This program demonstrates a simple doubly linked list used to store names and phone numbers. There are methods to add, search, edit, delete, and print records. This program ignores user input validation since pointer and memory manipulation are the key goals.

listExample.c

/*
 * Author: Tad DeVries <tad@splunk.net>
 * Date: 2013/02/27
 * 
 * Language: C
 * 
 * Description: This program demonstrates a simple doubly linked list 
 * used to store names and phone numbers. There are methods to add, search,
 * edit, delete, and print records. This program ignores user input validation
 * since pointer and memory manipulation are the key goals.
 * 
 * 
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/*
 * Structure: account
 *
 * Description: This structure contains a template for records.
 * Each record holds a Name, Phone number, and pointer to the next
 * item in the list.
 *
 */
struct account {
	int accountNumber;
	char *accountName;
	char *accountPhone;
	struct account *prev;
	struct account *next;
};

/* Linked List pointers to head and current items */
struct account *head = NULL;
struct account *tail = NULL;

/*
 * Function: flushInput
 *
 * Description: The will clear everything from STDIN.
 *
 * NOT USED
 */
//void flushInput() {
//	int c;
//	
//	while(c != '\n' && c != EOF) {
//		c = getchar();
//	}
//}

/*
 * Function: printMenu
 *
 * Description: Prints a menu to the user and asks for letter
 * choices based on the menu.
 *
 * Return: A single Character from the user;
 */
char printMenu() {
	char input;

	printf("*** Menu ***\n");
	printf("A: Add Record\n");
	printf("P: Print Records\n");
	printf("F: Find Record\n");
	printf("E: Edit Record\n");
	printf("D: Delete Record\n");
	printf("Q: Quit\n");
	printf("\nEnter Choice: ");
	input = getchar(); //get character input
	getchar(); //flush the newline/carrage return

	return input;
}

/*
 * Function: getInput
 *
 * Description: Grabs an input string from the user and stores it
 * into a dynamically allocated memory location.
 *
 * Variables: char* - String to output to the user describing what to input
 *
 * Return: A pointer to the string location in memory
 */ 
char *getInput(char *outputString) {
	char *userInput = NULL;
	size_t inputBuffer = 0;
	int inputCount = 0;

	//allocate the userInput
	//userInput = (char *)malloc(inputBytes+1);
	//not needed with the getline function since it
	//automatically calls realloc to manage the memory

	printf("%s", outputString);
	inputCount = getline(&userInput, &inputBuffer, stdin);

	//remove the newline character from the string
	userInput[inputCount-1] = '\0';

	//return the user input
	return userInput;
}
	
/*
 * Function: insertRecord
 *
 * Descprition: Allocates a new record then stores the user input
 * for the record name and phone number. This new item is then added to
 * the end of the list.
 */ 
void insertRecord(){
	struct account *newAccount = NULL;

	//allocate memory for the new record
	newAccount = (struct account*)malloc(sizeof(struct account));

	//get input and store it into each record variable
	newAccount->accountName = getInput("Input Name: ");
	newAccount->accountPhone = getInput("Input Phone Number: ");
	newAccount->next = NULL;

	//set this item as the start of a new list or
	//add it to the end of an existing list
	if(head == NULL) {
		newAccount->accountNumber = 1;
		newAccount->prev = NULL;
		head = newAccount;
		tail = head;
	}
	else {
		newAccount->accountNumber = tail->accountNumber + 1;
		newAccount->prev = tail;
		tail->next = newAccount;
		tail = tail->next;
	}
}

/*
 * Function: printOneRecord
 *
 * Description: Prints the record associated with the pointer passed in
 */
void printOneRecord(struct account *currentPointer){
	printf("*** Record: %d ***\n", currentPointer->accountNumber);
	printf("Name: %s\n", currentPointer->accountName);
	printf("Phone: %s\n", currentPointer->accountPhone);
}

/*
 * Function: printRecords
 *
 * Description: Traverses the linked list and prints each records contents
 */
void printAllRecords() {
	struct account *listPointer = NULL;

	//if the head pointer is NULL then it's an empty list
	//else print each record in order
	if(head == NULL){
		printf("No Records found\n");
	}
	else {
		//set a temp pointer to traverse the list with
		listPointer = head;

		printf("\n");

		//move through the list printing each record until
		//we reach the end
		while(listPointer != NULL) {
			printOneRecord(listPointer);
			listPointer = listPointer->next;
		}

		printf("\n");
	}
}

/*
 * Function: searchRecords
 *
 * Description: Searchs the list comparing each records name entry
 * with a search string. When a match is found the pointer to that 
 * entry is returned.
 *
 * Return: pointer to the found list entry or NULL if nothing is found
 */
struct account *searchRecords(char* searchName){
	struct account *listPointer = NULL;
	int temp;

	//if head is null then the list is empty
	if(head == NULL) {
		printf("Records empty\n");
	}
	else {
		listPointer = head; //start at the beginning
		
