Linked list C program produces no output

Firstly, you are attempting to define functions inside main() function. BuildOneTwoThree is defined inside main, while Length appears to be defined inside BuildOneTwoThree. Why did you do it that way? C language has no such feature. You cannot nest function definitions. All functions have to be defined separately and independently at file level.

Secondly, you never call any of the functions you defined. Your main() function does nothing besides return 0;.

Take a look at any valid C program, and you should figure out immediately how functions should be defined. Note that while some compilers support nested function definitions as a non-standard extension, you still have to call your functions at some point.

I think this sort of post belongs somewhere else (stackoverflow maybe?). In any case, you are defining a function BuildOneTwoThree() and you do not call it, so it does not output anything.

Try using standard C and formatting your code so it's readable.

If you decompose things, you get to something like this:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

typedef struct LLNODE
{
  struct LLNODE *next ;

  int payload ;

} LLNODE ;

LLNODE *create_node( int value )
{
  LLNODE *p = calloc( 1 , sizeof(LLNODE) ) ;

  p->next    = NULL  ;
  p->payload = value ;

  return p ;
}

LLNODE *create_list( int start_value , int count )
{
  LLNODE *root = NULL ;
  LLNODE *tail = NULL ;

  for ( int i = 0 , value = start_value ; i < count ; ++i )
  {
    LLNODE *node = create_node( value++ ) ;

    if ( root == NULL )
    {
      root = tail = node ;
    }
    else
    {
      tail->next = node ;
      tail = node ;
    }

  }

  return root ;
}

int compute_length( LLNODE *root )
{
  int len = 0 ;

  for ( LLNODE *p = root ; p != NULL ; p = p->next )
  {
    ++len ;
  }

  return len ;
}

int main( int argc, char *argv[] )
{
  LLNODE *root   = create_list( 101 , 50 ) ;
  int     length = compute_length( root ) ;

  printf( "list length is %d\n" , length ) ;

  int i = 0 ;
  for ( LLNODE *p = root ; p != NULL ; p = p->next )
  {
    printf( "Node #%d.\tcontains the value %d\n" , ++i , p->payload ) ;
  }

  return 0 ;
}

you're missing a curly brace to end the BuildOneTwoThree() function, then you must call that function in main. Try this:

#include <stdio.h>
#include <stdlib.h>
struct node {

int data;
struct node* next;

};

struct node* BuildOneTwoThree() {
struct node* head = NULL;
            struct node* second = NULL;
            struct node* third = NULL;
            head = malloc(sizeof(struct node)); // allocate 3 nodes in the heap
            second = malloc(sizeof(struct node));
            third = malloc(sizeof(struct node));
            head->data = 1; // setup first node
            head->next = second; // note: pointer assignment rule
            second->data = 2; // setup second node
            second->next = third;
            third->data = 3; // setup third link
            third->next = NULL;
            return head;
        }

int Length(struct node* head) {
            struct node* current = head;
            int count = 0;
            while (current != NULL) {
                count++;
                current = current->next;
                }
            return count;
        }

int main(){
    struct node* newNode = BuildOneTwoThree();
    printf("%d",Length(newNode));
}