CODEWITHSUNDEEP
cpointerssegmentation-faultscanf
minzey
minzey

Reputation: 116

Program immediately terminates with segfault when using scanf

When I use gets() or fgets() instead of scanf(), the program does execute completely but prints segmentation fault(core dumped) in the end! I don't understand why am I getting segfault in both the cases. Here is the code for converting an infix to postfix exp using stacks.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

typedef struct stack{
    int top;
    int capacity;
    int *array;
}stack;

stack* createstack(char *);
void push(stack* ,int );
int isempty(stack *);
int pop(stack *st);
int peek(stack *st);
int precedence(char c);

int main(){
char exp[100];
char post[100];
int k=-1;
stack *st;
int i=0,p=0;
printf("enter string expression: ");
//gets(exp); 
//fgets(exp, sizeof(exp), stdin);
scanf("%s",exp);
printf("Infix expression : %s",exp);
st=createstack(exp);

for(i=0;i<strlen(exp);i++){
    if( (exp[i]>='a' && exp[i]<='z') || (exp[i]>='A' && exp[i]<='Z'))
        post[++k]=exp[i];
    else if(exp[i]=='(')
        push(st,exp[i]);
    else if(exp[i]==')'){
        while(!isempty(st) && peek(st)!='(')
            post[++k]=pop(st);
        pop(st);
    }
    else{
        while(precedence(exp[i]) < precedence(peek(st)))
            post[++k]=pop(st);
        push(st,exp[i]);
    }
}
while(!isempty(st))
    post[++k]=pop(st);

//post[++k]='\0';
printf("Postfix expression :\n%s\n",post);
return 0;

}

stack* createstack(char *exp){
stack* st;
st->top=-1;
st->capacity=strlen(exp);
st->array=(int*)malloc(st->capacity * sizeof(int));
printf("Stack created successfully\n");
return st;
}

void push(stack* st,int val){
st->array[++st->top]=val;
}

int isempty(stack *st){
return st->top==-1;
}

int pop(stack *st){
return st->array[st->top--];
}

int peek(stack *st){
return st->array[st->top];
}

int precedence(char c){
switch(c){
    case '(':
        return 0;
        break;
    case '+':
        return 1;
        break;
    case '-':
        return 1;
        break;
    case '*':
        return 2;
        break;
    case '/':
        return 2;
        break;

    case '^':
        return 3;
        break;
    }
}

Upvotes: 0

Views: 316

Answers (1)

Sourav Ghosh
Sourav Ghosh

Reputation: 134326

In your code,

stack* st;
st->top=-1;

you're using st uninitialized which in turn invokes undefined behaviour.

You need to allocate memory to st before using it.

Try something like

stack* st = malloc(sizeof*st);  //also, check for malloc success

That said,

  1. Please see why not to cast the return value of malloc() and family in C.

  2. The recommended signature of main() is int main(void).

Upvotes: 3

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