usr.bin/make/cond.c

/*
 * Copyright (c) 1988, 1989, 1990 The Regents of the University of California.
 * Copyright (c) 1988, 1989 by Adam de Boor
 * Copyright (c) 1989 by Berkeley Softworks
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Adam de Boor.
 *
 * %sccs.include.redist.c%
 */

#ifndef lint
static char sccsid[] = "@(#)cond.c	5.6 (Berkeley) 06/01/90";
#endif /* not lint */

/*-
 * cond.c --
 *	Functions to handle conditionals in a makefile.
 *
 * Interface:
 *	Cond_Eval 	Evaluate the conditional in the passed line.
 *
 */

#include    "make.h"
#include    <buf.h>
#include    <ctype.h>

/*
 * The parsing of conditional expressions is based on this grammar:
 *	E -> F || E
 *	E -> F
 *	F -> T && F
 *	F -> T
 *	T -> defined(variable)
 *	T -> make(target)
 *	T -> exists(file)
 *	T -> empty(varspec)
 *	T -> target(name)
 *	T -> symbol
 *	T -> $(varspec) op value
 *	T -> $(varspec) == "string"
 *	T -> $(varspec) != "string"
 *	T -> ( E )
 *	T -> ! T
 *	op -> == | != | > | < | >= | <=
 *
 * 'symbol' is some other symbol to which the default function (condDefProc)
 * is applied.
 *
 * Tokens are scanned from the 'condExpr' string. The scanner (CondToken)
 * will return And for '&' and '&&', Or for '|' and '||', Not for '!',
 * LParen for '(', RParen for ')' and will evaluate the other terminal
 * symbols, using either the default function or the function given in the
 * terminal, and return the result as either True or False.
 *
 * All Non-Terminal functions (CondE, CondF and CondT) return Err on error.
 */
typedef enum {
    And, Or, Not, True, False, LParen, RParen, EndOfFile, None, Err
} Token;

/*-
 * Structures to handle elegantly the different forms of #if's. The
 * last two fields are stored in condInvert and condDefProc, respectively.
 */
static Boolean	  CondDoDefined(),
		  CondDoMake();

static struct If {
    char	*form;	      /* Form of if */
    int		formlen;      /* Length of form */
    Boolean	doNot;	      /* TRUE if default function should be negated */
    Boolean	(*defProc)(); /* Default function to apply */
} ifs[] = {
    "ifdef",	  5,	  FALSE,  CondDoDefined,
    "ifndef",	  6,	  TRUE,	  CondDoDefined,
    "ifmake",	  6,	  FALSE,  CondDoMake,
    "ifnmake",	  7,	  TRUE,	  CondDoMake,
    "if",	  2,	  FALSE,  CondDoDefined,
    (char *)0,	  0,	  FALSE,  (Boolean (*)())0,
};

static Boolean	  condInvert;	    	/* Invert the default function */
static Boolean	  (*condDefProc)(); 	/* Default function to apply */
static char 	  *condExpr;	    	/* The expression to parse */
static Token	  condPushBack=None;	/* Single push-back token used in
					 * parsing */

#define	MAXIF		30	  /* greatest depth of #if'ing */

static Boolean	  condStack[MAXIF]; 	/* Stack of conditionals's values */
static int  	  condTop = MAXIF;  	/* Top-most conditional */
static int  	  skipIfLevel=0;    	/* Depth of skipped conditionals */
static Boolean	  skipLine = FALSE; 	/* Whether the parse module is skipping
					 * lines */

static Token	  CondT(), CondF(), CondE();

/*-
 *-----------------------------------------------------------------------
 * CondPushBack --
 *	Push back the most recent token read. We only need one level of
 *	this, so the thing is just stored in 'condPushback'.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	condPushback is overwritten.
 *
 *-----------------------------------------------------------------------
 */
static void
CondPushBack (t)
    Token   	  t;	/* Token to push back into the "stream" */
{
    condPushBack = t;
}

