C malloc string

Question

I am trying to implement a dynamic array to my C stack, however I am a bit confused how char malloc works in such that it inserts garbage data as soon as it is created.

Here is the code used to initialize the array and push elements in.

typedef struct {
  char *array;
  int used;
  int size;
} Array;

void initArray(Array *a, int initialSize) {
  a->array = (char *)malloc(initialSize * sizeof(char)+1);
  a->used = 0;
  a->size = initialSize;
}

void pushArray(Array *a, int element) {
  if (a->used == a->size) {
    a->size +=1;
    a->array = (char *)realloc(a->array, a->size * sizeof(char));
  }
  a->array[a->used++] = element;
}

And here is the main snippet where I initialize and push elements into array

int main()
{
    Array a;
    int i=0;
    initArray(&a,0);
    char choice='a';
    char  exp[100]="";
    printf("Please enter an expression\n");
    scanf("%s",exp);
    for (i=0;i<strlen(exp);i++){

        pushArray(&a,exp[i]);

    }

Initializing array

After running the push function

chux · Accepted Answer · 2015-12-07 20:38:10Z

5

Unclear how OP determined "it inserts garbage data". Since ``malloc()` does not initialized the contents of the allocated data, insure data is set as needed.

Viewing field array as string is a problem as array is not null character terminated.

Change loop to <= to also push the '\0'.

for (i=0;i<=strlen(exp);i++){
  pushArray(&a,exp[i]);
}

Alternatively append the null character on each push. Note: little need for * sizeof(char) and casting the return value of *alloc()

void initArray(Array *a, int initialSize) {
  a->array = malloc(initialSize + 1);
  a->used = 0;
  a->size = initialSize;
  a->array[0] = '\0';
}

void pushArray(Array *a, int element) {
  if (a->used == a->size) {
    a->size +=1;
    a->array = realloc(a->array, a->size + 1);
  }
  a->array[a->used++] = element;
  a->array[a->used] = '\0';
}

edited Dec 7, 2015 at 20:38

answered Dec 7, 2015 at 19:59

chux

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Comments

Iharob Al Asimi · Accepted Answer · 2015-12-07 20:11:14Z

5

malloc() does not "put" garbage in the memory allocated, it just doesn't initialize it to anything, so you get whatever "garbage" is there. You can use either memset() to clear out the memory, of just use calloc(), which does it.

edited Dec 7, 2015 at 20:11

Iharob Al Asimi

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answered Dec 7, 2015 at 19:58

DBug

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2 Comments

Brian Pace Over a year ago

Is there a way to overwrite the contents, like a->array="" ?

DBug Over a year ago

no, that is not memory copy, but pointer assignment. If you want the malloced memory to represent a C string, set first element to 0, a->array[0] = '\0';

bruceg · Accepted Answer · 2015-12-07 19:59:28Z

1

Malloc just gives you a pointer to some place in memory, and it's contents will be whatever was leftover the last time some process used it. If you need to get the contents of the allocated memory initialized, you should use calloc instead of malloc.

answered Dec 7, 2015 at 19:59

bruceg

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