What does the error mean: realloc(): invalid Pointer: 0xb78094d8?

Question

My program, whose code is further down, has shown the following error:

I don’t understand because I have too much memory on my PC, and all the compiler sites have the same problem.

*** Error in `./prog': realloc(): invalid pointer: 0xb78094d8 ***
======= Backtrace: =========
/lib/i386-linux-gnu/i686/cmov/libc.so.6(+0x75e72)[0xb76d2e72]
/lib/i386-linux-gnu/i686/cmov/libc.so.6(realloc+0x275)[0xb76d6ad5]
./prog[0x804866c]
/lib/i386-linux-gnu/i686/cmov/libc.so.6(__libc_start_main+0xf5)[0xb76768f5]
./prog[0x80486ad]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:03 1295711    /home/MhQXj9/prog
08049000-0804a000 rw-p 00000000 08:03 1295711    /home/MhQXj9/prog
0914d000-0916e000 rw-p 00000000 00:00 0          [heap]
b763b000-b7656000 r-xp 00000000 08:03 1303883    /lib/i386-linux-gnu/libgcc_s.so.1
b7656000-b7657000 rw-p 0001a000 08:03 1303883    /lib/i386-linux-gnu/libgcc_s.so.1
b765b000-b765d000 rw-p 00000000 00:00 0 
b765d000-b7806000 r-xp 00000000 08:03 1303839    /lib/i386-linux-gnu/i686/cmov/libc-2.17.so
b7806000-b7807000 ---p 001a9000 08:03 1303839    /lib/i386-linux-gnu/i686/cmov/libc-2.17.so
b7807000-b7809000 r--p 001a9000 08:03 1303839    /lib/i386-linux-gnu/i686/cmov/libc-2.17.so
b7809000-b780a000 rw-p 001ab000 08:03 1303839    /lib/i386-linux-gnu/i686/cmov/libc-2.17.so
b780a000-b780d000 rw-p 00000000 00:00 0 
b780d000-b784e000 r-xp 00000000 08:03 1303836    /lib/i386-linux-gnu/i686/cmov/libm-2.17.so
b784e000-b784f000 r--p 00040000 08:03 1303836    /lib/i386-linux-gnu/i686/cmov/libm-2.17.so
b784f000-b7850000 rw-p 00041000 08:03 1303836    /lib/i386-linux-gnu/i686/cmov/libm-2.17.so
b7851000-b7856000 rw-p 00000000 00:00 0 
b7856000-b7857000 r-xp 00000000 00:00 0          [vdso]
b7857000-b7876000 r-xp 00000000 08:03 1303796    /lib/i386-linux-gnu/ld-2.17.so
b7876000-b7877000 r--p 0001f000 08:03 1303796    /lib/i386-linux-gnu/ld-2.17.so
b7877000-b7878000 rw-p 00020000 08:03 1303796    /lib/i386-linux-gnu/ld-2.17.so
bf99a000-bf9af000 rw-p 00000000 00:00 0          [stack]

---

the program below is a hash table and does the following:

I have a structured type Tab[int* vetor, int tam_vetor], and constantly the Tab[n].vetor needs to be reallocated to grow in size.

At the end of each num_de_caso I release the *vetores and the structure Tab. And then I create them again, until the cycle is over.

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

typedef struct tab
{
    int* vetor;
    short int tam_vetor;

}Tab;

void inicializa(Tab* tabela, int tam)
{
    int i;

    for(i=0; i < tam; i++)
        tabela[i].tam_vetor = 0;
}

int hash(int num,int tam, int i)//gera um codigo p a chave
{
    num = (num%tam + i);

    return num;
}

void insere(Tab* tabela, int chave, int pos)
{
    tabela[pos].vetor[tabela[pos].tam_vetor] = chave;
    tabela[pos].tam_vetor = tabela[pos].tam_vetor + 1;
}

void liberaTabela(Tab* tabela, int tam_tabela)
{
    int i;

    for(i=0; i<tam_tabela;i++)
        if(tabela[i].vetor != NULL)
            free(tabela[i].vetor);

    free(tabela);

}

void verificaAlocacao(Tab* tabela, int pos)
{
    if(pos < 0)
    {
        if(tabela == NULL)
        {
            printf("\nmemoria insuficiente");
            exit(1);
        }
    }
    else
    {
        if(tabela[pos].vetor == NULL)
        {
            printf("\nmemoria insuficiente");
            exit(1);
        }
    }
}

void printTabela(Tab* tabela, int tam_tabela)
{
    int i,j;

    for(i=0; i< tam_tabela; i++)
    {
        printf("%d ->",i);

        if(tabela[i].vetor != NULL)
        {
            for(j=0; j < tabela[i].tam_vetor; j++)
            {
                printf(" %d ->",tabela[i].vetor[j]);
            }
        }

        printf(" \\\n");
    }
}

int main()
{
    int num_de_casos,
        tam_tabela,
        num_chaves,
        chave,
        pos,
        i,
        j;

    Tab* tabela;

    scanf("%d", &num_de_casos);

    while(num_de_casos != 0)
    {
        scanf("%d",&tam_tabela);
        scanf("%d",&num_chaves);
        tabela = (Tab*) malloc(tam_tabela*sizeof(Tab));
        inicializa(tabela, tam_tabela);
        verificaAlocacao(tabela, -1);

        while(num_chaves != 0)
        {
            scanf("%d",&chave);

            i=0;

            pos = hash(chave,tam_tabela,i);

            while(1)
            {
                if(tabela[pos].vetor == NULL)
                {
                    tabela[pos].vetor = (int*) malloc(sizeof(int));
                    verificaAlocacao(tabela, pos);
                    insere(tabela,chave,pos);
                    break;
                }
                else
                {
                    for(j=0; j < tabela[pos].tam_vetor && chave != tabela[pos].vetor[j]; j++);//acha j<tam_vetor se: chave == algum elemento && acha j == tam_vetor se: chave não coincide com nenhum elemento

                    if(j == tabela[pos].tam_vetor)
                    {
                        tabela[pos].vetor = (int*) realloc(tabela[pos].vetor,(j+2)*sizeof(int));
                        verificaAlocacao(tabela, pos);
                        insere(tabela,chave,pos);
                        break;
                    }
                    else
                    {
                       i++;
                       pos = hash(chave,tam_tabela,i);
                    }
                }
            }

            num_chaves--;
        }

        printTabela(tabela, tam_tabela);
        liberaTabela(tabela, tam_tabela);
        num_de_casos--;
    }

    return 0;
}

Answer 1

Well... what I learned in this problem was: Never forget to initialize a pointer vector with NULL;

void inicializa(Tab* tabela, int tam)
{
    int i;

    for(i=0; i < tam; i++)
    {   
        tabela[i].vetor = NULL;//a falta dessa linha ocasionou o erro de memóriaN

        tabela[i].tam_vetor = 0;
    }
}