$ cat main.c
#include <stdio.h>
#include <stdlib.h>

typedef struct {
  int id_cinta;
} pmanager_t;

int main () {
  int i = 5, n_max_cintas = 10;
  pmanager_t *datos_entrada;

  datos_entrada = (pmanager_t *)malloc(sizeof(pmanager_t)*(n_max_cintas));
  datos_entrada[i].id_cinta = 7;
  fprintf(stdout, "%i", datos_entrada[i].id_cinta);
  free(datos_entrada);
  return 0;
}
$ gcc -Wall -pedantic main.c
$ valgrind ./a.out
==12727== Memcheck, a memory error detector
==12727== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==12727== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==12727== Command: ./a.out
==12727==
7==12727==
==12727== HEAP SUMMARY:
==12727==     in use at exit: 0 bytes in 0 blocks
==12727==   total heap usage: 1 allocs, 1 frees, 40 bytes allocated
==12727==
==12727== All heap blocks were freed -- no leaks are possible
==12727==
==12727== For counts of detected and suppressed errors, rerun with: -v
==12727== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)