Malloc does not allocate suficient memory

Question

When running the code I check if the pointer has been allocated suficient memory or if it stays at NULL but it can be assigned the whole quantity of memory we desired. We are using C11 with Clion.

First part of the code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "LS_allegro.h"

#define MAX_CHAR 100

typedef struct {
    char name[MAX_CHAR];
    char escuderia[MAX_CHAR];
    int dorsal;
    int reflejos;
    int c_fisica;
    int temperamento;
    int gest_neumaticos;
}DatosPiloto;

typedef struct {
    char type[26];
    int vel;
    int acc;
    int cons;
    int fiab;
}Pieza;

typedef struct {
    char name_cat[26];
    Pieza *pieza;
    int num_piezas;
}Categorias;

Int main and funcion calling:

int main (int argc, char **argv) {

    char sel[MAX_CHAR];
    int opt = 0;
    int error = 0, datosok = 0;
    // Opcion 1
    DatosPiloto datosPiloto;
    Categorias *categorias;

    if (argc == 5) {
        error = abreFichero(argv);
    }
    else {
        printf("Error. El programa tiene que recibir 4 argumentos.");
        error = 1;
    }


    if (error == 0) {
        leerArchivos(argv, &categorias);
    ...
    ...

argc is an integer with a value of 5 always and **argv is the pointer with the files imported in clion, where [0] is the first file and [4] is the last one.

And the function is:

void leerArchivos (char **argv, Categorias **categorias) {
    FILE *Piezas;
    FILE *GPs;
    FILE *Corredores;
    FILE *Base;

    int num_categorias = 0, lineas_leer = 0;
    int j = 0, i = 0, cat_count = 0;
    char basura;
    Piezas = fopen(argv[1], "r");
    GPs = fopen(argv[2], "r");
    Corredores = fopen(argv[3], "rb");
    Base = fopen(argv[4], "rb");


    fscanf(Piezas, "%d", &num_categorias);

    printf("%d\n", num_categorias);

    *categorias = (Categorias *) malloc(num_categorias * sizeof(Categorias));

    if (*categorias == NULL || sizeof(categorias) < num_categorias * sizeof(Categorias)) {
        printf("ERROR! Memory not allocated or smaller that desired.\n");
    }else {
    ...
    ...

sizeof does not tell you how mch memory has been allocated. There's no standard way to find that out. With malloc, you either get at least the amount of bytes you asked for or you get NULL. And sizeof(categorias) is the size of a pointer, no matter whether and how much memory is allocated to it. — M Oehm

Unknown Unknown · Accepted Answer · 2020-03-18T23:25:43

sizeof does not tell you how much memory has been allocated. There's no standard way to find that out. With malloc(), you either get at least the amount of bytes you asked for or you get NULL. sizeof(categorias) is the size of a pointer, no matter whether and how much memory is allocated to it. @M Oehm

The below is OK.

*categorias = (Categorias *) malloc(num_categorias * sizeof(Categorias));
if (*categorias == NULL) {
    printf("ERROR! Memory not allocated or smaller that desired.\n");
} else {

Cast not needed.

Allocating to the size of the referenced data is cleaner than the size of of the type.

*categorias = malloc(sizeof *categorias * num_categorias);

A value of less than 0 for num_categorias creates issues. Consider an unsigned type.

// int num_categorias = 0
unsigned  num_categorias = 0
// or 
size_t num_categorias = 0

A value of for num_categorias creates issues. Some older systems return NULL even when successful.

if (*categorias == NULL && num_categorias != 0) {

Malloc does not allocate suficient memory

1 Answers