Length of a Linked List in C

#include<stdio.h>

#include<conio.h>

#include<alloc.h>

#define newa (struct a*) malloc(sizeof(struct a))

struct a

{

int data;

struct a *next;

};

struct a *list();

void main()

{

struct a *f;

int len;

f = NULL;

clrscr();

f = list();

len = length(f);

printf("\n length = %d",len);

} 

struct a *list()

{

struct a *f,*c,*p;

int tdata;

f = NULL;

printf("\n Enter data ( use 0 to exit ) : ");

scanf("%d",&tdata);

while( tdata != 0 )

{

c = newa;

if( c == NULL)

{

printf("\n Insuf. mem. ");

exit(0);

}

c->data = tdata;

c->next = NULL;

if( f== NULL)

f = c;

else

p->next = c;

p = c;

printf("\n Enter data ( use 0 to exit ) : ");

scanf("%d",&tdata);

} 

return(f);

} 

int length(struct a *f)

{

int len=0;

struct a *t;

if( f == NULL)

return(0);

t = f;

while( t != NULL )

{

len++;

t = t->next;

}

return(len);

}