CODEWITHSUNDEEP
cstructmemset
Wipqozn
Wipqozn

Reputation: 1314

What is the proper way to use memset on a struct element?

I'm trying to use memset on a struct element like so:

memset( &targs[i]->cs, 0, sizeof( xcpu ) );

However, doing so gives me a segmentation fault. I neither understand why this is failing, nor how I can make it work. What is the proper way to use memset on an element of a struct, and why does my method not work?

Line which allocates memory for targs:

eargs **targs = (eargs **) malloc(p * sizeof(eargs *));

Struct definitions for struct element cs (xcpu_context) and struct targs (execute_args):

typedef struct xcpu_context {
  unsigned char *memory;              
  unsigned short regs[X_MAX_REGS];    
  unsigned short pc;                  
  unsigned short state;              
  unsigned short itr;                 
  unsigned short id;                 
  unsigned short num;                 
} xcpu;

typedef struct execute_args {
    int ticks;
    int quantum;
    xcpu cs;
} eargs;

Upvotes: 1

Views: 3114

Answers (2)

Adam Mihalcin
Adam Mihalcin

Reputation: 14468

You have allocated an array of pointers in the line

eargs **targs = (eargs **) malloc(p * sizeof(eargs *));

but you haven't initialized the elements themselves. So this segfault has nothing to do with properly using memset on the fields of a struct, but instead derives from using uininitialized memory (assuming that you don't have a loop to initialize each eargs object after you allocate the array of pointers).

Instead, if you wanted to allocate a dynamic array of p eargs objects (I'm using the term "objects" loosely here), you would write

eargs *args = malloc(p * sizeof(eargs));
if (!args) {
    /* Exit with an error message */
}
memset(&(args[i].cs), 0, sizeof(xcpu));

instead. Note that args is a dynamically allocated array of eargs objects, not a dynamically allocated array of pointers, so it's of type eargs * rather than eargs **.

Upvotes: 2

Carl Norum
Carl Norum

Reputation: 225042

Your memory allocation line doesn't allocate any memory for any structures, only for pointers to structures. If you want to allocate memory for that whole array, you need to add a loop to allocate memory for the structures themselves:

for (i = 0; i < p; i++)
    targs[i] = malloc(sizeof(eargs));

Once you actually have structures to operate on, your memset() call should be fine.

Upvotes: 1

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