Question

I want to create a new array with the same size of “chaine” but only after the function

 char chaine[Lenght];   //Lenght = 20
 function(chaine, sizeof(chaine));

When I called “function” the size of “chaine” is changing randomly.

The new array “chaine2″ also needs to be full of ” * ” characters.

Let met try to explain with some printf :

     printf("chaine = %s\n", chaine);

will show on screen something like : chaine = WORDS (5 characters)

And i want “chaine2” to be shown like this : chaine2 = ***** (5 stars)

I apologize for my english, thank you for reading

Answer 1

Remember that char arrays are special, in the sense that they have a size, which you specify when you declare them, and a length, which depends on their contents. The size of an array is the amount of memory that’s been allocated to it. The length of a string is the number of characters before a terminating null ('\0').

some_func() {
  int len = 20;      // Size of the array
  char chaine[len];  // Uninitialized array of size 20.

  memset(chaine, '\0', sizeof(chaine)); // Init to all null chars, len = 0
  strcpy(chaine, "WORDS"); // Copy a string, len = 5

  char *chaine2 = function(chaine, sizeof(chaine));
  printf("%s\n", chaine2);
  free (chaine2);
}

When you pass an array to a function, it’s treated like a pointer. So sizeof(str) inside the function will always return the size of pointer-to-char, and not the size of the original array. If you want to know how long the string is, make sure it’s null-terminated and use strlen() like this:

char *function(char *str, int len) {
  // Assume str = "WORDS", len = 20.
  char *new_str = malloc(len); // Create a new string, size = 20

  memset(new_str, '\0', len);  // Initialize to nulls
  memset(new_str, '*', strlen(str)); // Copy 5 '*' chars, len = 5

  return new_str;  // Pointer to 20 bytes of memory: 5 '*' and 15 '\0'
}