/*-
 *-----------------------------------------------------------------------
 * CondGetArg --
 *	Find the argument of a built-in function.
 *
 * Results:
 *	The length of the argument and the address of the argument.
 *
 * Side Effects:
 *	The pointer is set to point to the closing parenthesis of the
 *	function call.
 *
 *-----------------------------------------------------------------------
 */
static int
CondGetArg (linePtr, argPtr, func, parens)
    char    	  **linePtr;
    char    	  **argPtr;
    char    	  *func;
    Boolean 	  parens;   	/* TRUE if arg should be bounded by parens */
{
    register char *cp;
    int	    	  argLen;
    register Buffer buf;

    cp = *linePtr;
    if (parens) {
	while (*cp != '(' && *cp != '\0') {
	    cp++;
	}
	if (*cp == '(') {
	    cp++;
	}
    }

    if (*cp == '\0') {
	/*
	 * No arguments whatsoever. Because 'make' and 'defined' aren't really
	 * "reserved words", we don't print a message. I think this is better
	 * than hitting the user with a warning message every time s/he uses
	 * the word 'make' or 'defined' at the beginning of a symbol...
	 */
	*argPtr = cp;
	return (0);
    }

    while (*cp == ' ' || *cp == '\t') {
	cp++;
    }

    /*
     * Create a buffer for the argument and start it out at 16 characters
     * long. Why 16? Why not?
     */
    buf = Buf_Init(16);

    while ((index(" \t)&|", *cp) == (char *)NULL) && (*cp != '\0')) {
	if (*cp == '$') {
	    /*
	     * Parse the variable spec and install it as part of the argument
	     * if it's valid. We tell Var_Parse to complain on an undefined
	     * variable, so we don't do it too. Nor do we return an error,
	     * though perhaps we should...
	     */
	    char  	*cp2;
	    int		len;
	    Boolean	doFree;

	    cp2 = Var_Parse(cp, VAR_CMD, TRUE, &len, &doFree);

	    Buf_AddBytes(buf, strlen(cp2), (Byte *)cp2);
	    if (doFree) {
		free(cp2);
	    }
	    cp += len;
	} else {
	    Buf_AddByte(buf, (Byte)*cp);
	    cp++;
	}
    }

    Buf_AddByte(buf, (Byte)'\0');
    *argPtr = (char *)Buf_GetAll(buf, &argLen);
    Buf_Destroy(buf, FALSE);

    while (*cp == ' ' || *cp == '\t') {
	cp++;
    }
    if (parens && *cp != ')') {
	Parse_Error (PARSE_WARNING, "Missing closing parenthesis for %s()",
		     func);
	return (0);
    } else if (parens) {
	/*
	 * Advance pointer past close parenthesis.
	 */
	cp++;
    }

    *linePtr = cp;
    return (argLen);
}

/*-
 *-----------------------------------------------------------------------
 * CondDoDefined --
 *	Handle the 'defined' function for conditionals.
 *
 * Results:
 *	TRUE if the given variable is defined.
 *
 * Side Effects:
 *	None.
 *
 *-----------------------------------------------------------------------
 */
static Boolean
CondDoDefined (argLen, arg)
    int	    argLen;
    char    *arg;
{
    char    savec = arg[argLen];
    Boolean result;

    arg[argLen] = '\0';
    if (Var_Value (arg, VAR_CMD) != (char *)NULL) {
	result = TRUE;
    } else {
	result = FALSE;
    }
    arg[argLen] = savec;
    return (result);
}

/*-
 *-----------------------------------------------------------------------
 * CondStrMatch --
 *	Front-end for Str_Match so it returns 0 on match and non-zero
 *	on mismatch. Callback function for CondDoMake via Lst_Find
 *
 * Results:
 *	0 if string matches pattern
 *
 * Side Effects:
 *	None
 *
 *-----------------------------------------------------------------------
 */
static int
CondStrMatch(string, pattern)
    char    *string;
    char    *pattern;
{
    return(!Str_Match(string,pattern));
}

