Why does this give an error saying "request for data element in something not a structure"?

What's the problem with the commented line?

#include<stdio.h>


typedef struct _stack
{
    char data;
    struct stack* next;开发者_StackOverflow中文版
}stack;

int main()
{
    char *s="4$2*3-3+8/4/(1+1)";
    char * prefix= (char*) malloc(strlen(s+1)); 
    convertinfixtoprefix(s,prefix);
    int len=strlen(prefix);
    int i;
    for(i=0;i<len/2;i++)
    {
        char temp;
        temp=prefix[i];
        prefix[i]=prefix[len-i-1];
        prefix[len-i-1]=prefix[i];
    }
    printf("The Prefix expression is %s",prefix);
}


void convertinfixtoprefix(char* infix,char*prefix)
{
    stack *s=NULL;
    int pr;
    char c=NULL;
    char d;
    int i=strlen(infix)-1;
    int index=0;

    while(i>=0)
    {
        c=infix[i];
        if(c==')')
        {
            push(&s,c);
        }
        else
        if(c=='(')
        {
            while((d=pop(&s))!=')')
            {
                prefix[index]=d;
                index++;
            }
        }

        if( (c!='+') && (c!='-') && (c!='*') && (c!='/') && (c!='$') ) 
        {
            prefix[index]=c;
            index++;
        }   
        else
        {
            pr=priority(d=pop(&s));
            while(pr>priority(c))
            {
                prefix[index]=d;
                index++;
                pr=priority(d=pop(&s));
            }
            push(&s,c);
        }
    }
}


void push(stack** s,char c)
{
    if(*s==NULL)
    {
    *s=(stack*)malloc(sizeof(stack));
    ((*s)->data)=c;// HERE ITS SHOWS ERROR WHY?
    ((*s)->next)=NULL;
        }
    else
    {
        stack* temp=(stack*)malloc(sizeof(stack));
        temp->data=c;
        temp->next=*s;
        (*s)=temp;
    }
}


char pop(stack**s)
{
    if(*s==NULL)
    printf("Stack khaali hai bhai\n");
    else
    {
    char c=*s->data;
    stack* temp=*s;
    *s=*s->next;
    free(temp);
        return c;   
    }
    return 0;
}


int priority(char c)
{
    if(c=='+'||c=='-')
    {   return 1;
    }
    else if(c=='*'||c=='/'||c=='%')
    {   return 2;
    }
    else if(c=='$')
    {
        return 3;
    }
}

Your error is not where you indicated it.

It's these lines:

char c=*s->data;
stack* temp=*s;
*s=*s->next;

Which should be:

char c=(*s)->data;
stack* temp=*s;
*s=(*s)->next;

There's a handful other bad stuff too, e.g. you need to incude the stdlib.h and string.h header - and a handful of your functions needs a prototype before you call them. Compile with warnings on, and you'll see these errors.

I'm afraid the recursion in your struct _stack isn't right. The pointer to next should be also declared as struct _stack *next, instead of referencing to the type stack you created...

Very often that error pops up when your struct/class (C/C++/ObjectiveC/...) isn't defined in the scope of the access to a member.

Check your included files, might be the simple cause.