		//loop through the list looking for a match
		//if a match is found return the pointer
		//if not print an error and return NULL
		while(listPointer != NULL) {
			if(strcmp(listPointer->accountName, searchName) == 0) {
				return listPointer;
			}
			else {
				listPointer = listPointer->next;
			}
		}
		printf("Record %s not found\n", searchName);
	}
	
	return NULL;
}

/*
 * Function: editOneRecord
 *
 * Description: Allows a single record to be changed. This functions is 
 * extremely simplfied and lacks any serious error correction. It is intended
 * as a proof of concept only.
 */
void editOneRecord(struct account *listPointer) {
	char menuInput = '\0';	
	char *newName = NULL;
	char *newPhone = NULL;
	
	//print out the current record information
	printf("Current Record Information\n");
	printOneRecord(listPointer);

	//loop to display menu and process options
	do {
		printf("*** Edit Menu ***\n");
		printf("N: Change Record Name\n");
		printf("P: Change Record Phone Number\n");
		printf("S: Save Changes\n");
		printf("Selection: ");
		menuInput = getchar();
		getchar();

		switch(menuInput) {
			case 'n':
			case 'N': //change get a new name
				printf("Changing Name...\n");
				newName = getInput("Enter New Name: ");	
				break;
			
			case 'p':
			case 'P': //get a new phone number
				printf("Changing Phone Number...\n");
				newPhone = getInput("Enter New Phone Number: ");
				break;

			case 's': //verify that new items have been stored
			case 'S': //and save them to the record
				printf("Saving Record...\n");
				if(newName != NULL) {
					listPointer->accountName = newName;
				}

				if(newPhone != NULL) {
					listPointer->accountPhone = newPhone;
				}
				break;

			default:
				printf("Invalid Input\n");
		}
	}
	while(menuInput != 's' && menuInput != 'S'); 
}

/*
 * Function: deleteOneRecord
 * 
 * Description: Delete record entry referenced by pointer.  
 */
void deleteOneRecord(struct account *listPointer) {

	if(head == tail) {
		//delete single item list
		head = NULL;
		tail = NULL;
	}
	else if(listPointer == head) {
		//delete head
		head = listPointer->next;		
		head->prev = NULL;
	} 
	else if(listPointer == tail) {
		//delete tail
		tail = listPointer->prev;
		tail->next = NULL;
	}
	else {
		//delete non head/tail record
		listPointer->next->prev = listPointer->prev;
		listPointer->prev->next = listPointer->next;
	}	

	//free the memory location
	free(listPointer);
}

/*
 * Function: cleanupList
 * 
 * Description: Moves through a linked list freeing all memory locations
 */
void cleanupList() {
	struct account *tempPointer = NULL;

	//loop through the list clearing each record
	while(head != NULL) {
		tempPointer = head;
		head = head->next;
		free(tempPointer);
	}
}
		

/*
 * Function: main
 * 
 * Description: Handles menu options and calls all worker function
 */
int main() {
	char menuInput = '\0';
	struct account *tempPointer = NULL;
	char *searchName;

	//loop through the program until the user selects Q
	do {
		//get the user input
		menuInput = printMenu();	

		//work on the user input
		switch(menuInput) {
			case 'a':
			case 'A': //add a record to the list
				printf("Adding User...\n");
				insertRecord();	
				break;	
	
			case 'p':
			case 'P': //print all records in the list
				printf("Printing Records...\n");	
				printAllRecords();
				break;	

			case 'f':
			case 'F': //search by name matching
				printf("Finding Record...\n");
				if(head == NULL) {
					printf("Empty List\n");
				}
				else {
					searchName = getInput("Search Name: ");
					tempPointer = searchRecords(searchName);
					
					//if the pointer is null, nothing was returned 
					//so don't try to print
					if(tempPointer != NULL) {
						printOneRecord(tempPointer);
						tempPointer = NULL;
					}
				}
				break;	

			case 'e':
			case 'E': //edit a record
				printf("Editing Record...\n");	
				if(head == NULL) {
					printf("Empty List\n");
				}
				else {
					searchName = getInput("Search Name: ");
					tempPointer = searchRecords(searchName);
					
					//if the pointer is null, nothing was returned 
					//so don't try to print
					if(tempPointer != NULL) {
						editOneRecord(tempPointer);
						printOneRecord(tempPointer);
						tempPointer = NULL;
					}
				}
				break;	

			case 'd':
			case 'D'://delete a record
				printf("Deleting Record...\n");	
				if(head == NULL) {
					printf("Empty List\n");
				}
				else {
					searchName = getInput("Delete Name: ");
					tempPointer = searchRecords(searchName);
					
					//if the pointer is null, nothing was returned 
					//so don't try to print
					if(tempPointer != NULL) {
						deleteOneRecord(tempPointer);
						tempPointer = NULL;
					}
				}
				break;	

			case 'q':
			case 'Q': //quit the program loop
				printf("Goodbye!\n");	
				cleanupList();
				break;	

			default:
			 	printf("ERROR: Invalid Menu Option\n");
				break;	
		}
	}
	while(menuInput != 'q' && menuInput != 'Q');

	return 0;
}