * @brief Reallocate memory block
+ *
+ * Changes the size of the memory block pointed to by rmem.
+ * The function may move the memory block to a new location
+ * (whose address is returned by the function).
+ * The content of the memory block is preserved up to the
+ * lesser of the new and old sizes, even if the block is
+ * moved to a new location. If the new size is larger,
+ * the value of the newly allocated portion is indeterminate.
+ *
+ * @param[in,out] rmem pointer to a memory block previously allocated with
+ *             malloc, calloc or realloc to be reallocated.
+ *             If this is RT_NULL, a new block is allocated and
+ *             a pointer to it is returned by the function.
+ * @param[in] newsize new size for the memory block, in bytes.
+ *
+ * @return A pointer to the reallocated memory block, which may be either
+ *         the same as the rmem pointer or a new location.
+ *         If the system is configured without heap (RT_USING_HEAP is not defined),
+ *         the function will assert and return RT_NULL.
+ */
 void *realloc(void *rmem, size_t newsize)
 {
 #ifdef RT_USING_HEAP
@@ -49,6 +86,21 @@ void *realloc(void *rmem, size_t newsize)
 }
 RTM_EXPORT(realloc);
 
+/**
+ * @brief Allocate and zero-initialize array
+ *
+ * Allocates a block of memory for an array of nelem elements, each of them
+ * elsize bytes long, and initializes all its bits to zero.
+ * The effective result is the allocation of a zero-initialized memory block
+ * of (nelem*elsize) bytes.
+ *
+ * @param[in] nelem number of elements to allocate.
+ * @param[in] elsize size of each element.
+ *
+ * @return On success, a pointer to the memory block allocated by the function.
+ *         If the system is configured without heap (RT_USING_HEAP is not defined),
+ *         the function will assert and return RT_NULL.
+ */
 void *calloc(size_t nelem, size_t elsize)
 {
 #ifdef RT_USING_HEAP
@@ -60,6 +112,19 @@ void *calloc(size_t nelem, size_t elsize)
 }
 RTM_EXPORT(calloc);
 
+/**
+ * @brief Deallocate memory block
+ *
+ * A block of memory previously allocated by a call to malloc, calloc or realloc
+ * is deallocated, making it available again for further allocations.
+ *
+ * @param[in] rmem pointer to a memory block previously allocated with malloc,
+ *             calloc or realloc to be deallocated. If a null pointer is
+ *             passed as argument, no action occurs.
+ *
+ * @note If the system is configured without heap (RT_USING_HEAP is not defined),
+ *       the function will assert.
+ */
 void free(void *rmem)
 {
 #ifdef RT_USING_HEAP