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how to malloc for this structure
typedef struct testMsg_ {
    unsigned char opCode;
    unsigned int  Count;
    char    *macsStrList[MAC_ADDR_STR_LEN];
} testMsg_t;

testMsg_t *pInput = (testMsg_t *) malloc(sizeof(testMsg_t) );

testMsg_t *pInput = (testMsg_t *) malloc(sizeof(testMsg_t) );
pInput->macsStrList[0] = (char *) malloc( MAC_ADDR_STR_LEN+1 );
pInput->macsStrList[1] = (char *) malloc( MAC_ADDR_STR_LEN+1 );
pInput->macsStrList[2] = (char *) malloc( MAC_ADDR_STR_LEN+1 );
...

typedef struct testMsg_ {
    unsigned char opCode;
    unsigned int  Count;
    char    macsStrList[NUMBER_OF_MAC_ADDRESSES][MAC_ADDR_STR_LEN];
} testMsg_t;

typedef struct testMsg_ {
        unsigned char opCode;
        unsigned int  Count;
        char    macsStrList[1][MAC_ADDR_STR_LEN];
    } testMsg_t;

testMsg_t *pInput = (testMsg_t *) malloc(sizeof(testMsg_t) + (countOfMacsAddresses * MAC_ADDR_STR_LEN) );

/* assuming we only need macStrList[0] ... [Count-1] */
struct testMsg
{
    unsigned char opCode;
    unsigned int  Count;
    char *macsStrList[];
};

struct testMsg *allocate_testMsg(int count)
{
    char *string_storage;
    struct testMsg *msg;

    size_t size = sizeof(struct testMsg)   /* base object */
                + (count * sizeof(char *)) /* char* array */
                + (count * (MAC_ADDR_STR_LEN+1)) /* char storage */
                ;

    msg = malloc(size);
    msg->Count = count;
    string_storage = (char *)&(msg->macStrList[count]);

    /* note msg->macStrList points to UNINITIALIZED but allocated storage.
       it might be sensible to zero-fill string_storage, depending on how you'll
       initialize it
    */
    for (count=0; count < msg->Count;
         ++count, string_storage += (MAC_ADDR_STR_LEN+1))
    {
        msg->macStrList[count] = string_storage;
    }

    return msg;
}

testMsg_t* myPointer = (testMsg_t*) malloc(sizeof(testMsg_t));