Welcome to the MacNN Forums.

Ghoser777 · Jun 15, 2002, 05:11 PM

Okay, so I found this very basic regular math expression parser, and it works great!... except that if you enter an expression long enough (about 25 characters in length) it dies. The code isn't that complicated, but I can't see where the crash is happening.

Anyone see what's wrong?

<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">
/* file calcu.c */
/* (c) Donald Axel GPL - license */
/* ANSI - C program demonstration, command line calculator */
/* Recursive descent parser */
/* Improve: Make a HELP command. Add more variables. */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include "calcu.h"

#define stricmp strcasecmp
#define strnicmp strncasecmp

#define MAXL 8196
char gs[MAXL];
char *cp;
char *errorp;
double oldval;

int nextchar()
{
++cp;
while (*cp == ' ')
++cp;
return *cp;
}

int eatspace()
{
while (*cp == ' ')
++cp;
return *cp;
}

/* More local prototypes. This could, of course, be a separate file. */
double expression();
double product();
double potens();
double signedfactor();
double factor();
double stdfunc();

int evaluate(char *s, double *r)
{
cp = s;
eatspace();
*r = expression();
eatspace();
if (*cp == '\n' && !errorp)
return (0);
else
return (cp - s) + 11;
}

double expression()
{
double e;
int opera2;

e = product();
while ((opera2 = *cp) == '+' || opera2 == '-') {
nextchar();
if (opera2 == '+')
e += product();
else
e -= product();
}
eatspace();
return e;
}

double product()
{
double dp;
int ope;

dp = potens();
while ((ope = *cp) == '*' || ope == '/') {
nextchar();
if (ope == '*')
dp *= potens();
else
dp /= potens();
}
eatspace();
return dp;
}

double potens()
{
double dpo;

dpo = signedfactor();
while (*cp == '^') {
nextchar();
dpo = exp(log(dpo) * signedfactor());
}
eatspace();
return dpo;
}

double signedfactor()
{
double ds;
if (*cp == '-') {
nextchar();
ds = -factor();
} else
ds = factor();
eatspace();
return ds;
}

double factor()
{
double df;

switch (*cp) {
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
df = strtod(cp, &cp);
break;
case '(':
nextchar();
df = expression();
if (*cp == ')')
nextchar();
else
errorp = cp;
break;
case 'X':
nextchar();
df = oldval;
break;

default:
df = stdfunc();
}

eatspace();
return df;
}

char *functionname[] =
{
"abs", "sqrt", "sin", "cos", "tan", "asin", "acos" "atan", "log", "exp", "sinh", "cosh",
"tanh", "\0"
};

double stdfunc()
{
double dsf;
char **fnptr;
int jj;

eatspace();
jj = 0;
fnptr = functionname;
while (**fnptr) {
if (strncmp(*fnptr, cp, strlen(*fnptr)) == 0)
break;

++fnptr;
++jj;
}
if (!**fnptr) {
errorp = cp;
return 1;
}
cp += (strlen(*fnptr) - 1);
nextchar();
dsf = factor();
switch (jj) {
case 0: dsf = abs(dsf); break;
case 1: dsf = sqrt(dsf); break;
case 2: dsf = sin(dsf); break;
case 3: dsf = cos(dsf); break;
case 4: dsf = tan(dsf); break;
case 5: dsf = asin(dsf); break;
case 6: dsf = acos(dsf); break;
case 7: dsf = atan(dsf); break;
case 8: dsf = log(dsf); break;
case 9: dsf = exp(dsf); break;
case 10: dsf = sinh(dsf); break;
case 11: dsf = cosh(dsf); break;
case 12: dsf = tanh(dsf); break;
default:{
errorp = cp;
return 4;
}
}
eatspace();
return dsf;
}

/* end calcu.c */</pre><hr /></blockquote>This is how I was using it:

<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">+(NSString *)evaluateExpression

NSString *)string
{
char gs;
int rv;
double oldval;

[string getCString:&gs];
rv = evaluate(&gs, &oldval);

return [NSString stringWithFormat:@"%g", oldval];
}</pre><hr /></blockquote>Thanx in advance,
F-bacher

SpotBug · Jun 15, 2002, 06:38 PM

<blockquote>quote:<hr />Originally posted by Ghoser777:
<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">+(NSString *)evaluateExpression

NSString *)string
{
char gs;
int rv;
double oldval;

[string getCString:&gs];
rv = evaluate(&gs, &oldval);

return [NSString stringWithFormat:@"%g", oldval];
}</pre><hr /></blockquote><hr /></blockquote>I could be very wrong (I have limited Objective-C experience), but this doesn't look right to me. You're trying to pass the address of the passed-in NString's buffer to evaluate(), but I don't see how you're getting that address. &gs certainly isn't the address of the string's buffer, it's the address of the single char you've got on the stack. My guess is you're call to evaluate() is trouncing on the stack.

Detrius · Jun 15, 2002, 06:51 PM

<blockquote>quote:<hr />Originally posted by SpotBug:
 <blockquote>quote:<hr />Originally posted by Ghoser777:
<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">+(NSString *)evaluateExpression

NSString *)string
{
char gs;
int rv;
double oldval;

[string getCString:&gs];
rv = evaluate(&gs, &oldval);

return [NSString stringWithFormat:@"%g", oldval];
}</pre><hr /></blockquote><hr /></blockquote>I could be very wrong (I have limited Objective-C experience), but this doesn't look right to me. You're trying to pass the address of the passed-in NString's buffer to evaluate(), but I don't see how you're getting that address. &gs certainly isn't the address of the string's buffer, it's the address of the single char you've got on the stack. My guess is you're call to evaluate() is trouncing on the stack.<hr /></blockquote>I would agree with this. You are passing the address of a single character instead of a string. Thus, you are writing over memory that you shouldn't be. gs is globally defined in the code given, and it is a HUGE array. I would recommend removing YOUR definition of gs, as this is creating a local gs that overrides the global gs. The other option is to make your local declaration of gs into a large array, somewhat like this:

<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;"> char gs[5120]; </pre><hr /></blockquote>then you can put 5k of text in to be evaluated.

Ghoser777 · Jun 15, 2002, 07:02 PM

Excellent. Thank you so much! My C knowledge just grew

F-bacher

lindberg · Jun 16, 2002, 12:51 AM

Is there any reason this wouldn't work?

<blockquote>code:<hr /><pre style="font-size:x-small; font-family: monospace;">+(NSString *)evaluateExpression

NSString *)string
{
int rv;
double oldval;

rv = evaluate([string cString], &oldval);

return [NSString stringWithFormat:@"%g", oldval];
}</pre><hr /></blockquote>At any rate, yes, -getCString assumes that you have allocated storage for it to put the c-string into. You want to make sure that the storage size is at least [string length]+1 long, or do something like:

char gs[1024];

...

[string getCString:gs maxLength:sizeof(gs)-1];

otherwise the getCString method will write into unknown memory locations if the length is longer than you were expecting. [This is what was happening in your original code btw.]