/*-
 *-----------------------------------------------------------------------
 * CondDoMake --
 *	Handle the 'make' function for conditionals.
 *
 * Results:
 *	TRUE if the given target is being made.
 *
 * Side Effects:
 *	None.
 *
 *-----------------------------------------------------------------------
 */
static Boolean
CondDoMake (argLen, arg)
    int	    argLen;
    char    *arg;
{
    char    savec = arg[argLen];
    Boolean result;

    arg[argLen] = '\0';
    if (Lst_Find (create, (ClientData)arg, CondStrMatch) == NILLNODE) {
	result = FALSE;
    } else {
	result = TRUE;
    }
    arg[argLen] = savec;
    return (result);
}

/*-
 *-----------------------------------------------------------------------
 * CondDoExists --
 *	See if the given file exists.
 *
 * Results:
 *	TRUE if the file exists and FALSE if it does not.
 *
 * Side Effects:
 *	None.
 *
 *-----------------------------------------------------------------------
 */
static Boolean
CondDoExists (argLen, arg)
    int	    argLen;
    char    *arg;
{
    char    savec = arg[argLen];
    Boolean result;
    char    *path;

    arg[argLen] = '\0';
    path = Dir_FindFile(arg, dirSearchPath);
    if (path != (char *)NULL) {
	result = TRUE;
	free(path);
    } else {
	result = FALSE;
    }
    arg[argLen] = savec;
    return (result);
}

/*-
 *-----------------------------------------------------------------------
 * CondDoTarget --
 *	See if the given node exists and is an actual target.
 *
 * Results:
 *	TRUE if the node exists as a target and FALSE if it does not.
 *
 * Side Effects:
 *	None.
 *
 *-----------------------------------------------------------------------
 */
static Boolean
CondDoTarget (argLen, arg)
    int	    argLen;
    char    *arg;
{
    char    savec = arg[argLen];
    Boolean result;
    GNode   *gn;

    arg[argLen] = '\0';
    gn = Targ_FindNode(arg, TARG_NOCREATE);
    if ((gn != NILGNODE) && !OP_NOP(gn->type)) {
	result = TRUE;
    } else {
	result = FALSE;
    }
    arg[argLen] = savec;
    return (result);
}


/*-
 *-----------------------------------------------------------------------
 * CondCvtArg --
 *	Convert the given number into a double. If the number begins
 *	with 0x, or just x, it is interpreted as a hexadecimal integer
 *	and converted to a double from there. All other strings just have
 *	atof called on them.
 *
 * Results:
 *	The double value of string.
 *
 * Side Effects:
 *
 *
 *-----------------------------------------------------------------------
 */
static double
CondCvtArg(str)
    register char    	*str;
{
    int	    	  	sign = 1;
    double  	  	atof();

    if (*str == '-') {
	sign = -1;
	str++;
    } else if (*str == '+') {
	str++;
    }
    if (((*str == '0') && (str[1] == 'x')) ||
	(*str == 'x'))
    {
	register int i;

	str += (*str == 'x') ? 1 : 2;

	i = 0;

	while (isxdigit(*str)) {
	    i *= 16;
	    if (*str <= '9') {
		i += *str - '0';
	    } else if (*str <= 'F') {
		i += *str - 'A' + 10;
	    } else {
		i += *str - 'a' + 10;
	    }
	    str++;
	}
	if (sign < 0) {
	    return((double)(-i));
	} else {
	    return((double)i);
	}
    } else if (sign < 0) {
	return(- atof(str));
    } else {
	return(atof(str));
    }
}

