exemplo simples realloc

realloc_string.c

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

int main(){

char *buffer = NULL;
char palavra;

printf("Alocando memoria\n");
buffer = (char *) malloc(1 + sizeof(char));

while (1){

        printf("Digite a string para concatena-la: \n");
        scanf("%s", &palavra);

        if (strcmp(&palavra, "exit") == 0)
                break;

        /* Alocacao dinamica */
        buffer = (char *)realloc(buffer, strlen(buffer) + strlen(&palavra) + sizeof(char));
        printf("Memoria alocada\n");

        if (!buffer) {
                printf("Deu pau");
                free(buffer);
                return 0;
        }
        /* Concatenando a parada */
        strcat(buffer, &palavra);
        printf("String concatenada %s\n", buffer);

}

free(buffer);
return 0;