CODEWITHSUNDEEP
c++
Hyunan Kwon
Hyunan Kwon

Reputation: 575

Why does it cause a run-time error, character to int without initializing?

First of all, I apologize about poor english.

At next simple program,

void fx(int *a){
    for(int i=*a; i<='Z'; i++)
    printf("%c", i);
}
int main(){
    int a;
    scanf("%c", &a);
    fx(&a);
    return 0;
}

I entered a capital letter at run-time, it caused FATAL error and was solved by killing proccess.

It does not cause any problem at next codes.

//except fx()
int main(){
    int a;
    scanf("%c", &a);
    return 0;
}

or

//initialize int a
void fx(int *a){
    for(int i=*a; i<='Z'; i++)
    printf("%c", i);
}
int main(){
    **int a = 0;**
    scanf("%c", &a);
    fx(&a);
    return 0;
}

I know it should be 'char' to input character. but I cannot understand about above situation.

What happened?

PS. I worked with VS2010, c++

Upvotes: 2

Views: 182

Answers (2)

jpinto3912
jpinto3912

Reputation: 1465

You've declared an uninitialized int a, and set it's lower-most byte to something. The result may be a very large number, because the upper-most bytes (whether 16bit or 32bit integers) were left unassigned/uninitialized.

When you passed it to the function, this one will use the full extent of the int a representation. You've then setup a cycle, with stop condition "till it gets to 'Z'", which, by the way, will be correctly promoted to an integer (i.e. all upper-most non-used bytes as 0).

In that cycle you'll be forcing poor printf to try output a byte spanning from 0 to 0xff, several gazillion of times, depending on how long it may take the i to rollover into 'Z'... apparently somewhere in that printf code, someone didn't like non-printable (not just non-ascii) codes.

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258568

The difference between this

int a;
scanf("%c", &a);

and this

int a = 0;
scanf("%c", &a);

is that int a; declares an uninitialized a. So it can be anything. When you write

scanf("%c", &a);

on an uninitialized int, you're only setting the topmost bits, because %c tells scanf to write the input to a char, so only the first byte will be written. This can lead to some weird behavior, include what you just stated.

Upvotes: 8

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