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<span style="font-size: 10pt"><a href="/plan9/">Plan 9 from Bell Labs</a>&rsquo;s /usr/web/sources/contrib/aiju/mathfs.c</span></p>
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Copyright © 2009 Alcatel-Lucent.<br />
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<a href="/plan9/download.html">Download the Plan 9 distribution.</a>
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#include &lt;u.h&gt;
#include &lt;libc.h&gt;
#include &lt;thread.h&gt;
#include &lt;fcall.h&gt;
#include &lt;9p.h&gt;
#include &lt;ctype.h&gt;

void
mathattach(Req *req)
{
	req-&gt;ofcall.qid = (Qid) {1, 0, 0x80};
	req-&gt;fid-&gt;qid = req-&gt;ofcall.qid;
	respond(req, nil);
}

char *
mathwalk(Fid *fid, char *name, Qid *qid)
{
	static int qidcount = 2;
	char opstack[10], *tops, tok, *p;
	double nstack[20], *tns;
	
	tops = opstack + nelem(opstack);
	tns = nstack + nelem(nstack);
	p = name;

	for(;;) {
		for(; *p &amp;&amp; *p == ' '; p++);
		if(isdigit(*p)) {	
			if(tns &lt;= nstack)
				goto error;
			*--tns = strtod(p, &amp;p);
			continue;
		}
		tok = *p++;
		switch(tok) {
		case '+': case '-': case '*': case '%': case ')': case 0:
			while(tops &lt; opstack + nelem(opstack) &amp;&amp;
			   (tok != '*' &amp;&amp; tok != '%' || *tops != '+' &amp;&amp; *tops != '-')) {
			   	if(*tops == '(') {
			   		if(tok != ')')
			   			break;
			   		tok = -1;
			   		tops++;
			   		continue;	
			   	}
				if(tns &gt;= nstack + nelem(nstack) - 1)
					goto error;
				switch(*tops) {
				case '+':
					tns[1] += *tns;
					break;
				case '-':
					tns[1] -= *tns;
					break;
				case '*':
					tns[1] *= *tns;
					break;
				case '%':
					tns[1] /= *tns;
					break;
				}
				tops++;
				tns++;
			}
			if(tok &amp;&amp; tok != -1 &amp;&amp; tok != ')') {
		case '(':
				if(tops &lt;= opstack)
					goto error;
				*--tops = tok;
			}
			if(tok == 0)
				goto out;
			break;
		default:
			goto error;
		}
	}

out:
	if(tops != opstack + nelem(opstack))
		goto error;
	if(tns != nstack + nelem(nstack) - 1)
		goto error;
	*qid = (Qid) {qidcount++, 0, 0};
	fid-&gt;qid = *qid;
	fid-&gt;aux = smprint("%g\n", *tns);
	return nil;

error:
	return "syntax error";
}

void
mathread(Req *req)
{
	if(req-&gt;fid-&gt;aux == nil) {
		respond(req, nil);
		return;
	}
	readstr(req, (char *) req-&gt;fid-&gt;aux);
	respond(req, nil);
}

void
mathdestroy(Fid *fid)
{
	free(fid-&gt;aux);
}

Srv mathsrv = {
	.attach = mathattach,
	.walk1 = mathwalk,
	.read = mathread,
	.destroyfid = mathdestroy,
};

void
main()
{
	postmountsrv(&amp;mathsrv, "math", "/n/math", MREPL);
}
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