A general protection exception failure

Hi, I am programming something to change from infix to postfix notation, However I get a general protection exception when i try to call the function that checs if the notation is correct.
I don´t even know what a general protection exception is, can someone tell me please what is it, and also, can someone help me to get rid of it, i´ll place the first part of the code:

Code:

void cambio(int,int);    //esta funcion cambia entre las bases
char *pedir_infija();    //esta funcion pide la expresion de forma infija
char *posfija(char*);   //esta funcion cambia la expresion a postfija
int comprobar(char*);   //comprueba que la expresi¢n se haya dado de manera correcta
int eval(char *);        //Da el resultado de la postfija
int convnum(char);       //Convierte el caracter a su numero equivalente

void main(){
	clrscr();
	int comp=0,resul=0,op=0,z=1,yapos=0,yain=0,sicon=0,base=10;
	char *expre, *postex;
	while(op!=4){
		op=0;
		while(op>z||op<1){
			z=1;
			printf("\n\n---------Convertidor de expresiones---------\n");
			printf("\n%d. Aceptar infija",z++);
			if(yain==1){
				printf("\n%d. Convertir a postfija",z++);
			}
			if (yapos==1){
				if(resul==-60){
					printf("\nOperacion invalida division entre cero");
				}else{
					printf("\n%d. Evaluar postfija",z++);
				}
			}
			printf("\n%d. Salir\n\n Opcion: ",z);
			scanf("%d",&op);
			getchar();
		}
		if(op==z)op=4;
		switch(op){
			case 1:
				comp=0;
				while(comp==0){   //mientras no sea valida la expresion la pedirI am silly de nuevo
					printf("\nDame la expresion infija: ");
					scanf("%s",&expre);
					getchar();
					comp=comprobar(expre);      /* general protectio exception is here*/
				}
				//printf("\nLa expresi¢n infija es: %s",expre);
				yain=1;
				yapos=0;
				break;
			case 2:
				postex=posfija(expre);   //llamada a la conversion a postfija
				printf("Postfija: ");
				printf("%s",postex);
				resul=eval(postex);
				yapos=1;
				break;
			case 3:
				//calcula el resultado
				printf("Resultado: %d",resul);
				sicon=0;
				while(sicon!=2){
					sicon=0;
					while(sicon>2||sicon<1){
						printf("\nConvertir? (1.Si,2.No): ");
						scanf("%d",&sicon);
						getchar();
					}
					if(sicon!=2){
						printf("\nEscribe la base: ");
						scanf("%d",&base);
						cambio(resul,base);
					}
				}
				break;
		}
	}
}





int comprobar(char *expre){      /*here is where the function begins*/


	int x,y=1,abr=0,cer=0;
	for(x=0;x<strlen(expre);x++){   //va comprobando cada caracter
		//comprueba de la lista de posibles caracteres que no haya ninguno
		if(expre[x]!='+'&&expre[x]!='-'&&expre[x]!='*'&&expre[x]!='/'&&expre[x]!='%'&&expre[x]!='('&&expre[x]!=')'&&!isdigit(expre[x])) y=0;
		if(expre[x]=='+'||expre[x]=='-'||expre[x]=='%'||expre[x]=='/'||expre[x]=='*'){     //En caso de que sea operador
			if((!isdigit(expre[x-1])&&expre[x-1]!=')')||x==0){      //si el anterior a l operador no es un digito
				y=0;
			}
		}
		if(expre[x]=='('){     //cuenta parentesis abiertos
			abr++;
		}
		if(expre[x]==')'){     //cuenta parentesis cerrados
			cer++;
		}
		if(expre[x]==')'){     //verifica que cuando cierre parentesis lo anterior sea digito
			if(!isdigit(expre[x-1])&&expre[x-1]!=')') y=0;
		}
	}
	if(abr!=cer) y=0;  //si no son la misma cantidad de parentesis abiertos que cerrados
	if(!isdigit(expre[strlen(expre)-1])&&expre[strlen(expre)-1]!=')') y=0;   //si el ultimo caracter es invalido
	if(y==0) printf("Expresion Invalida\n\n");
	return y;
}

}

I wont´paste the rest of the code, since it is very long as you can see

thank you

Use a debuger and see what happens step by step. The fault will happen somewhere in your code and you'll see where.

I have added in the code the place where it marks the mistake the step by step debugging process,

Code:

comp=comprobar(expre);      /* general protection exception is here*/

However i can't figure out what the mistake is.

I have also marked the function it is calling.

help please!

I am really mad about this mistake

Please post code that compiles, and post ALL the functions, otherwise there's no way to us to find tou what's wrong. Don't repost your code, edit the 1st post.

Bad:

Code:

	char *expre;
	scanf("%s",&expre);

Better:

Code:

	char expre[100];
	scanf("%.99s", expre);

Unreadable:

Code:

if(expre[x] != '+' &&expre[x]!='-'&&expre[x]!='*'&&expre[x]!='/'&&expre[x]!='%'&&expre[x]!='('&&expre[x]!=')'&&!isdigit(expre[x])) y=0;

Readable:

Code:

if (!strchr("+-*/%()", expre[x]) && !isdigit(expre[x]))
{
	y = 0;
}

Code:

if((!isdigit(expre[x-1])&&expre[x-1]!=')')||x==0){

What happens if x is 0?


--

[edit]
nvm
my bad
(i cant find my reading glasses )

> void main(){
> clrscr();
Wow - void main, and the need to attempt to use a C++ compiler to compile this C code.