CSO101: Computer Programming

Lecture – 23

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O.S.L. Bhavana

Memory Allocation

• Constant length arrays may often waste space
• C provides library functions for on-demand memory location
• The library stdlib.h has the memory allocation functions malloc,

calloc, free, realloc etc.

malloc

• Takes the number of bytes of memory to be allocated as input
• Returns a pointer to the first byte in the memory block allocated
• This pointer is a void* pointer which needs to be type casted later
• Ex: int * pa = (int *) malloc(20); //
• allocates a memory block of twenty bytes;
• Returns the address of first byte as a pointer
• This pointer is later type casted as an integer pointer in this example

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• Note that the allocated memory cells may have garbage value
• The input to malloc can also be an expression.

Ex: int n; scanf(“%d”, &n); int * pa = (int *) malloc(n * sizeof(int)); // is valid

malloc

• The allocated memory can then be used to store data using the returned pointer and the dereferencing operator.

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• If there is no sufficient space for allocation a NULL pointer will be returned

calloc

• calloc stands for contigous allocation
• It has two parameters
• number of elements
• Size of each element
• It returns a void pointer for the starting address of a memory block sufficient to store the input number of elements with the given size.
• int * a = (int *) calloc(10, sizeof(int)); //return a pointer to a block of memory to hold 10 elements each of size 4bytes
• Each element in the block allocated by calloc is initialized to 0.
• When there is no sufficient space for allocation a null pointer will be returned

calloc

free()

• Used to free the memory allocated using malloc, calloc, realloc
• Takes a pointer as input

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free

• free should not be used on pointers not returned by malloc, calloc, realloc
• Such use results in errors.

realloc

• Re-allocates memory
• Suppose the amount of memory allocated by malloc/calloc needs to be modified, the realloc function may be used
• realloc takes two parameters:
• a pointer // this pointer should be a pointer returned by malloc/calloc
• Number of bytes to be allocated
• realloc works as follows:
• Allocates a new memory block with the requested number of bytes
• Copies the data of the block (given by the input pointer) into this new block
• Frees the block with starting address given by the pointer

realloc

realloc

• Re-allocates memory
• Suppose the amount of memory allocated by malloc/calloc needs to be modified, the realloc function may be used
• realloc takes two parameters:
• a pointer // this pointer should be a pointer returned by malloc/calloc
• Number of bytes to be allocated
• realloc works as follows:
• Allocates a new memory block with the requested number of bytes
• Copies the data of the block (given by the input pointer) into this new block
• Frees the block with starting address given by the pointer

realloc

getline( )

• getline is a C library to read a full line from input stream (till \n)
• It uses (dynamic) memory allocation to store the characters of line of variable length. (i.e the length of the line isn’t fixed)
• getline code uses realloc to do this memory allocation.

getline( )

• getline takes the following parameters as input
• Address of a pointer given by malloc/calloc
• Address of a variable to store length
• Input stream (will be stdin for now)
• To start with assume the input line will have 12 characters
• size_t len = 13; // a variable to store length
• We now allocate memory required to store these 13 char

char *str = (char *) malloc(13*sizeof(char));

• Call getline(&str, &len, stdin);

getline( )

• getline takes the following parameters as input
• Address of a pointer given by malloc/calloc
• Address of an variable to store length
• Input stream (will be stdin for now)
• Call getline(&str, &len, stdin);
• If the input line has more than 12 characters getline reallocates memory required to store the full line.
• Stores the address of the new re-allocated pointer as address of str.
• It stores the length of re-allocated buffer in the address &len
• This is not the length of the input string. Length of the input string should be calculated using strlen. This is only the buffer length

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getline()