Thread: Questions About Stack

Yes it's declared, but the error message is about initialisation, not declaration.

Code:

#include<stdio.h>
#include<stdlib.h> /*!! malloc.h is obsolete, use stdlib for malloc */

struct snode {
    int data;
    struct snode *next;
};

void begin(struct snode *head);
void push(struct snode *head, int t);
void pop(struct snode *head, int *d);

int main(void)  /*!! main returns int, not void */
{
    struct snode *first = NULL; /*!! make sure it's initialised */
    begin(first);
    int i, x;
    for (i = 1; i < 10; i = i + 2) {
        push(first, i);
    }
    while (first->next != NULL) {
        pop(first, &x);
        printf("%d", x);
    }

    return 0;
}

void begin(struct snode *head)
{
    head = malloc(sizeof(struct snode));
    head->next = NULL;
}

void push(struct snode *head, int t)
{
    struct snode *p;
    p = malloc(sizeof(struct snode));
    p->data = t;
    p->next = head->next;
    head->next = p;
}

void pop(struct snode *head, int *d)
{
    struct snode *p;
    p = head->next;
    head->next = p->next;
    *d = p->data;
    free(p);
}

Be consistent about your indentation.
Indent style - Wikipedia, the free encyclopedia
Note that multiple closing braces on the same line are particularly hard to follow.