using malloc for char inputs in C

Question

For an assignment, I have to declare a struct as follows:

struct Food
{
    char *name;
    int weight, calories;
} lunch[5] = {
    {
     "apple", 4, 100
    },
    { 
     "salad", 2, 80
    }
};

In my main, I am trying to ask the user for the rest of the inputs to fill the struct to print them out. I figure I would try to use malloc. Would I do something like this?

int main(void) 
{
    char *str1;
    printf("Please enter a food, weight, and calories of the food: ");
    scanf("%s", (char *)malloc(str1));
    return(EXIT_SUCCESS);
}

Answer 1

You should look at your course material, read the books on the reading-list, or ask your lecturer.

If you must do your own research online, you should perhaps try to understand this example and google for similar how-tos.

Generally, you shouldn't use scanf for parsing user input. Its much more straightforward and robust to use getline to retrieve a line of user input, then utilities such as strdup and atoi to extract your values, one per line rather than on the same line, and check for error values and such.

Answer 2

Well ... Not quite.

You just pass the result of malloc() to scanf(), and that function won't return it, you lose the pointer. This is generally a bad idea. Also, investigate what argument malloc() expects, you're not doing it right.

Consider first allocating the memory, using a pointer variable to store it, and then passing the value of that pointer to scanf(). Hint: you already have the pointer variable, in your array.

Also, you shouldn't cast the return value of malloc() in C, and return is not a function, so it shouldn't have parenthesis around its value.

Answer 3

You should rather allocate space for a new instance of Food, then allocate space for name. If all allocations succeed, then you can start asking the user for data.

Answer 4

Here is a small example of how I would go about this. Unfortunatley I cannot test the code and I am learning C myself at the moment, but maybe this will give you some ideas. This would just initialize the instance at lunch[2]. So you should add some sort of loop to fill the other instances.

int main(void) 
{
    lunch[2].name = (char*) malloc(/*The size of the string you want for the 
                                        name of your food. Every character has 
                                        the size of one byte (+'\0' at the end)*/)



    printf("Please enter a name for your food: ");
    scanf("%s", lunch[2].name);
    printf("Please enter the weight of your food: ");
    scanf("%d", &lunch[2].weight;
    printf("Please enter calories of your food: ");
    scanf("%d", &lunch[2].calories);
    return EXIT_SUCCESS;
}

Answer 5

getline, strtok, strdup, atoi

the rest is left as an exercise :-)