结构和内存分配

Question

When you allocate space for memory how do you tell if you need to allocate more space for it? Is there a check or so you can do on your new memory to make sure it is doing OK? ( allocated memory for a struct).

Because what i am thinking is that a struct is a set amount of data and even though I pass it around a lot it should never need more than the size of the struct correct?

Answer 1

If you're just using a simple struct , you don't need more memory allocated for it as time goes on. You just create the struct , use it, and clean it up if required. If you are dynamically allocating your struct (ie: with malloc ), then you test the value of the pointer-to-struct you create and see if it is NULL . If it is NULL , then the memory allocation failed, and you can either retry, or abandon further operations (ie: exit on error condition).

#include <stdio.h>

typedef struct myStruct {
  int i;
  char c;
} myStruct;

int main(void) {
  // Static allocation, no cleanup required
  myStruct staticStruct;
  staticStruct.i = 0;
  staticStruct.c = 'c';

  // Dynamic allocation, requires cleanup
  myStruct* dynamicStruct;
  dynamicStruct = malloc(sizeof(myStruct));
  if (dynamicStruct == NULL) {
    printf("Memory allocation error!\n");
    return (-1);
  } else {
    printf("Successfully allocated memory!\n");
  }

  dynamicStruct->i = 1;
  dynamicStruct->c = 'd';
  free(dynamicStruct);  // Release allocated memory
  dynamicStruct = NULL; // Somewhat common practise, though not 100% necessary
  return 0;
}

Now, if you need to create an array of dynamically allocated structs, and you've used them all up, and need more, you'd likely be best off with a slightly more complicated approach, like a dynamically allocated linked list of structs. A good example can be found in the "References" section below. Also, I've included a link to a somewhat related question I answered on memory allocation in C. It has some good examples that might also help clear up this topic for you.

References

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1. C Linked List Data Structure Explained with an Example C Program , Accessed 2014-03-25, <http://www.thegeekstuff.com/2012/08/c-linked-list-example/>
2. Difference between declared string and allocated string , Accessed 2014-03-25, <https://stackoverflow.com/questions/16021454/difference-between-declared-string-and-allocated-string>