c - Program immediately terminates with segfault when using scanf -

when use gets() or fgets() instead of scanf(), program execute prints segmentation fault(core dumped) in end! don't understand why getting segfault in both cases. here code converting infix 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;     } }        

in code,

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

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

you need allocate memory st before using it.

try like

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

that said,

1. please see why not cast return value of malloc() , family in c.

2. the recommended signature of main() int main(void).