Allocate memory for a struct in a function

Question

I need to make an adfgx machine (Code language from WWII) for a project in school. But I am running into some problems.

There is a struct along with some functions defined in adfgx.h which looks like this:

typedef struct {
   char* alphabet;         
   char* symbols;          
   char* dictionary;       
   char* transposition;    
} adfgx;

In adfgx.c we include the header and I have to write a function that allocates memory for this struct with a predefined signature:

/* Creates a new ADFGX machine */
adfgx* create_adfgx(const char* alphabet, const char* symbols, const char* dictionary, const char* transposition);

So what I am supposed to do here is allocate memory for the struct in that function. I don't get how I am supposed to do this, because I don't now the size of alphabet, symbols, dictionary and transposition, so how can I now hom much memory I need to allocate?

WhozCraig · Accepted Answer · 2015-03-10 18:08:07Z

1

I don't now the size of alphabet, symbols, dictionary and transposition

Since the size is not passed in, the API must assume that const char* parameters represent C strings. This makes it possible to compute the desired size by calling strlen, and adding one to the result for the null terminator.

To avoid doing the same thing multiple times, I suggest defining your own function for string duplication:

static char *duplicate(const char *s) {
    if (s == NULL) return NULL;
    size_t len = strlen(s)+1;
    ... // Call malloc, check results, make a copy
    return res;
}

Now you can use this function to populate the struct:

adfgx *res = malloc(sizeof(adfgx));
if (res == NULL) ...
res->alphabet = duplicate(alphabet);
...
return res;

edited Mar 10, 2015 at 18:08

WhozCraig

66.5k11 gold badges82 silver badges148 bronze badges

answered Mar 10, 2015 at 18:01

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jonathan Leffler Over a year ago

Any particular reason to suggest writing your own variant of strdup() without mentioning that it is what you're suggesting?

Sergey Kalinichenko Over a year ago

@JonathanLeffler This is almost certainly a learning exercise, and writing their own version of strdup is probably its centerpiece. If professor wanted them to use strdup, OP would probably never asked this question :-)

Jonathan Leffler Over a year ago

One possible interpretation, yes. It isn't the one that occurred to me, but it is far from impossible. Another possible interpretation is that the function groups its arguments into a structure for convenience; the strings pointed at must last as long as code will be using the structure. Then all that's needed is to copy the pointers into the allocated structure. In that case, strdup() is not needed at all. Still, on average, it is more likely that the strings must be duplicated, but I'm not sure whether strdup() is verboten. I would have mentioned it -- these comments now suffice.

user3458 · Accepted Answer · 2015-03-10 18:07:33Z

1

The size of adfgx does not depend on "the size of alphabet, symbols, dictionary and transposition" - it's the size of 4 pointers.

As to assigning values to newAdfgx->alphabet and other members, you can use strdup. Just be sure to free() the strduped strings when you free the instance of adfgx.

answered Mar 10, 2015 at 18:07

user3458

Collectives™ on Stack Overflow

Allocate memory for a struct in a function

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related