Memory allocation excercise

Question

Write a program to ask the user to enter the total number of float data. Then use the calloc() and malloc() functions to allocate two memory blocks with the same size specified by the number, and print out the initial values of the two mem- ory blocks.

My solution is:

float *p_1,*p_2;
int i,num_of_floats=0;
printf("the total number of float numbers: ");
scanf("%d",&num_of_floats);
if((p_1=calloc(num_of_floats,sizeof(float)))==NULL||(p_2=malloc(num_of_floats*sizeof(float)))==NULL){
  printf("error at alllocating!!\n");
  exit(1);
}
else{
  for(i=0; i<num_of_floats ;i++){
    printf("%f",*(p_1+i));
  }
  printf("\n");
  for(i=0; i<num_of_floats ;i++){
    printf("%f",*(p_2+i));
  }
}
free(p_1);
free(p_2);

return 0;

output:

the total number of float numbers: 5
0.0000000.0000000.0000000.0000000.000000
0.0000000.0000000.0000000.0000000.000000

The malloc function doesn't Initialize the values to zero, and still i get zeros.

That could be a coincidence - zero is just as probable as any other indeterminate value. Or it could be caused by you running a debug build with a runtime that zeroes all memory in debug. — molbdnilo, Nov 07 '14 at 15:24
The malloc function doesn't Initialize the values to zero, and still I get zeros. So the output is incorrect. And it's not coincidence, I have tried many inputs. — Mor Haham, Nov 07 '14 at 15:24
@MorHaham Neither language stipulates that `malloc` is required to make sure that the memory block it returns isn't filled with zeroes. — molbdnilo, Nov 07 '14 at 15:27
Is it possible to get for every input only zeros? So many times? — Mor Haham, Nov 07 '14 at 15:29
If you get 0 the 1st time and re-launch the program many times, as long as you don't write things at this address, you will still get 0. See http://stackoverflow.com/a/6445794/3194340 — n0p, Nov 07 '14 at 15:33
@Coconop: That's not really true. Each time you run the process, you get a fresh batch of virtual memory and it could map to a completely different place in physical memory. What happens on a given run of your program has essentially nothing to do with what happens on a subsequent run. — Lightness Races in Orbit, Nov 07 '14 at 15:34

score 1 · Accepted Answer · answered Nov 07 '14 at 15:24

1

If you do not initialize your values, you can get any data. Zeroes are a valid kind of "any data".

You're more likely to see it in debug builds, wherein some the C++ runtimes zero out a good portion of its heap memory at program startup in order to make it easier for you to see when you're using it. Although, in my opinion, it makes it less clear that you're failing to initialize your data, as shown by this very question.

answered Nov 07 '14 at 15:24

Lightness Races in Orbit

378,754
76
643
1,055

Is it possible to get for every input only zeros? So many times? – Mor Haham Nov 07 '14 at 15:30
@MorHaham: Yes. Is it possible that you did not read the second paragraph of my answer? – Lightness Races in Orbit Nov 07 '14 at 15:33

Memory allocation excercise

1 Answers1