Memory error trying to free pointers in struct - C

Question

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct Nodo{
    int valor;
    struct Nodo* hijo1;
    struct Nodo* hijo2;
}Nodo;

typedef Nodo* Arreglo;

Arreglo INIC_ARR(int longitud);
void IMP_ARR(Arreglo A,int longitud);
void INIC_ARBOL(Arreglo A, int longitud);
void FREE_ARBOL(Arreglo A, int longitud);


int main(){
    Arreglo A;
    int longitud = 10;
    A = INIC_ARR(10);
    INIC_ARBOL(A,longitud);
    IMP_ARR(A,longitud);
    FREE_ARBOL(A,longitud);
    return 0;
}

Arreglo INIC_ARR(int longitud){
    int i;
    Arreglo A = (Arreglo)calloc(longitud,sizeof(Nodo));
    for(i = 0; i < longitud; i++){
        A[i].valor = rand()%10;
    }
    return A;
}

void IMP_ARR(Arreglo A,int longitud){
    int i;
    for(i = 0;i < longitud; i++){
        printf("[%d,",A[i].valor);
        if(A[i].hijo1 == NULL){
            printf("-,");
        }
        else{
            printf("%d,",(*(A[i].hijo1)).valor);
        }
        if(A[i].hijo2 == NULL){
            printf("-]");
        }
        else{
            printf("%d]",(*(A[i].hijo2)).valor);
        }

    }
    printf("\n");
}

void INIC_ARBOL(Arreglo A, int longitud){
    int i;
    for(i = 0; i < longitud; i++){
        if(2*i + 1 < longitud)
            A[i].hijo1 = &A[2*i + 1];
        if(2*i + 2 < longitud)
            A[i].hijo2 = &A[2*i + 2];
    }
}


void FREE_ARBOL(Arreglo A, int longitud){
    int i;
    for(i = 0; i < longitud; i++){
        free(A[i].hijo1);
        free(A[i].hijo2);
    }
    free(A);
}

I created an arrary of structs (Nodo); each have two pointers that are initialised in INIC_ARBOL function. I tried to print and then free them using FREE_ARBOL, and then this shows up:

[3,6,7][6,5,3][7,5,6][5,2,9][3,1,-][5,-,-][6,-,-][2,-,-][9,-,-][1,-,-]
*** Error in `./a.out': free(): invalid pointer: 0x0000000000a31028 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x77725)[0x7f3f8adea725]
/lib/x86_64-linux-gnu/libc.so.6(+0x7ff4a)[0x7f3f8adf2f4a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7f3f8adf6abc]
./a.out[0x40096b]
./a.out[0x400696]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f3f8ad93830]
./a.out[0x400579]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:0a 140095                             /home/luis/Documentos/Programacion/C/a.out
00600000-00601000 r--p 00000000 08:0a 140095                             /home/luis/Documentos/Programacion/C/a.out
00601000-00602000 rw-p 00001000 08:0a 140095                             /home/luis/Documentos/Programacion/C/a.out
00a31000-00a52000 rw-p 00000000 00:00 0                                  [heap]
7f3f84000000-7f3f84021000 rw-p 00000000 00:00 0 
7f3f84021000-7f3f88000000 ---p 00000000 00:00 0 
7f3f8ab5d000-7f3f8ab73000 r-xp 00000000 08:09 135547                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7f3f8ab73000-7f3f8ad72000 ---p 00016000 08:09 135547                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7f3f8ad72000-7f3f8ad73000 rw-p 00015000 08:09 135547                     /lib/x86_64-linux-gnu/libgcc_s.so.1
7f3f8ad73000-7f3f8af33000 r-xp 00000000 08:09 141748                     /lib/x86_64-linux-gnu/libc-2.23.so
7f3f8af33000-7f3f8b132000 ---p 001c0000 08:09 141748                     /lib/x86_64-linux-gnu/libc-2.23.so
7f3f8b132000-7f3f8b136000 r--p 001bf000 08:09 141748                     /lib/x86_64-linux-gnu/libc-2.23.so
7f3f8b136000-7f3f8b138000 rw-p 001c3000 08:09 141748                     /lib/x86_64-linux-gnu/libc-2.23.so
7f3f8b138000-7f3f8b13c000 rw-p 00000000 00:00 0 
7f3f8b13c000-7f3f8b162000 r-xp 00000000 08:09 141744                     /lib/x86_64-linux-gnu/ld-2.23.so
7f3f8b340000-7f3f8b343000 rw-p 00000000 00:00 0 
7f3f8b35e000-7f3f8b361000 rw-p 00000000 00:00 0 
7f3f8b361000-7f3f8b362000 r--p 00025000 08:09 141744                     /lib/x86_64-linux-gnu/ld-2.23.so
7f3f8b362000-7f3f8b363000 rw-p 00026000 08:09 141744                     /lib/x86_64-linux-gnu/ld-2.23.so
7f3f8b363000-7f3f8b364000 rw-p 00000000 00:00 0 
7fffddc61000-7fffddc82000 rw-p 00000000 00:00 0                          [stack]
7fffddd39000-7fffddd3b000 r--p 00000000 00:00 0                          [vvar]
7fffddd3b000-7fffddd3d000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Abortado (`

core' generado)

I have zero knowledge on tools like valgrind so if you could explain me slowly what to do to fix I'd appreciate it.

Answer 1

Short story: You only have one calloc, so you should only have one free.

Additionally, since you have those ifs (e.g.if (2*i + 1 < longitud)) inside INIC_ARBOL, half nodes in your array don't even point to anything. And I would remove typedef Nodo *Arreglo completely and just use Node* within the rest of the code, it would make reading the code much easier.

Answer 2

Do not pass what is neither returned from memory management functions nor null pointer, or you will invoke undefined behavior.

Remove the problematic part

for(i = 0; i < longitud; i++){
    free(A[i].hijo1);
    free(A[i].hijo2);
}

from the function FREE_ARBOL().

Answer 3

In this function

Arreglo INIC_ARR(int longitud){
    int i;
    Arreglo A = (Arreglo)calloc(longitud,sizeof(Nodo));
    for(i = 0; i < longitud; i++){
        A[i].valor = rand()%10;
    }
    return A;
}

there is allocated one extent of memory using statement

Arreglo A = (Arreglo)calloc(longitud,sizeof(Nodo));

You may not free elements of the dynamically allocated array

void FREE_ARBOL(Arreglo A, int longitud){
    int i;
    for(i = 0; i < longitud; i++){
        free(A[i].hijo1);
        free(A[i].hijo2);
    }
    free(A);
}

because they are parts of the extent and were not allocated themselves dynamically.