C struct with struct pointer

Question

I am trying to define an array using pointer instead of size declaration because I do not know how many elements a map might have. Tried linked list but was not successful. I am sorry if this is a report. I am a newbie please excuse if it looks like a stupid question.

#include<stdio.h>

typedef struct _keyValue
{
    char *key;
    char *value;
} _keyValue;

typedef struct _keyValues
{
    /* _keyValue keyValue[5];  - This works*/
    _keyValue *keyValue;
    int size;
} _keyValues;

_keyValues map;

main()
{
    map.keyValue[0].key     = "Key One";
    map.keyValue[0].value   = "Value One";

    map.keyValue[1].key     = "Key Two";
    map.keyValue[1].value   = "Value Two";

    map.size = 2;

    printf("Key: %s Value: %s", map.keyValue[0].key, map.keyValue[0].value);
}

Answer 1

map.keyValue in your example is an uninitialised pointer. You need to provide storage for the array by allocating memory using malloc

main()
{
    map.keyValue = malloc(sizeof(*map.keyValue) * 2);
    map.size = 2;

    map.keyValue[0].key     = "Key One";
    map.keyValue[0].value   = "Value One";

You can later extend the array using realloc

int newMapSize = ...
_keyValue* temp = realloc(map.keyValue, sizeof(*map.keyValue) * newMapSize);
if (temp == NULL) {
    /* allocation failed.  Handle out of memory error and exit */
}
map.keyValue = temp;
map.size = newMapSize;
// map.keyValue[0..newMapSize-1] are now available

Answer 2

If you don't know how big they will be, then use dynamic allocation. Key functions here are malloc and free .

Here's a suggestion on how you can use your structs if you don't know how big they will be:

First have the following includes in your code:

#include <stdio.h>   /* for printf */ 
#include <string.h>  /* for strcpy, as you cannot directly assign strings to a malloc'd pointer */
#include <stdlib.h>  /* for malloc and free, for managing memory dynamically */

Then we define the size of the keys and values:

const int key_size   = 10; /* let's define how big our keys and values will be */
const int value_size = 25;

And here is how you can use your structs:

map.size = 30; /* decide how many keyValues we will have */

map.keyValue = malloc(sizeof(_keyValue) * map.size);   /* create storage big enough for 30 _keyValue structs
                                                        * malloc will allow you to assign memory to key and treat it as an array
                                                        * malloc assigns memory from the heap
                                                        * equal to the size specified (30), 
                                                        * this can be potentially as large as your computer's memory */

map.keyValue[0].key = malloc(sizeof(char) * key_size); /* let's create a key at position 0 */

strcpy(map.keyValue.key, "some key"); /* copying some values into key */

map.keyValue[0].value = malloc(sizeof(char) * value_size); /* let's create some space for a value for the 0th element */

strcpy(map.keyValue.value, "some value");


... /* you process and work with those values as you see fit */

free(map.keyValue[0]) /* malloc assigned memory needs to be returned to the OS as it's manually managed, 
                       * here we free the element at position 0 we created earlier
                       * if you have more than element here use a loop e.g: 
                       * for (int i = 0; i < map.size; i++) { free(map.KeyValue[i]) }
                       */


free(map.keyValue); /* free the keyValue itself that stored all the keyValue structs*/

One tip, declarations starting with an underscore are discouraged as they are reserved for the language.

Answer 3

#include <stdlib.h>

int main(void){
    map.size = 2;
    map.keyValue = malloc(sizeof(_keyValue)*map.size);

    map.keyValue[0].key     = "Key One";
    map.keyValue[0].value   = "Value One";

    map.keyValue[1].key     = "Key Two";
    map.keyValue[1].value   = "Value Two";


    printf("Key: %s Value: %s", map.keyValue[0].key, map.keyValue[0].value);

    return 0;
}

if I do not know the size

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

int main(void){
    char buff[128] = "";

    map.size = 5;//decide the size temporarily
    map.keyValue = malloc(sizeof(_keyValue)*map.size);

    int count = 0, retv;
    while(1){
        printf("input key : ");
        retv=scanf(" %127s", buff);
        if(retv != 1 || strcmp(buff, "end")==0) break;
        map.keyValue[count].key = strdup(buff);
        printf("input value : ");
        scanf(" %127s", buff);
        map.keyValue[count].value = strdup(buff);
        ++count;
        if(count == map.size)//full
            map.keyValue = realloc(map.keyValue, sizeof(_keyValue)*(map.size+=5));//increase the size
    }

    int i;
    for(i=0;i<count;++i)
        printf("Key: %s Value: %s\n", map.keyValue[i].key, map.keyValue[i].value);

    //dealloc
    for(i=0;i<count;++i){
        free(map.keyValue[i].key);
        free(map.keyValue[i].value);
    }
    free(map.keyValue);

    return 0;
}