/*-
 *-----------------------------------------------------------------------
 * CondToken --
 *	Return the next token from the input.
 *
 * Results:
 *	A Token for the next lexical token in the stream.
 *
 * Side Effects:
 *	condPushback will be set back to None if it is used.
 *
 *-----------------------------------------------------------------------
 */
static Token
CondToken(doEval)
    Boolean doEval;
{
    Token	  t;

    if (condPushBack == None) {
	while (*condExpr == ' ' || *condExpr == '\t') {
	    condExpr++;
	}
	switch (*condExpr) {
	    case '(':
		t = LParen;
		condExpr++;
		break;
	    case ')':
		t = RParen;
		condExpr++;
		break;
	    case '|':
		if (condExpr[1] == '|') {
		    condExpr++;
		}
		condExpr++;
		t = Or;
		break;
	    case '&':
		if (condExpr[1] == '&') {
		    condExpr++;
		}
		condExpr++;
		t = And;
		break;
	    case '!':
		t = Not;
		condExpr++;
		break;
	    case '\n':
	    case '\0':
		t = EndOfFile;
		break;
	    case '$': {
		char	*lhs;
		char	*rhs;
		char	*op;
		int	varSpecLen;
		Boolean	doFree;

		/*
		 * Parse the variable spec and skip over it, saving its
		 * value in lhs.
		 */
		t = Err;
		lhs = Var_Parse(condExpr, VAR_CMD, doEval,&varSpecLen,&doFree);
		if (lhs == var_Error) {
		    /*
		     * Even if !doEval, we still report syntax errors, which
		     * is what getting var_Error back with !doEval means.
		     */
		    return(Err);
		}
		condExpr += varSpecLen;

		/*
		 * Skip whitespace to get to the operator
		 */
		while (isspace(*condExpr)) {
		    condExpr++;
		}
		/*
		 * Make sure the operator is a valid one. If it isn't a
		 * known relational operator, pretend we got a
		 * != 0 comparison.
		 */
		op = condExpr;
		switch (*condExpr) {
		    case '!':
		    case '=':
		    case '<':
		    case '>':
			if (condExpr[1] == '=') {
			    condExpr += 2;
			} else {
			    condExpr += 1;
			}
			break;
		    default:
			op = "!=";
			rhs = "0";

			goto do_compare;
		}
		while (isspace(*condExpr)) {
		    condExpr++;
		}
		if (*condExpr == '\0') {
		    Parse_Error(PARSE_WARNING,
				"Missing right-hand-side of operator");
		    goto error;
		}
		rhs = condExpr;
do_compare:
		if (*rhs == '"') {
		    /*
		     * Doing a string comparison. Only allow == and != for
		     * operators.
		     */
		    char    *string;
		    char    *cp, *cp2;
		    Buffer  buf;

		    if (((*op != '!') && (*op != '=')) || (op[1] != '=')) {
			Parse_Error(PARSE_WARNING,
		"String comparison operator should be either == or !=");
			goto error;
		    }

		    buf = Buf_Init(0);

		    for (cp = rhs+1; (*cp != '"') && (*cp != '\0'); cp++) {
			if ((*cp == '\\') && (cp[1] != '\0')) {
			    /*
			     * Backslash escapes things -- skip over next
			     * character, if it exists.
			     */
			    cp++;
			    Buf_AddByte(buf, (Byte)*cp);
			} else if (*cp == '$') {
			    int	len;
			    Boolean freeIt;

			    cp2 = Var_Parse(cp, VAR_CMD, doEval,&len, &freeIt);
			    if (cp2 != var_Error) {
				Buf_AddBytes(buf, strlen(cp2), (Byte *)cp2);
				if (freeIt) {
				    free(cp2);
				}
				cp += len - 1;
			    } else {
				Buf_AddByte(buf, (Byte)*cp);
			    }
			} else {
			    Buf_AddByte(buf, (Byte)*cp);
			}
		    }

