Memory and Files

Memory Management

Memory Layout

stack: funtion call stack, local variables

heap: dynamically allocated data objects

initialized data: global or static variables (initialized)
uninitialized data: global or static variables (uninitialized) a.k.a BSS (block started by symbol)

text: stores source code

// use size command to get memory layout
> size test.exe

Dynamic Allocation Fucntions

Reference

  • <stdlib.h> functions: returns pointer to allocated block or NULL.
  1. malloc(): memory allocation, fills with garbage
Syntax: void* malloc(size_t size)

Usage: int* p = (int*) malloc(5 * sizeof(int));
  1. calloc(): continuous allocation of n objects, each has 0
Syntax: void* calloc(n, size_t size)

Usage:int* p = (int*) calloc (5, sizeof(int))
  1. realloc(): Reallocates memory block of a previously allocated block
Syntax: void* realloc(void* p, size_t size)

Usage:int* pNew = (int*) realloc (p, 10 * sizeof(int)) 		// more memory was allocated, may not start from the same location
  1. free(): frees memory. Takes only one argument (pointer) since above functions implicitly store size information that free() can read.
free(p);

Avoids memory leaks.

Tips

  • calloc() is just malloc() followed by memset() to 0

  • we can free allocated memory by free() or by realloc(0, size)

  • the above two are equivalent in outcome but not at all interchangeable because of optimization and surprises.

  • Other functions

  1. memset(): originally made for setting characters but can be used to set memory to only 0.
Syntax: void memset(void* ptr, int c, size_t size)

Usage: memset(arr, 0, sizeof arr);

File Management

A file is always terminated by an EOF character. And file functions check for this character often.

Operations

To open a file: fopen(const char* filename, const char* mode)
To read a file: fscanf() or fgets()
To write to a file: fputs() or fprintf()
To close a file: fclose(FILE* fp)

// Returns NULL if erorr

Checking end of file (returns non-zero value only if EOF is reached): int feof(FILE* fp)

Return file pointer position: long ftell(FILE* fp)

Going to specific position in file: void rewind(FILE* fp)
									int fseek(FILE *fp, long int offset, int whence)
										whence constants = SEEK_SET (beginning of file)	
														   SEEK_CUR (current position of file pointer)
														   SEEK_END  (end of file)

Set file pointer position: int fsetpos(FILE *fp, const fpos_t *position)


Rename file: int rename (const char *old_name, const char *new_name)

Remove file: int remove(FILE* fp)

Create temporary files: FILE *tmpfile(void) 
// Perform file operations on it and closes automatically, no need to use fclose()
// rewind() can't tell if operation was a success or otherwise

fget/fput functions

int fgetc(FILE *pointer)
int fputc(int char, FILE *pointer)

char* fgets(char *str, int n, FILE *stream)		//safer than gets() since it specifies maximum number of characters to be read
char* gets (char *str)

File Writing Modes

  • Exclusive create-and-open mode (wx): fopen() returns NULL if file already exists or could not open.