How to make a function return a pointer to array struct

Question

I'm working on a function in C that should return a pointer to an array struct. The struct is

struct corr{
  char nome[128];
  char cognome[128];
  char password[10];
  float importo;
};
typedef struct corr correntista;

The function that return a pointer to this struct is

correntista *max_prelievo(FILE *fcorrentisti, FILE *fprelievi){
    correntista *corr_max[2];
    corr_max[0] = (correntista *)malloc(sizeof(correntista));
    corr_max[1] = (correntista *)malloc(sizeof(correntista));
    /*.........*/
    return *corr_max;
}

In the main program I want to print the returned value in the following way:

*c_max = max_prelievo(fc, fp);
printf("Correntista con max prelievi:\n");
printf("Nome: %s\n", c_max[0]->nome);
printf("Cognome: %s\n", c_max[0]->cognome);
printf("Max prelievo: %f\n\n", c_max[0]->importo);

printf("Correntista con max versamenti:\n");
printf("Nome: %s\n", c_max[1]->nome);
printf("Cognome: %s\n", c_max[1]->cognome);
printf("Max versamento: %f\n", c_max[1]->importo);

but only the first struct c_max[0] has the expected value. c_max[1] has garbage values. What should I change in my program?

What is the type of c_max? Try changing correntista *max_prelievo(FILE *fcorrentisti, FILE *fprelievi){ to correntista **max_prelievo(FILE *fcorrentisti, FILE *fprelievi){ and return *corr_max; to return corr_max;. and from the calling function, correntista **c_max = max_prelievo(fc, fp); — Spikatrix
– Spikatrix, Commented Jun 10, 2015 at 14:09
You don't need to cast the result of malloc - stackoverflow.com/questions/605845/… — Treesrule14
– Treesrule14, Commented Jun 10, 2015 at 14:11
@CoolGuy returning a pointer into an array that isn't guaranteed to exist once the function returns is obviously UB. — autistic
– autistic, Commented Jun 10, 2015 at 14:21

autistic · Accepted Answer · 2015-06-10 14:44:57Z

There are several solutions, but they all require an overhaul of how your function is to operate. Perhaps the best solution is to consider how scanf works; It returns no objects, instead it operates solely on objects that are pointed to by parameters passed to it. Hence:

void max_prelievo(correntista *corr_max, FILE *fcorrentisti, FILE *fprelievi){
    /* NOTE: You should make a habit of separating your allocation from this logic;
     *       it makes debugging and testing so much easier... */

    corr_max[0] = (correntista) { .nome = "Michael",
                                  .cognome = "Anderson",
                                  .importo = 42.0 };

    corr_max[1] = (correntista) { .nome = "Undefined",
                                  .cognome = "Behaviour",
                                  .importo = -1.0 };


    /*.........*/
}

int main(void) {
    correntistia c_max[2] = { 0 };
    FILE *f1 = fopen(...), *f2 = fopen(...);

    max_prelievo(c_max, f1, f2);

    printf("Correntista con max prelievi:\n");
    printf("Nome: %s\n", c_max[0].nome);
    printf("Cognome: %s\n", c_max[0].cognome);
    printf("Max prelievo: %f\n\n", c_max[0].importo);

    printf("Correntista con max versamenti:\n");
    printf("Nome: %s\n", c_max[1].nome);
    printf("Cognome: %s\n", c_max[1].cognome);
    printf("Max versamento: %f\n", c_max[1].importo);
}

What scanf does return is a single integer, indicating success. Perhaps, if you are to use return values in the future, you could resort to using a similar pattern? Look at the bright side: There's no malloc or free here and no chance you could leak any memory; the code is as simple as it could possibly get.

Alternatively, as horrific as it seems, if you must resort to using such a pattern then less calls to malloc will serve you well. Consider something like this:

correntista *max_prelievo(FILE *fcorrentisti, FILE *fprelievi) {
    correntista *corr_max = malloc(2 * sizeof *corr_max);

    corr_max[0] = (correntista) { .nome = "Michael",
                                  .cognome = "Anderson",
                                  .importo = 42.0 };

    corr_max[1] = (correntista) { .nome = "Undefined",
                                  .cognome = "Behaviour",
                                  .importo = -1.0 };

    /*.........*/

    return corr_max;
}

The main entry point will be almost identical to the example I gave earlier... except for one thing: Don't forget to free that memory! :(

@CoolGuy Thanks for the edits :) my astigmatism is too horrific to notice that I accidentally typed , instead of ....
I would also say that this is the preferred solution, as how and where the memory is allocated has nothing to do with the actual algorithm of that function.

Gopi · Accepted Answer · 2015-06-10 14:21:30Z

1

Remember what you have is array of pointers so when you exit the function the array is out of scope. Have a pointer to pointer for this purpose as shown below.

correntista **max_prelievo(FILE *fcorrentisti, FILE *fprelievi){
    correntista **corr_max = malloc(sizeof(correntista *) * 2);
    corr_max[0] = malloc(sizeof(correntista));
    corr_max[1] = malloc(sizeof(correntista));
    /*.........*/
    return corr_max;
}

The call should be

correntista **c_max = max_prelievo(fc, fp);

answered Jun 10, 2015 at 14:21

Gopi

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3 Comments

Lundin Over a year ago

This isn't correct though. A pointer to pointer is not a pointer to an array and you aren't allocating an array, you are allocating a segmented heap fiasco, which is something you should never do. Instead, allocate an array. correntista* corr_max = malloc(2 * sizeof(*corr_max)); or alternatively use an array pointer (although the syntax for returning array pointers from functions is very ugly).

autistic Over a year ago

@user3700643 Indeed. Technically speaking, this doesn't meet your requirements as specified in your comment to Cool Guy: "I am obliged to define the function as correntista *max_prelievo(FILE *fcorrentisti, FILE *fprelievi). c_max type is "correntista" as the struct"

Gopi Over a year ago

@Lundin I agree!! I don't know where I was when I wrote this code.. Just going through it again I feel pointer to pointer is unnecessary here.. Anyways even though this approach is ugly it is not wrong so I keep this answer :)

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