C LinkedList example doesn't compile - c

Question

The following C code is my own way of writing a primitive linked list. It uses a struct called lnode. I know this is not the best/most efficient way to do it but my idea is this: create the base node, use an "iterator" pointer, here q, that points to that last node in the list and then add a new node.
The following code will not compile. I can't find the cause but it hates this line
struct lnode *q= malloc(sizeof(struct lnode));
Any advice on making this idea work? Thanks in advance.
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
struct lnode{
int value;
struct lnode *nextnode;
};
int main(){
struct lnode *startnode = malloc(sizeof(struct lnode));
startnode->value=0;
startnode->nextnode=NULL;
struct lnode *q= malloc(sizeof(struct lnode));
int i = 0;
for(i=0;i<10;i++){
struct lnode *p = malloc(sizeof(struct lnode));
p= q->nextnode;
p->value=i;
p->nextnode=NULL;
q=p;
}
return 0;
}
I would like to point out that I'm a novice. I'm using the Watcom compiler (Why? My computer is old and its all I need for these practice porgrams) The log output is
structure1.c(17): Error! E1063: Missing operand structure1.c(17):
Warning! W111: Meaningless use of an expression structure1.c(17):
Error! E1009: Expecting ';' but found 'struct' structure1.c(17):
Error! E1011: Symbol 'lnode' has not been declared structure1.c(17):
Error! E1011: Symbol 'q' has not been declared structure1.c(17):
Error! E1014: Left operand must be an 'lvalue' structure1.c(19):
I followed the advice given and changed the code the new code is this:
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
struct lnode{
int value;
struct lnode *nextnode;
};
int main(){
struct lnode *startnode = (struct lnode *)malloc(sizeof(struct lnode));
struct lnode *q;
startnode->value=0;
startnode->nextnode=NULL;
q = malloc(sizeof(struct lnode));
doLoop(q);
return 0;
}
void doLoop(struct lnode *q){
int i = 0;
for(i=0;i<10;i++){
struct lnode *p = (struct lnode *)malloc(sizeof(struct lnode));
q->nextnode=p;
p->value=i;
p->nextnode=NULL;
printf("%i, %i\n",p->value,q->value);
q=p;
}
}
I printed the "value" values of each node in the list along with the previous value. It works except the first iteration which gives a weird output.

Answer 1

I suspect the compiler (Microsoft compilers for example) supports C89 standard only, which does not permit the intermingling of code and declarations. Move declaration of q to top of scope:
int main(){
struct lnode *startnode = (struct lnode *)malloc(sizeof(struct lnode));
struct lnode *q
startnode->value=0;
startnode->nextnode=NULL;
q = malloc(sizeof(struct lnode));

Answer 2

The code compiles - http://ideone.com/j6fGe - but the logic is wrong:
struct lnode *p = (struct lnode *)malloc(sizeof(struct lnode));
p= q->nextnode;
Besides the fact that you have a memory leak, I'm sure this is not what you intended.
q->nextnode doesn't point to a valid node, just some random memory. Which you then try to overwrite with p->value=i;.

Answer 3

The error messages is due to the mixing of code and declarations.
Further; You switch p and q around in the for loop.
p = q->next_node; /* here you set p to an undefined area.
* q->next_node is not malloc'd */
p->value = i; /* here you cause undefined / erronous behaviour
* Most probably a SIGSEGV */
So to sum it up, perhaps something like:
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
struct lnode{
int value;
struct lnode *nextnode;
};
int main(void)
{
struct lnode *startnode;
struct lnode *p;
size_t z;
int i;
z = sizeof(struct lnode);
if ((startnode = malloc(z)) == NULL) {
fprintf(stderr, "Unable to malloc %d bytes.\n", z);
return 1;
}
/* Fill list */
p = startnode;
for (i = 0; i < 10; i++) {
if ((p->nextnode = malloc(z)) == NULL) {
fprintf(stderr, "Unable to malloc %d bytes.\n", z);
return 1;
}
p->value = i;
p = p->nextnode;
p->nextnode = NULL;
}
/* Print values */
p = startnode;
while (p->nextnode != NULL) {
printf("value: %2d\n", p->value);
p = p->nextnode;
}
/* Free */
p = startnode;
while (p != NULL) {
p = p->nextnode;
free(startnode);
startnode = p;
}
return 0;
}