CODEWITHSUNDEEP
cscanf
user3595632
user3595632

Reputation: 5730

Is there anything that i missed in signed number in C language?

This is my basic C test program. After I built it, I just entered negative number like -1, -2, etc. in console. But the result is "oh", not "another number". I don't know why this happens, because negative numbers should make the 'if' statement true.

int main(int argc, char* argv[]){
    long int num;

    scanf("%d", &num);

    if(num ==1 || num < 0){
        printf("another number\n");
    }else{
        printf("oh\n");
    }
}

Upvotes: 0

Views: 120

Answers (3)

Jasen
Jasen

Reputation: 12402

/tmp$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:4:5: warning: implicit declaration of function ‘scanf’ [-Wimplicit-function-declaration]
foo.c:4:5: warning: incompatible implicit declaration of built-in function ‘scanf’ [enabled by default]
foo.c:4:5: warning: format ‘%d’ expects argument of type ‘int *’, but argument 2 has type ‘long int *’ [-Wformat]
foo.c:7:9: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
foo.c:7:9: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
foo.c:9:9: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
foo.c:11:1: warning: control reaches end of non-void function [-Wreturn-type]

fix the causes of those warnings and all the bugs will go away.

Upvotes: 1

Ben Voigt
Ben Voigt

Reputation: 283624

When you use the %d format string with scanf, the corresponding argument will be treated as int*. But you have passed a long int*. The value scanf stores will not be the same size as what your if statement reads.

Formally, you get undefined behavior. In practice, on most platforms scanf will write only part of the variable, and the rest will be left with an arbitrary value, with the usual bad effects on future use.

Upvotes: 2

John Kugelman
John Kugelman

Reputation: 361565

Use %ld for long variables, %d for ints. Change your code to one of these:

int num;
scanf("%d", &num);

or

long int num;
scanf("%ld", &num);

Upvotes: 2

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