		    Buf_AddByte(buf, (Byte)0);

		    string = (char *)Buf_GetAll(buf, (int *)0);
		    Buf_Destroy(buf, FALSE);

		    if (DEBUG(COND)) {
			printf("lhs = \"%s\", rhs = \"%s\", op = %.2s\n",
			       lhs, string, op);
		    }
		    /*
		     * Null-terminate rhs and perform the comparison.
		     * t is set to the result.
		     */
		    if (*op == '=') {
			t = strcmp(lhs, string) ? False : True;
		    } else {
			t = strcmp(lhs, string) ? True : False;
		    }
		    free(string);
		    if (rhs == condExpr) {
			condExpr = cp + 1;
		    }
		} else {
		    /*
		     * rhs is either a float or an integer. Convert both the
		     * lhs and the rhs to a double and compare the two.
		     */
		    double  	left, right;
		    char    	*string;

		    left = CondCvtArg(lhs);
		    if (*rhs == '$') {
			int 	len;
			Boolean	freeIt;

			string = Var_Parse(rhs, VAR_CMD, doEval,&len,&freeIt);
			if (string == var_Error) {
			    right = 0.0;
			} else {
			    right = CondCvtArg(string);
			    if (freeIt) {
				free(string);
			    }
			    if (rhs == condExpr) {
				condExpr += len;
			    }
			}
		    } else {
			right = CondCvtArg(rhs);
			if (rhs == condExpr) {
			    /*
			     * Skip over the right-hand side
			     */
			    while(!isspace(*condExpr) && (*condExpr != '\0')) {
				condExpr++;
			    }
			}
		    }

		    if (DEBUG(COND)) {
			printf("left = %f, right = %f, op = %.2s\n", left,
			       right, op);
		    }
		    switch(op[0]) {
		    case '!':
			if (op[1] != '=') {
			    Parse_Error(PARSE_WARNING,
					"Unknown operator");
			    goto error;
			}
			t = (left != right ? True : False);
			break;
		    case '=':
			if (op[1] != '=') {
			    Parse_Error(PARSE_WARNING,
					"Unknown operator");
			    goto error;
			}
			t = (left == right ? True : False);
			break;
		    case '<':
			if (op[1] == '=') {
			    t = (left <= right ? True : False);
			} else {
			    t = (left < right ? True : False);
			}
			break;
		    case '>':
			if (op[1] == '=') {
			    t = (left >= right ? True : False);
			} else {
			    t = (left > right ? True : False);
			}
			break;
		    }
		}
error:
		if (doFree) {
		    free(lhs);
		}
		break;
	    }
	    default: {
		Boolean (*evalProc)();
		Boolean invert = FALSE;
		char	*arg;
		int	arglen;

		if (strncmp (condExpr, "defined", 7) == 0) {
		    /*
		     * Use CondDoDefined to evaluate the argument and
		     * CondGetArg to extract the argument from the 'function
		     * call'.
		     */
		    evalProc = CondDoDefined;
		    condExpr += 7;
		    arglen = CondGetArg (&condExpr, &arg, "defined", TRUE);
		    if (arglen == 0) {
			condExpr -= 7;
			goto use_default;
		    }
		} else if (strncmp (condExpr, "make", 4) == 0) {
		    /*
		     * Use CondDoMake to evaluate the argument and
		     * CondGetArg to extract the argument from the 'function
		     * call'.
		     */
		    evalProc = CondDoMake;
		    condExpr += 4;
		    arglen = CondGetArg (&condExpr, &arg, "make", TRUE);
		    if (arglen == 0) {
			condExpr -= 4;
			goto use_default;
		    }
		} else if (strncmp (condExpr, "exists", 6) == 0) {
		    /*
		     * Use CondDoExists to evaluate the argument and
		     * CondGetArg to extract the argument from the
		     * 'function call'.
		     */
		    evalProc = CondDoExists;
		    condExpr += 6;
		    arglen = CondGetArg(&condExpr, &arg, "exists", TRUE);
		    if (arglen == 0) {
			condExpr -= 6;
			goto use_default;
		    }
		} else if (strncmp(condExpr, "empty", 5) == 0) {
		    /*
		     * Use Var_Parse to parse the spec in parens and return
		     * True if the resulting string is empty.
		     */
		    int	    length;
		    Boolean doFree;
		    char    *val;

