How to initialize a pointer outside of main? - c

Question

I am trying to implement nodelist in c:
#include <stdio.h>
#include <stdlib.h>
typedef struct{
int value;
void *next;
} node_t;
void printlist(node_t*head){
for(node_t*i=head;i;i=i->next)
printf("%i\n",i->value);
}
node_t create(int value){
node_t *ret = malloc(sizeof(node_t));
ret->value=value;
ret->next=0;
return *ret;
}
int main(){
int i=0;
node_t*head=0;
for(node_t*tmp;i++<10;head=tmp)
{
*tmp=create(i+100);
tmp->next=head;
}
printlist(head);
}
and in the for loop, I am using node_t* pointer which is assign to already initialized (via malloc) struct outside in create function. But the program emits:
warning: ‘tmp’ may be used uninitialized in this function [-Wmaybe-uninitialized]
*tmp=create(i+100);
Command terminated
So how to assign a dereferenced pointer to already initialized data in c?

Answer 1

When you first enter the loop, tmp doesn't point to anything. Because of that, it's invalid to dereference the pointer.
Also, you have a memory leak in create. You dynamically allocate memory for a node but then return a copy of that node, leaving nothing to point to the allocated memory.
You can fix these issues by returning a pointer from the function:
node_t *create(int value){
node_t *ret = malloc(sizeof(node_t));
ret->value=value;
ret->next=0;
return ret;
}
And subsequently saving that pointer in tmp in the calling function:
tmp=create(i+100);

Answer 2

For starters the function create should return pointer to the created dynamically node. Otherwise you will be unable to free allocated nodes because the function returns a copy of a created node.
So rewrite the function at least like
node_t * create(int value){
node_t *ret = malloc(sizeof(node_t));
ret->value=value;
ret->next=0;
return ret;
}
In the for loop in main you are using an uninitialized pointer tmp. So with the function implementation of create as you defined it the loop invokes undefined behavior due to the underlined assignment
for(node_t*tmp;i++<10;head=tmp)
{
*tmp=create(i+100);
^^^^^^^^^^^^^^^^^^
tmp->next=head;
}
So after changing the function create change the loop like
for(node_t*tmp;i++<10;head=tmp)
{
tmp=create(i+100);
tmp->next=head;
}
Here is your updated program.
#include <stdio.h>
#include <stdlib.h>
typedef struct{
int value;
void *next;
} node_t;
void printlist(node_t*head){
for(node_t*i=head;i;i=i->next)
printf("%i\n",i->value);
}
node_t * create(int value){
node_t *ret = malloc(sizeof(node_t));
ret->value=value;
ret->next=0;
return ret;
}
int main(){
int i=0;
node_t*head=0;
for(node_t*tmp;i++<10;head=tmp)
{
tmp=create(i+100);
tmp->next=head;
}
printlist(head);
}
Its output is
110
109
108
107
106
105
104
103
102
101