Memory

Last modified Monday, 06-Oct-2025 18:06:00 UTC.

Memory allocation
- ANSI C provides several functions for heap manipulation, declared in stdlib.h.
- void *malloc(size_t size)
  - malloc returns a pointer to a block of memory whose size (as measured by sizeof) is size, or NULL if unsuccessful.
  - The memory is uninitialised.
  - Returns a void pointer, which must be coerced to an appropriate type.
  - void free(void *space)
    - Frees memory assigned by malloc (and realloc and calloc)
    - space must be a pointer returned by such a function.
  - MallocPrimitive.c - Program that mallocs an int
  - MallocFunction.c - Program that uses a function for malloc
  - RaggedArray.c - Dynamic ragged array
- void *realloc(void *old,size_t size)
  - realloc changes the size of a block of memory previously allocated with malloc or calloc whilst preserving the contents.
  - The new size is size and old points to the currently allocated block.
  - Returns a pointer to the new block or NULL if unsuccessful, in which case old is unchanged.
  - DynamicArray.c - Program that reallocs an array
  - If the new space is larger, the extra memory is uninitialised.
  - The new block may be in a different place - beware if you have multiple pointers to the old block.
- Memory leaks
  - If not freed, allocated space remains allocated until the program ends. If there is no pointer to it, it is useless; this is called a memory leak.
  - MemoryLeak.c - Program that leaks
  - The valgrind utility is useful for checking for memory leaks.
Memory Operations
- Defined in memory.h and string.h
- void *memchr(void *s,int c,size_t n) - Search for a character in a buffer.
- int memcmp(void *s1,void *s2,size_t n) - Compare two buffers (bytewise version of strncmp).
- void *memcpy(void *dest,void *src,size_t n) - Copy one buffer into another (bytewise version of strncpy).
- void *memmove(void *dest,void *src,size_t n) - Move a number of bytes from one buffer to another. Same as memcpy except the buffers may overlap.
- void *memset(void *s,int c,size_t n) - Set all bytes of s buffer to c.

Exercises

Write a program last that prints the last n lines of its text input. By default n should be 5, but your program should allow an optional argument so that last -n prints out the last n lines, where n is any integer. Your program should make the best use of available storage. (Input of text could be by reading a file specified from the command or reading a file from standard input)
Write a program that reads a number that says how many integer numbers are to be stored in an array, creates an array to fit the exact size of the data and then reads in that many numbers into the array.
Write a program to sort a sequence of numbers using a binary tree (Using Pointers). A binary tree is a tree structure with only two (possible) branches from each node. Each branch then represents a false or true decision. To sort numbers simply assign the left branch to take numbers less than the node number and the right branch any other number (greater than or equal to). To obtain a sorted list simply search the tree in a depth first fashion. Your program should:
- Create a binary tree structure.
- Create routines for loading the tree appropriately.
- Read in integer numbers terminated by a zero.
- Sort numbers into numeric ascending order.
- Print out the resulting ordered values, printing ten numbers per line as far as possible.
Typical output should be
```
The sorted values are:
2  4  6  6  7  9 10 11 11 11
15 16 17 18 20 20 21 21 23 24
27 28 29 30
     
```

Exam Style Questions

Any of the short exercises.
Write a program that declares an array of 10 pointers to char, then reads 10 lines from the user, dynamically allocates enough memory for each line after it is read, and makes successive members of the array point to the dynamically allocated memory. When done, the program must then release all the dynamically allocated memory.
Explain what realloc does, including the cases when the new size does not equal the old size.
Explain the difference between memcpy and memmove.