		    condExpr += 5;

		    for (arglen = 0;
			 condExpr[arglen] != '(' && condExpr[arglen] != '\0';
			 arglen += 1)
		    {
			/* void */ ;
		    }
		    if (condExpr[arglen] != '\0') {
			val = Var_Parse(&condExpr[arglen - 1], VAR_CMD,
					doEval, &length, &doFree);
			if (val == var_Error) {
			    t = Err;
			} else {
			    t = (*val == '\0') ? True : False;
			}
			if (doFree) {
			    free(val);
			}
			/*
			 * Advance condExpr to beyond the closing ). Note that
			 * we subtract one from arglen + length b/c length
			 * is calculated from condExpr[arglen - 1].
			 */
			condExpr += arglen + length - 1;
		    } else {
			condExpr -= 5;
			goto use_default;
		    }
		    break;
		} else if (strncmp (condExpr, "target", 6) == 0) {
		    /*
		     * Use CondDoTarget to evaluate the argument and
		     * CondGetArg to extract the argument from the
		     * 'function call'.
		     */
		    evalProc = CondDoTarget;
		    condExpr += 6;
		    arglen = CondGetArg(&condExpr, &arg, "target", TRUE);
		    if (arglen == 0) {
			condExpr -= 6;
			goto use_default;
		    }
		} else {
		    /*
		     * The symbol is itself the argument to the default
		     * function. We advance condExpr to the end of the symbol
		     * by hand (the next whitespace, closing paren or
		     * binary operator) and set to invert the evaluation
		     * function if condInvert is TRUE.
		     */
		use_default:
		    invert = condInvert;
		    evalProc = condDefProc;
		    arglen = CondGetArg(&condExpr, &arg, "", FALSE);
		}

		/*
		 * Evaluate the argument using the set function. If invert
		 * is TRUE, we invert the sense of the function.
		 */
		t = (!doEval || (* evalProc) (arglen, arg) ?
		     (invert ? False : True) :
		     (invert ? True : False));
		free(arg);
		break;
	    }
	}
    } else {
	t = condPushBack;
	condPushBack = None;
    }
    return (t);
}

/*-
 *-----------------------------------------------------------------------
 * CondT --
 *	Parse a single term in the expression. This consists of a terminal
 *	symbol or Not and a terminal symbol (not including the binary
 *	operators):
 *	    T -> defined(variable) | make(target) | exists(file) | symbol
 *	    T -> ! T | ( E )
 *
 * Results:
 *	True, False or Err.
 *
 * Side Effects:
 *	Tokens are consumed.
 *
 *-----------------------------------------------------------------------
 */
static Token
CondT(doEval)
    Boolean doEval;
{
    Token   t;

    t = CondToken(doEval);

    if (t == EndOfFile) {
	/*
	 * If we reached the end of the expression, the expression
	 * is malformed...
	 */
	t = Err;
    } else if (t == LParen) {
	/*
	 * T -> ( E )
	 */
	t = CondE(doEval);
	if (t != Err) {
	    if (CondToken(doEval) != RParen) {
		t = Err;
	    }
	}
    } else if (t == Not) {
	t = CondT(doEval);
	if (t == True) {
	    t = False;
	} else if (t == False) {
	    t = True;
	}
    }
    return (t);
}

