Write a C program to implement a Generic Linked List. 

Discuss it!          

Here is a C program which implements a generic linked list. This is also one of the very popular interview questions thrown around. The crux of the solution is to use the void C pointer to make it generic. Also notice how we use function pointers to pass the address of different functions to print the different generic data. 


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

typedef struct list { 
void *data; 
struct list *next; 
} List; 

struct check { 
int i; 
char c; 
double d; 
} chk[] = { { 1, 'a', 1.1 }, 
{ 2, 'b', 2.2 }, 
{ 3, 'c', 3.3 } }; 

void insert(List **, void *, unsigned int); 
void print(List *, void (*)(void *)); 
void printstr(void *); 
void printint(void *); 
void printchar(void *); 
void printcomp(void *); 

List *list1, *list2, *list3, *list4; 

int main(void) 
{ 
char c[] = { 'a', 'b', 'c', 'd' }; 
int i[] = { 1, 2, 3, 4 }; 
char *str[] = { "hello1", "hello2", "hello3", "hello4" }; 

list1 = list2 = list3 = list4 = NULL; 

insert(&list1, &c[0], sizeof(char)); 
insert(&list1, &c[1], sizeof(char)); 
insert(&list1, &c[2], sizeof(char)); 
insert(&list1, &c[3], sizeof(char)); 

insert(&list2, &i[0], sizeof(int)); 
insert(&list2, &i[1], sizeof(int)); 
insert(&list2, &i[2], sizeof(int)); 
insert(&list2, &i[3], sizeof(int)); 

insert(&list3, str[0], strlen(str[0])+1); 
insert(&list3, str[1], strlen(str[0])+1); 
insert(&list3, str[2], strlen(str[0])+1); 
insert(&list3, str[3], strlen(str[0])+1); 

insert(&list4, &chk[0], sizeof chk[0]); 
insert(&list4, &chk[1], sizeof chk[1]); 
insert(&list4, &chk[2], sizeof chk[2]); 

printf("Printing characters:"); 
print(list1, printchar); 
printf(" : done\n\n"); 

printf("Printing integers:"); 
print(list2, printint); 
printf(" : done\n\n"); 

printf("Printing strings:"); 
print(list3, printstr); 
printf(" : done\n\n"); 

printf("Printing composite:"); 
print(list4, printcomp); 
printf(" : done\n"); 

return 0; 
} 

void insert(List **p, void *data, unsigned int n) 
{ 
List *temp; 
int i; 

/* Error check is ignored */ 
temp = malloc(sizeof(List)); 
temp->data = malloc(n); 
for (i = 0; i < n; i++) 
*(char *)(temp->data + i) = *(char *)(data + i); 
temp->next = *p; 
*p = temp; 
} 

void print(List *p, void (*f)(void *)) 
{ 
while (p) 
{ 
(*f)(p->data); 
p = p->next; 
} 
} 

void printstr(void *str) 
{ 
printf(" \"%s\"", (char *)str); 
} 

void printint(void *n) 
{ 
printf(" %d", *(int *)n); 
} 

void printchar(void *c) 
{ 
printf(" %c", *(char *)c); 
} 

void printcomp(void *comp) 
{ 
struct check temp = *(struct check *)comp; 
printf(" '%d:%c:%f", temp.i, temp.c, temp.d); 
}