Merge sort 2 sorted linked list

My coding skill is pretty rusty and moreover, I have never paid much attention to writing elegant code. I want to start on this. Problem is to merge 2 sorted linked list. Algorithmically this is trivial, but looks a good problem to practice coding.

struct node
{
 char *data;
 struct node *nextp;
}

typedef struct node node;
typedef struct node list; //I do 2 typedefs here to signify when conceptually I
//mean a whole list and when just a single node.is this a good practice? or only  
//one that confuses.

node * add(node *listp, char *data)
{
 node * newp=(node *)malloc(sizeof(node));
 assert(newp != NULL);
 newp->data=(char *)malloc(sizeof(strlen(data));
 assert(newp->data != NULL);
 strcpy(newp->data,data); // allocate space in newp->data . should there be +1?
 newp->next=NULL;
 if(listp==NULL) 
  listp=nextp ;
 else
  listp->nextp=newp;
 return newp;
}

list *mergelist(list * list1p, list *list2p)
{
// initialize mergedlistp;
node dummy;
node* mergedlistendp=&dummy;
list* leftlistp = NULL;

if (list1p == NULL) leftlistp=list2p;
if(list2p == NULL) leftlistp = list1p;

if(list1p != NULL && list2p!= NULL)
{
while(1)
{

 if(strcmp(list1p -> data,list2p -> data) data);
  list1p=list1p -> next;
  if(list1p == NULL)
  {leftlistp=list2p; break;}
 }
 else
 {
  mergedlistendp=add (mergedlistendp,list2p->data);
  list2p=list2p -> next;
  if(list2p == NULL)
  {leftlistp=list1p; break;}
 }

}

for(leftlistp!=NULL;leftlistp = leftlistp->next) 
add(mergedlistendp,leftlistp->data);

return mergedlistendp; // I have to return mergedlistp here (i.e. head of list)
//. How to store address of mergedlistendp the first time it is assigned a value?
}

I would like my code to be reviewed.Any feedback on naming, program flow, correctness, better code, indenting, idioms etc is appreciated. Please advise on how corner cases are handled more elegantly.

You should have added error checking:

node * newp=(node *)malloc(sizeof(node));
 newp->data=(char *)malloc(sizeof(strlen(data));

malloc can return NULL if there wont be enough Virtual Adress Space. Would be great if function return error code.

add {} it could bite your next time

if(listp==NULL) 
   listp=nextp ;
  else
   listp->nextp=newp;
   //next live goes here, looks like next statment in else, but it's not.

Use PascalCase or camelCase

// initialize mergedlistp;
node* mergedlistendp=NULL; // mergedList is a better name
list* leftlistp = NULL;

Stick to one style, once you have:

if (list1p == NULL) leftlistp=list2p;
if(list2p == NULL) leftlistp = list1p;

another one

if(listp==NULL) 
   listp=nextp ;

or

if(list2p == NULL)
   {leftlistp=list1p; break;}

One statment per line

if(list2p == NULL)
 {leftlistp=list1p; break;} // put break in the next line.

StackExchange Code Review Q#2693, answer score: 6