CODEWITHSUNDEEP
cstringpointersarrays
phunehehe
phunehehe

Reputation: 8778

how to use char* as char[]

I have a struct like this

typedef struct bookStruct
{
   char title[80];
   char author[80];
} BookType;

And I have two strings like this

char *title = "A Book on C";
char *author = "A. Kelly";

Now I can't create a BookType like this

BookType book = {title, author};

Can anyone tell me what is wrong? How can I do that?

Upvotes: 1

Views: 3665

Answers (5)

Alex Budovski
Alex Budovski

Reputation: 18436

You must use strcpy (if you know the length of the input) or a safe function instead.

A lot of the other answers made the same mistake of leaving un-terminated strings, a major source of security vulnerabilities.

The correct way is to use a safe string copy function, like StringCbCopy or roll your own (albeit not as robust):

// Copy at most n-1 characters to dst, truncating if strlen(src) > n-1
// and guaranteeing NUL-termination.
void safe_strcpy(char *dst, const char *src, size_t n) {
  strncpy(dst, src, n-1);
  dst[n-1] = 0;  // Guarantee NUL-termination.
}

Then you may use it as follows

void f(const char *title, const char *author) {
  BookType book;
  safe_strcpy(book.title, title, sizeof book.title);
  safe_strcpy(book.author, author, sizeof book.author);
}

Upvotes: 2

N 1.1
N 1.1

Reputation: 12524

void InitBookStruct(BookType *book, const char *title, const char *author){
   size_t title_length = sizeof book->title;
   size_t author_length = sizeof book->author;

   strncpy(book->title, title, title_length - 1); //-1, make way for null byte
   strncpy(book->author, author, author_length - 1);

   book->title[title_length - 1] = 0;
   book->author[author_length - 1] = 0;
}

Many ways to do, above is one of them.

From man pages,

char *strncpy(char *dest, const char *src, size_t n);

If the length of src is less than n, strncpy() pads the remainder of dest with null bytes.

So, specifying (one less than) the size of the dest is sufficient.

Upvotes: 2

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208323

There are two possible solutions to your problem. The first of which is using the string literals in the place of construction:

BookType book = { "A book on C", "A. Kelly" };

In this case the compiler will copy the literals to the appropriate variables. If you cannot use the literals in the initialization, then you must copy the elements yourself:

BookType book = { 0 }; // 0 initalize
strncpy( book.title, title, sizeof(book.title)-1 );   
strncpy( book.author, author, sizeof(book.author)-1 );

Upvotes: 5

Ed.
Ed.

Reputation: 1952

If you change your structure to this it should work

typedef struct bookStruct
{
   char* title;
   char* author;
} BookType;

Upvotes: 1

anon
anon

Reputation:

As far as I'm aware, there is no way of doing that in C. Your best bet is probably to use a macro:

#define TITLE "A Book On C"
#define AUTHOR "A. Kelley"

BookType book {TITLE, AUTHOR};

though this of course does not have exactly the same effect.

Upvotes: 1

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