Passing array of struct to function in C

Question

Do I need a * in front of my struct array (in the function arguments) when passing the struct to a function by reference? The reason I am thinking we don't is because an array is essential going to pass the address in which the first object is located.

I feel like I just got lucky my code is working:

#include <stdio.h>

struct member {
    char lastName[30];
    char gender;
    int age;
};

void readAndUpdate(struct member *people[]); 

// begin main function
int main(void){

    struct member *people[30]; 

    readAndUpdate(people);


} // end main function

// begin function which reads a .dat file and propogates the array with the data in the .dat file
void readAndUpdate(struct member *people[]){



}

I worked on my code some more from the help of the commentors and I have the following which works properly. I accidentally created an array of pointers.

#include <stdio.h>
#define MAXPEOPLE 3 

struct member {
    char lastName[30];
    char gender;
    int age;
};

void readAndUpdate(struct member *person, size_t maxpeople); 
void populateDatFile();
void displayMembers(struct member *person, size_t maxpeople);

// begin main function
int main(void){

    struct member people[2]; 

    populateDatFile(); // program will first populate the .dat file with the given specs

    readAndUpdate(people, MAXPEOPLE);

    printf("The data was read and input as follows:\n\n");
    displayMembers(people, MAXPEOPLE);


} // end main function

// function which displays the entire array of struct members
void displayMembers(struct member *person, size_t maxpeople){

    int i=0;

    for (i=0;i<3;i++){

    printf("%s ", person[i].lastName);
    printf("%c ", person[i].gender);
    printf("%d ", person[i].age);
    printf("\n");
    }

} // end displayMembers function

// function which loads the .dat file with hardcoded structs
void populateDatFile(){

    struct member person1={"Gates", 'M', 60};
    struct member person2={"Jobs", 'M', 55};
    struct member person3={"Jane", 'F', 45};    

    FILE *file;
    file = fopen("question3.dat","w");
    if(file == NULL)
        printf("question3.dat cannot be opened!\n");
    else
        printf("question3.dat was opened successfully.\n");

    fprintf(file, "%s %c %d\n", person1.lastName, person1.gender, person1.age);
    fprintf(file, "%s %c %d\n", person2.lastName, person2.gender, person2.age);
    fprintf(file, "%s %c %d\n", person3.lastName, person3.gender, person3.age);

    fclose(file);
} // end function populateDatFile

// begin function which reads a .dat file and propogates the array with the data in the .dat file
void readAndUpdate(struct member *person, size_t maxpeople){

    int i=0;

    FILE *file;
    file = fopen("question3.dat","r");
    if(file == NULL)
        printf("question3.dat cannot be opened!\n");
    else
        printf("question3.dat was opened successfully.\n");

    fscanf(file, "%s", &person->lastName);
    fscanf(file, " %c", &person->gender);
    fscanf(file, "%d", &person->age);

    fscanf(file, "%s", &person[1].lastName);
    fscanf(file, " %c", &person[1].gender);
    fscanf(file, "%d", &person[1].age);

    fscanf(file, "%s", &person[2].lastName);
    fscanf(file, " %c", &person[2].gender);
    fscanf(file, "%d", &person[2].age);

    fclose(file);

} // end function readAndUpdate

Given that your function does not do anything, you did in fact get lucky. Your function is declared and defined to accept an array of pointers, not a pointer/array. Similarly, in main, you have an array of pointers, not an array of structs. — Mad Physicist
– Mad Physicist, Commented Nov 18, 2018 at 23:41
You have array of pointers not array of structs. I think you wanted to have struct number people[30]; and then populate the array by passing the array pointer together with size readAndUpdate(struct member *people, size_t maxpeople). — KamilCuk
– KamilCuk, Commented Nov 18, 2018 at 23:46
With struct member *people[30];, people is an array of pointers to member structures, but no actual memory has been allocated for member structures. Maybe this happens in the readAndUpdate() function. It looks like a minimal reproducible example is in order.... — ad absurdum
– ad absurdum, Commented Nov 18, 2018 at 23:46
An array has a base address and size - but decays to a pointer on functional call, losing the size information, when passed to a function as in this case. Search StackOverflow for Array decay to pointer. Plenty of reading. FWIW, C++ has precisely the same behaviour. — marko
– marko, Commented Nov 18, 2018 at 23:51
In your revised code, you have: fscanf(file, "%s", &person->lastName); … fscanf(file, "%s", &person[1].lastName); For consistency, use &person[0].lastName in the first block of fscanf() calls. Then replace that code by a loop. And test each fscanf() to ensure it succeeded. You could also combine three consecutive calls into one: if (fscanf(file, "%s %c %d", &person[0].lastName, &person[0].gender, &person[0].age) != 3) { …oops — error handling… }. Replace the 0 by a loop index when you add the loop. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 19, 2018 at 0:34

Jonathan Leffler · Accepted Answer · 2018-11-19 00:30:12Z

The code you have is 'OK, but…'. And there are some quite significant "buts" to be worried about.

The first issue is whether what you wrote is what you intended to write. You've defined an array of pointers to structures, but not initialized it at all. You may have intended to define an array of structures rather than an array of pointers, which then alters the rest of the discussion. For the time being, I'm taking what you wrote as "it's OK — that's what I intended to write".

You pass the array to the function correctly. The function has no idea how big an array you passed, though. You should get into the habit of telling functions how big the array is.

You don't reference the array inside the function. That's not all bad; you haven't defined the memory that each of the pointers in the array is pointing to. You'll presumably dynamically allocate the items as you add them, and then reference them correctly using arrows -> and not dots .:

void readAndUpdate(size_t max, struct member *people[max])
{
    for (size_t i = 0; i < max; i++)
    {
        people[i] = malloc(sizeof(*people[i]));
        if (people[i] == NULL)
            …handle error appropriately…
        strcpy(people[i]->lastName, "Unknown");
        people[i]->gender = 'N';   // Neuter — unknown
        people[i]->age = 0;        // Babies only
    }
}

int main(void)
{
    struct member *people[30] = { NULL }; 
    readAndUpdate(30, people);
    return 0;
}

If the number of entries isn't actually fixed, then the readAndUpdate() function should report how many were initialized.

I didn't intend to create an array of pointers.

OK; then the rules of the game change:

void readAndUpdate(size_t max, struct member people[max])
{
    for (size_t i = 0; i < max; i++)
    {
        strcpy(people[i].lastName, "Unknown");
        people[i].gender = 'N';   // Neuter — unknown
        people[i].age = 0;        // Babies only
    }
}

int main(void)
{
    struct member people[30] = { { "", 0, 0 } }; 
    readAndUpdate(30, people);
    return 0;
}

The structures are already allocated, and initialized to all bytes zero. The code in the function uses . instead of -> to reference members. The * goes from the variable and parameter definitions.

@Jonathan_Leffler I didn't intend to create an array of pointers. I have however updated my code and it works as intended. I will edit it into my original post

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