fscanf and pointer address question

[haloman]

Why does:
char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", myString) != EOF;

work yet

char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", &myString) != EOF;

doesn't (crashes, doesn't result in a compiler error)? I thought the
prior was incorrect, and the latter correct.

Many thanks.

P.S. Yes, I know I shouldn't be using fscanf like this, there are
buffer overflow problems and that I probably should be shot already,
but that's not the question being asked

 

[Peter Nilsson]

Why does:
char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", myString) != EOF;

work yet

char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", &myString) != EOF;

doesn't (crashes, doesn't result in a compiler error)? I thought the
prior was incorrect, and the latter correct.

<snip>

You're confusing the situation of arrays as opposed to pointers. See
the FAQ:

http://c-faq.com/stdio/scanf1a.html
http://c-faq.com/aryptr/aryvsadr.html

Taking the address of a pointer yeilds a pointer to _that_ pointer,
not what that pointer points to.

 

[santosh]

Why does:
char *myString = (char *) malloc(WORDLIMIT*sizeof(char));

In C, you don't need to cast the return value of malloc. Also take the
size of the object using sizeof, rather than the size of the object's
type.

fscanf(openFile, "%s", myString) != EOF;

work yet

char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", &myString) != EOF;

doesn't (crashes, doesn't result in a compiler error)? I thought the
prior was incorrect, and the latter correct.

fscanf expects a pointer to the object meant to hold the result of each
conversion. In the second snippet, you've passed it a value of type
pointer to pointer to char.

 

[haloman]

Okay, I understand now. Many thanks

 

[Nick Keighley]

(e-mail address removed) wrote:

char *myString = (char *) malloc(WORDLIMIT*sizeof(char));

the cast is unnecessary, and may hide an error. sizeof(char) is 1 by
definition
hence some would omit it.

char *myString = malloc (WORDLIMIT);

<snip>

 

[Richard Heathfield]

(e-mail address removed) said:

P.S. Yes, I know I shouldn't be using fscanf like this, there are
buffer overflow problems and that I probably should be shot already,
but that's not the question being asked

In my experience, it's best to fix the stuff you know how to fix before
worrying about the stuff you don't know how to fix. That way, you don't get
distracted.

 

[David T. Ashley]

Why does:
char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", myString) != EOF;

work yet

char *myString = (char *) malloc(WORDLIMIT*sizeof(char));
fscanf(openFile, "%s", &myString) != EOF;

doesn't (crashes, doesn't result in a compiler error)? I thought the
prior was incorrect, and the latter correct.

Many thanks.

P.S. Yes, I know I shouldn't be using fscanf like this, there are
buffer overflow problems and that I probably should be shot already,
but that's not the question being asked

Problem is "pointer to pointer to char" rather than "pointer to char", as
I'm sure you discovered.

One final note: "pointer to pointer to thingie" has its place. Sometimes
when you are using data structures where either:

a)The location might change in memory, OR

b)The canonical form of the empty set is a NULL pointer,

it is necessary to pass a pointer to a pointer to a data structure to a
function. That way, the function can swap out the pointer pointed to if
necessary.

But, as I'm sure you discovered ... not with scanf.