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Telematic/Audiovisual Syst./Communication Syst. Engineering

Systems Architecture

September 2017 - January 2018

17.3.  Self-assessment questions
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17.3.  Self-assessment questions

  1. You have compiled a program with the sentence gcc -Wall -o main main.c. You execute it with Valgrind and obtain memory leaks errors, but Valgrind does not specify the code lines where these errors appear.

    • You have to indicate, in the compiling sentence, that the executable file must have detailed information, such as line numbers: gcc -l -Wall -o main main.c

    • You have to indicate, in the compiling sentence, that the executable file must have detailed information, such as line numbers: gcc -g -Wall -o main main.c

    • You have to indicate, in the Valgrind sentence, that you want line numbers printed out in the error messages output: valgrind -l --leak-check=yes ./main

    • You have to indicate, in the Valgrind sentence, that you want line numbers printed out in the error messages output: valgrind -g --leak-check=yes ./main

  2. Given the following function:

    #define SIZE 10

void f(void)
{
  int *x = malloc(SIZE * sizeof(int));
  x[SIZE-1] = 0;
}

    • Valgrind detects a memory leak.

    • Valgrind detects a memory error (invalid write).

    • Valgrind detects a memory error (invalid read).

    • Valgrind detects an invalid free.

    • Valgrind detects no memory errors or leaks.

  3. Given the following function:

    #define SIZE 10

void f(void)
{
  char *x = malloc(SIZE * sizeof(char));
  x[SIZE] = '\0';
  free(x);
}

    • Valgrind detects a memory leak.

    • Valgrind detects a memory error (invalid write).

    • Valgrind detects a memory error (invalid read).

    • Valgrind detects an invalid free.

    • Valgrind detects no memory errors or leaks.

  4. Given the following code:

    #define SIZE 4
...
  char string[SIZE] = "Test";
  printf(string);
...

    • Valgrind detects a memory leak.

    • Valgrind detects a memory error (invalid write).

    • Valgrind detects a memory error (invalid read).

    • Valgrind detects an invalid free.

    • Valgrind detects no memory errors or leaks.

  5. Given the following code:

    #define SIZE 6
...
  char string[SIZE] = "Test";
  printf(string);
...

    • Valgrind detects a memory leak.

    • Valgrind detects a usage of uninitialized value(s) or byte(s).

    • Valgrind detects an invalid free.

    • Valgrind detects no memory errors or leaks.

  6. Given the following program extract:

    #define SIZE 2
void f(int *number)
{
  int *numbers = realloc(number,SIZE*sizeof(int));
  numbers[0] = 64;
  numbers[1] = 65;
  free(numbers);
}
int main(void)
{
  int *aux = malloc (sizeof(int));
  f(aux);
  free(aux);
  return 0;
}

    • Valgrind detects a memory leak.

    • Valgrind detects a usage of uninitialized value(s) or byte(s).

    • Valgrind detects an invalid free.

    • Valgrind detects no memory errors or leaks.

  7. Given the following piece of code:

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       //compilation in linux with gcc -pthread option
 
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>       
#include <unistd.h>

struct struct_counter{
  int i;
  pthread_mutex_t mutex_i;
};

int increment_counter(struct struct_counter* counter)
{  int to_return=0;
   pthread_mutex_unlock(&((counter)->mutex_i));
   to_return=(*counter).i++;
   pthread_mutex_unlock(&((counter)->mutex_i));
   return to_return;
 }
 
 
void *counter_thread(void *ctr)
{  printf("In thread: running...\n");
   sleep(1);
   printf("[_THREAD_1]Counter is %d \n", increment_counter((struct struct_counter*)ctr) );
   printf("In thread: exiting .............\n");
   pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
  struct struct_counter int_counter;
  int_counter.i=0;
  pthread_mutex_init(&int_counter.mutex_i,NULL);
  
  pthread_t threads[1];
  int rc=0;
  printf("(log) In main: creating thread %i\n", 1);
  rc = pthread_create(&threads[0], NULL, counter_thread, (struct struct_counter *)&int_counter);
  if (rc){
      printf("ERROR; return code from pthread_create() is %d\n", rc);
      exit(-1);
     }
  sleep(1);
  int res_counter=increment_counter(&int_counter);
  pthread_join(threads[0],NULL);     
  pthread_mutex_destroy(&int_counter.mutex_i);
  printf("[_MAIN___] Counter is %i \n", res_counter);
  return 0;
}

    • Helgrind detects a race condition.

    • Helgrind warns about bad behaviour of POSIX API.

    • Helgrind detects issues in the lock ordering.

    • Helgrind does not detect any issues.

  8. Given the following piece of code

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       //compilation in linux with gcc -pthread option
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>       
#include <unistd.h>

int increment_counter(int *counter)
 {
   (*counter)++;
   return *counter;
 }
 
 
 void *counter_thread(void *ctr)
{  printf("In thread: running...\n");
   sleep(1);
   printf("In thread: exiting .............\n");
   pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
  int int_counter=0;
  pthread_t threads[1];
  int rc=0;
  printf("(log) In main: creating thread %i\n", 1);
  rc = pthread_create(&threads[0], NULL, counter_thread, (void *)&int_counter);
  if (rc){
      printf("ERROR; return code from pthread_create() is %d\n", rc);
      exit(-1);
     }
  sleep(1);
  int res_counter=increment_counter(&int_counter);
  pthread_join(threads[0],NULL);     
  printf("[_MAIN___] Counter is %i \n", res_counter);
  return 0;
}

    • Helgrind detects a race condition.

    • Helgrind warns about bad behaviour of POSIX API.

    • Helgrind detects issues in the lock ordering.

    • Helgrind does not detect any issues.

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