Распознать и вычислить математическое выражение, указанное в текстовом файле

[C]#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <ctype.h>

int tok;
double tokval;
int ReadFile(char *fname);

int next() 
{
	for(;;)
	{  
		int c = getchar();
		if (c == EOF || strchr("+-*/^()\n", c) != NULL) return tok = c;
		if (isspace(c)) continue;
		if (isdigit(c) || c == '.') 
		{
			ungetc(c, stdin);
			scanf(" %lf", &tokval);
			return tok = 'n';
		}
		fprintf(stdout, "Некорректный ввод: %c\n", c); abort();
	}
	// return 0;
}

void skip(int t) 

	{if (tok == t) next();}

double expr();

double numpar() 
{
	if (tok == 'n') 
	{ double x = tokval; skip('n'); return x; }
	skip('('); double x = expr(); skip(')'); 
	return x;
}

double factor() 
{
	double x = numpar();
	if (tok == '^') 
	{ skip('^'); x = pow(x, factor()); }
	return x;
}

double term() 
{
	double x = factor();
	for (;;) 
	{
		if (tok == '*') { skip('*'); x *= factor(); }
		else 
		if (tok == '/') { skip('/'); x /= factor(); }
		else 
		return x;
	}
}

double expr() 
{
	 double x = term();
	 for (;;) 
	 {
		if (tok == '+') { skip('+'); x += term(); }
		else 
		if (tok == '-') { skip('-'); x -= term(); }
		else 
		return x;
	 }
}

int ReadFile(char *fname)
{
	FILE *fo;
	int z;
	
	if((fo=fopen(fname,"rf"))==NULL)
	{
		printf("\nОшибка: ");
		perror(fname);
		return 1;
	}
	else
	fscanf(fo,"%d\n", &z);
	fclose(fo);
	return 0;
}

int main() 
{	 
	 char *fname= "message.txt";
	 int r=ReadFile(fname);
	 
	 if (r == 0)
	 {
	 next();
		while (tok != EOF) 
		for (;;)
		{
			if (tok == r) 
			{skip(r);continue;}
			printf("%.9g\n", expr()); continue;
		}
	 }
	 else
	 next();
		while (tok != EOF) 
		for (;;)
		{
			if (tok == '\n') 
			{skip('\n');continue;}
			printf("%.9g\n", expr());
		}
	 return 0;
}[/C]