/*-
 *-----------------------------------------------------------------------
 * CondF --
 *	Parse a conjunctive factor (nice name, wot?)
 *	    F -> T && F | T
 *
 * Results:
 *	True, False or Err
 *
 * Side Effects:
 *	Tokens are consumed.
 *
 *-----------------------------------------------------------------------
 */
static Token
CondF(doEval)
    Boolean doEval;
{
    Token   l, o;

    l = CondT(doEval);
    if (l != Err) {
	o = CondToken(doEval);

	if (o == And) {
	    /*
	     * F -> T && F
	     *
	     * If T is False, the whole thing will be False, but we have to
	     * parse the r.h.s. anyway (to throw it away).
	     * If T is True, the result is the r.h.s., be it an Err or no.
	     */
	    if (l == True) {
		l = CondF(doEval);
	    } else {
		(void) CondF(FALSE);
	    }
	} else {
	    /*
	     * F -> T
	     */
	    CondPushBack (o);
	}
    }
    return (l);
}

/*-
 *-----------------------------------------------------------------------
 * CondE --
 *	Main expression production.
 *	    E -> F || E | F
 *
 * Results:
 *	True, False or Err.
 *
 * Side Effects:
 *	Tokens are, of course, consumed.
 *
 *-----------------------------------------------------------------------
 */
static Token
CondE(doEval)
    Boolean doEval;
{
    Token   l, o;

    l = CondF(doEval);
    if (l != Err) {
	o = CondToken(doEval);

	if (o == Or) {
	    /*
	     * E -> F || E
	     *
	     * A similar thing occurs for ||, except that here we make sure
	     * the l.h.s. is False before we bother to evaluate the r.h.s.
	     * Once again, if l is False, the result is the r.h.s. and once
	     * again if l is True, we parse the r.h.s. to throw it away.
	     */
	    if (l == False) {
		l = CondE(doEval);
	    } else {
		(void) CondE(FALSE);
	    }
	} else {
	    /*
	     * E -> F
	     */
	    CondPushBack (o);
	}
    }
    return (l);
}

/*-
 *-----------------------------------------------------------------------
 * Cond_Eval --
 *	Evaluate the conditional in the passed line. The line
 *	looks like this:
 *	    #<cond-type> <expr>
 *	where <cond-type> is any of if, ifmake, ifnmake, ifdef,
 *	ifndef, elif, elifmake, elifnmake, elifdef, elifndef
 *	and <expr> consists of &&, ||, !, make(target), defined(variable)
 *	and parenthetical groupings thereof.
 *
 * Results:
 *	COND_PARSE	if should parse lines after the conditional
 *	COND_SKIP	if should skip lines after the conditional
 *	COND_INVALID  	if not a valid conditional.
 *
 * Side Effects:
 *	None.
 *
 *-----------------------------------------------------------------------
 */
Cond_Eval (line)
    char    	    *line;    /* Line to parse */
{
    struct If	    *ifp;
    Boolean 	    isElse;
    Boolean 	    value;
    int	    	    level;  	/* Level at which to report errors. */

    level = PARSE_FATAL;

    for (line++; *line == ' ' || *line == '\t'; line++) {
	continue;
    }

    /*
     * Find what type of if we're dealing with. The result is left
     * in ifp and isElse is set TRUE if it's an elif line.
     */
    if (line[0] == 'e' && line[1] == 'l') {
	line += 2;
	isElse = TRUE;
    } else if (strncmp (line, "endif", 5) == 0) {
	/*
	 * End of a conditional section. If skipIfLevel is non-zero, that
	 * conditional was skipped, so lines following it should also be
	 * skipped. Hence, we return COND_SKIP. Otherwise, the conditional
	 * was read so succeeding lines should be parsed (think about it...)
	 * so we return COND_PARSE, unless this endif isn't paired with
	 * a decent if.
	 */
	if (skipIfLevel != 0) {
	    skipIfLevel -= 1;
	    return (COND_SKIP);
	} else {
	    if (condTop == MAXIF) {
		Parse_Error (level, "if-less endif");
		return (COND_INVALID);
	    } else {
		skipLine = FALSE;
		condTop += 1;
		return (COND_PARSE);
	    }
	}
    } else {
	isElse = FALSE;
    }

