C string input overflows other string input - c

Question

I'm doing a simple console type command system, and inputting a command will scanf an integer and then will scanf a string, but the contents of the second string overflows the original string
while (exit == 0) {
scanf("%s", input);
if (strcmp(input, "parent") == 0) {
free(input);
ptemp = malloc(sizeof(node_p));
printf("Id: ");
scanf("%d", &ptemp->itemid);
printf("\nElement:");
scanf("%s", ptemp->element);
add_parent_node(parent, ptemp->itemid, ptemp->element);
free(ptemp);
}
}
ptemp is a pointer to a struct containing:
int itemid;
char *element;
I've tried using arrays with predefined size, but nothing seems to work...

Answer 1

Someone that made a comment about nothing overflowing is correct. What you were missing were (in laymans terms) a reservation for characters. Declaring something as char* instead of char[xx] means that you're prepared to reference another part of memory that you're allowed to manipulate with your characters. To keep things simple, I rewritten your code so your program works. Keep in mind that this code relies on users entering strings that are less than 100 to 200 characters long. Feel free to increase the number in square brackets if you need more characters.
I also made an add_parent_node function to verify that the data processing works.
If you want to get a little paranoid and you feel your systems implementation of scanf is screwy, then you can place the following under the while statement:
memset(ptemp,0,sizeof(ptemp));
What that does is floods the entire struct with null characters. This means the value of itemid would be zero since zero is null, and element would be 200 null characters.
Here's the code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct{
int itemid;
char element[200]; //fixed array of chars to actually store a string
}mycollection;
void add_parent_node(char* parentnodename,int itemid,char* element){
printf("Added node as follows\n");
printf("Parent: %s\n",parentnodename);
printf("Item ID: %d\n",itemid);
printf("Element: %s\n\n",element);
}
int main(){
char input[100]; //limit command to 99 characters
mycollection ptemp[1];
while(1){ //while(1) = endless loop
printf("\nEnter command: ");
scanf("%s", input);
if (strcmp(input, "parent") == 0) {
printf("\nId: ");
scanf("%d", &ptemp->itemid);
printf("\nElement:");
scanf("%s", ptemp->element);
add_parent_node("im_the_parent", ptemp->itemid, ptemp->element);
}
if (strcmp(input, "exit") == 0) {
return 0; //return 0 = exit
}
}
}

Answer 2

If you don't want to change anything outside your while loop I think this is what you can do.
while (exit == 0) {
scanf("%s", input);
if (strcmp(input, "parent") == 0) {
if(0 == ptemp ){
/* Allocate memory only once and reuse it.*/
ptemp = malloc(sizeof(node_p));
/* Allocate memory for element. */
ptemp->element = malloc(sizeof(char) * 1024 /* Max string len + 1 */)
}
printf("Id: ");
scanf("%d", &ptemp->itemid);
printf("\nElement:");
scanf("%s", ptemp->element);
add_parent_node(parent, ptemp->itemid, ptemp->element);
}
else if (strcmp(input, "exit") == 0) {
if(0 != ptemp){
/* If memory is allocated for ptemp, free it. */
if(0 != ptemp->element){
free(ptemp->element);
ptemp->element = 0;
}
free(ptemp);
ptemp = 0;
}
free(input);
input = 0;
exit = 1;
break;
}
}