Passing dynamic array struct through function

Question

struct aPoint {
 int somaVertical;
 int somaHorizontal;
 int valor;
};

I have an array of structs dynamically created in main(), like so:

struct aPoint *ps = malloc( sizeof(struct aPoint) * columns * rows )

And I want to work with its struct values outside of main() in a function that has sscanf(). The initialization of the array is also taken care of on the main().

How can I pass the array of structs through that function and set some struct values of it aswell? Argh I hate pointering!

Thanks!

• Doug T.: +1 for coining the term "pointering" :)
• user325117: Why are you multiplying the size by colunas and linhas? This will give you an array of structs, not just one.
• Qosmo: Because I need an array of structs :P
• user50049: We had The Riddler, now we have The Pointerer. I vote that all pointer related questions should subsequently begin with POINTER ME THIS!
• user325117: I meant use calloc in main ;-).

Answer 1

That would be:

 int readStuff(struct aPoint *ps, size_t len, const char *someVar)
 {
 unsigned int i;
 for (i = 0; i < len; i++) {
 sscanf(someVar, "%d", &(ps[i].somaVertical));
 /* And so on for the other fields */
 }
 /* Return whatever you're returning here */
 }
 const size_t len = colunas * linhas;
 struct aPoint *ps = calloc(len, sizeof(struct aPoint));
 int success = readStuff(ps, len, arrayOfNumbers);

• Qosmo: Most of the answers were pretty good and worked. I just choose the one that had more upvotes. Thanks everyone for the help.

Answer 2

This works for me

/* #include <assert.h> */
#include <stdio.h>
#include <stdlib.h>
struct aPoint {
 int somaVertical;
 int somaHorizontal;
 int valor;
};
int readStuff(struct aPoint *data, int rows, int cols) {
 sscanf("42", "%d", &data[3].somaVertical);
 sscanf("142", "%d", &data[3].somaHorizontal);
 sscanf("-42", "%d", &data[3].valor);
 return 0;
}
int main(void) {
 struct aPoint *ps;
 int colunas, linhas;
 colunas = 80;
 linhas = 25;
 ps = malloc(sizeof *ps * colunas * linhas);
 /* assert(ps); */ /* thanks Tim */
 if (ps) {
 readStuff(ps, linhas, colunas);
 printf("%d %d %d\n", ps[3].somaVertical, ps[3].somaHorizontal, ps[3].valor);
 free(ps);
 } else {
 fprintf(stderr, "no memory.\n");
 exit(EXIT_FAILURE);
 }
 return 0;
}

• user50049: Why the assertion instead of explicit malloc() failure checking? You know what malloc() returns when it fails. Why assert unless checking a condition that could happen beyond the return of a function? For instance, assert(i != 3); .. Your example means something like NDEBUG turns off checking the failure of malloc(), which is why assertions are sometimes categorized as evil.
• pmg: @Tim: +1 ... I just added the assert (and the free) because I always say "check the return value of malloc" and I had failed to do that in this post. Thank you for pointing it out; I'm editing the post
• user50049: upvoted, this answer took time and is the most helpful thus far. +2 if I could, as you made your example match the comments under it :)

Answer 3

I think you need either

readStuff(ps); 
...
sscanf(someVar, "%d", &(ps[index].valor)); // using index in readStuff

or

readStuff(ps + index); // using index in main
...
sscanf(someVar, "%d", &(ps->valor)); // or &ps[0].valor, that's equivalent 

Answer 4

All functions in C are passed arguments by value, so you can pass a pointer to the struct array you wish to modify:

int readStuff(struct aPoint *p, int numStruct)
{
 ...
 for(i=0; i<numStruct; i++)
 {
 sscanf(someVar, "%d", &(*(p+i).valor) );
 }
 ...
}

You can call this function with:

struct aPoint *ps = malloc( sizeof(struct aPoint) * columns * rows );
...
readStuff(ps, columns * rows);

• user191776: @Queops, sorry for the delay, I have edited the answer accordingly.