    /*
     * Figure out what sort of conditional it is -- what its default
     * function is, etc. -- by looking in the table of valid "ifs"
     */
    for (ifp = ifs; ifp->form != (char *)0; ifp++) {
	if (strncmp (ifp->form, line, ifp->formlen) == 0) {
	    break;
	}
    }

    if (ifp->form == (char *) 0) {
	/*
	 * Nothing fit. If the first word on the line is actually
	 * "else", it's a valid conditional whose value is the inverse
	 * of the previous if we parsed.
	 */
	if (isElse && (line[0] == 's') && (line[1] == 'e')) {
	    if (condTop == MAXIF) {
		Parse_Error (level, "if-less else");
		return (COND_INVALID);
	    } else if (skipIfLevel == 0) {
		value = !condStack[condTop];
	    } else {
		return (COND_SKIP);
	    }
	} else {
	    /*
	     * Not a valid conditional type. No error...
	     */
	    return (COND_INVALID);
	}
    } else {
	if (isElse) {
	    if (condTop == MAXIF) {
		Parse_Error (level, "if-less elif");
		return (COND_INVALID);
	    } else if (skipIfLevel != 0) {
		/*
		 * If skipping this conditional, just ignore the whole thing.
		 * If we don't, the user might be employing a variable that's
		 * undefined, for which there's an enclosing ifdef that
		 * we're skipping...
		 */
		return(COND_SKIP);
	    }
	} else if (skipLine) {
	    /*
	     * Don't even try to evaluate a conditional that's not an else if
	     * we're skipping things...
	     */
	    skipIfLevel += 1;
	    return(COND_SKIP);
	}

	/*
	 * Initialize file-global variables for parsing
	 */
	condDefProc = ifp->defProc;
	condInvert = ifp->doNot;

	line += ifp->formlen;

	while (*line == ' ' || *line == '\t') {
	    line++;
	}

	condExpr = line;
	condPushBack = None;

	switch (CondE(TRUE)) {
	    case True:
		if (CondToken(TRUE) == EndOfFile) {
		    value = TRUE;
		    break;
		}
		goto err;
		/*FALLTHRU*/
	    case False:
		if (CondToken(TRUE) == EndOfFile) {
		    value = FALSE;
		    break;
		}
		/*FALLTHRU*/
	    case Err:
	    err:
		Parse_Error (level, "Malformed conditional (%s)",
			     line);
		return (COND_INVALID);
	}
    }
    if (!isElse) {
	condTop -= 1;
    } else if ((skipIfLevel != 0) || condStack[condTop]) {
	/*
	 * If this is an else-type conditional, it should only take effect
	 * if its corresponding if was evaluated and FALSE. If its if was
	 * TRUE or skipped, we return COND_SKIP (and start skipping in case
	 * we weren't already), leaving the stack unmolested so later elif's
	 * don't screw up...
	 */
	skipLine = TRUE;
	return (COND_SKIP);
    }

    if (condTop < 0) {
	/*
	 * This is the one case where we can definitely proclaim a fatal
	 * error. If we don't, we're hosed.
	 */
	Parse_Error (PARSE_FATAL, "Too many nested if's. %d max.", MAXIF);
	return (COND_INVALID);
    } else {
	condStack[condTop] = value;
	skipLine = !value;
	return (value ? COND_PARSE : COND_SKIP);
    }
}

/*-
 *-----------------------------------------------------------------------
 * Cond_End --
 *	Make sure everything's clean at the end of a makefile.
 *
 * Results:
 *	None.
 *
 * Side Effects:
 *	Parse_Error will be called if open conditionals are around.
 *
 *-----------------------------------------------------------------------
 */
void
Cond_End()
{
    if (condTop != MAXIF) {
	Parse_Error(PARSE_FATAL, "%d open conditional%s", MAXIF-condTop,
		    MAXIF-condTop == 1 ? "" : "s");
    }
    condTop = MAXIF;
}