CODEWITHSUNDEEP
cpointersstrcpy
thesmizzle
thesmizzle

Reputation: 21

What i should use: strcpy or pointers?

typedef struct node{
char *name;
//etc...
}N

If i had char *string got from stdin and i wanted to pass it to N, i could do easily N->name=string, or it would be bettere to use strcpy? which is more correct? which is more efficient? why?

Upvotes: 0

Views: 216

Answers (2)

AugustinLopez
AugustinLopez

Reputation: 388

You want to use a buffer to read your input from stdin. But if you don't make a copy, your string will be lost at the next read iteration. If you are going to allocate memory, and if you don't know the size of the string you receive, you should use a function like strdup instead of strcpy. 

node->name = strdup(string_to_copy);
if (node->name == NULL)
     printf("My allocation has failed!\n");

The allocated memory will have to be freed afterward. If the allocation fails, you will have a message (you should always check the success of your memory allocation).

If you don't want to allocate memory and if you know that the size of your input will never be higher than say, 100, you could get away with something like this:

typedef struct node{
char name[100];
//etc...
}N

In that case you could use the following. Note that strcpy is a dangerous function that should be used with extra-care:

strncpy(node->name, buffer, 100);
strcpy(node->name, buffer);

EDIT:

Here is a way to free your memory properly:

if (node->name != NULL)
{
   free(node->name);
   node->name = NULL;
}

This construction will free your memory and helps preventing double-free.

Upvotes: 2

Joshua
Joshua

Reputation: 43327

Answering "the result is the same, but what's the difference?" (from comment)

Starting from:

N *node = malloc(sizeof(N));
char input_from_stdin[80];

If we do

strcpy(node->name, input_from_stdin);

we invoke undefined behavior, but it might appear to work.

If we do 

node->name = input_from_stdin;

than node->name only contains its expected value until input_from_stdin is reused, which is typically when the next line is read form input.

If we do

node->name = malloc(strlen(input_from_stdin) + 1);
strcpy(node->name, input_from_stdin);

we make a copy of the input that lives until we free it; but we must remember to free node->name before freeing node.

We can also do

typedef struct node{
    char name[MAXSIZE];
    //etc...
 }N;

and then expect

strncpy(node->name, input_from_stdin, MAXSIZE-1)[MAXSIZE-1] = 0;

to work.

Some poeple do

strncpy(node->name, input_from_stdin, MAXSIZE);

which is valid but getting it back out again is touchier. This is rarely seen out of fixed-width records on disk now. We don't like MAXSIZE much anymore and prefer everything stretchy.

Upvotes: 0

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