12 September 2011

How to determine the amount of memory required by my char* in struct?

Suppose you have the following C struct:
  1. typedef struct
  2. {
  3.     char* name;
  4. }Emp;
I asked in SO on how to determine the amount of memory to allocate to the char* struct member. and the answer was to use lazy/delayed allocation as of:
  1. void setName(Emp* emp, char* newName)
  2. {
  3.     free(emp->name);
  4.     emp->name = malloc(strlen(newName) + 1);
  5.     strcpy(emp->name, newName);
  6. }
(It seems to me to be the same idea used for Objective-C, the setter is release the previous pointer and then retain (in C to copy) the parameter pointer. So, the whole program will be:
  1. #include <stdio.h>
  2. #include <string.h>
  3. #include <stdlib.h>
  4. 
  5. typedef struct
  6. {
  7.     char* name;
  8. }Emp;
  9. 
 10. void init(Emp** emp)
 11. {
 12.     *emp = malloc(sizeof(Emp));
 13.     (*emp)->name = NULL;
 14.     (*emp)->name = malloc(sizeof(char*));
 15. }
 16. 
 17. void release(Emp** emp)
 18. {
 19.     free((*emp)->name);
 20.     free(*emp);
 21. }
 22. 
 23. void setName(Emp* emp, char* newName)
 24. {
 25.     free(emp->name);
 26.     emp->name = malloc(strlen(newName) + 1);
 27.     strcpy(emp->name, newName);
 28. }
 29. char* getName(Emp* emp)
 30. {
 31.     return emp->name;
 32. }
 33. 
 34. int main(void)
 35. {
 36.     Emp* emp;
 37.     init(&emp);
 38.     setName(emp, "Muhammad                              Abdullah");
 39.     printf("%s", getName(emp));
 40.     release(&emp);
 41.     
 42.     return 0;
 43. }
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