a1rwint.c

来自「TCP-IP红宝书源代码」· C语言 代码 · 共 74 行

/* a1rwint.c - a1readint, a1writeint */

#include <conf.h>
#include <kernel.h>
#include <network.h>

#ifdef	SNMP

#include <snmp.h>
#include <asn1.h>

#define h2asn 	memcpy

/*------------------------------------------------------------------------
 * a1readint - convert an ASN.1 encoded integer into a machine integer
 *------------------------------------------------------------------------
 */
int
a1readint(u_char *pack, unsigned len)
{
	register int	tot;
	u_char 		neg;
	unsigned	tlen;
	
	if ((tlen = len) > sizeof(int))
		return 0;
	tot = *pack & ~CHAR_HIBIT;
	neg = *pack & CHAR_HIBIT;
	for (tlen--, pack++ ; tlen > 0 ; tlen--, pack++)
		tot = (tot << CHAR_BITS) | (int) *pack;
	if (neg)
		tot -= (1 << ((len * CHAR_BITS) - 1));
	return tot;
}

/*------------------------------------------------------------------------
 * a1writeint - convert an integer into its ASN.1 encoded form
 *------------------------------------------------------------------------
 */
int
a1writeint(int val, u_char *buffp, int a1type)
{
	unsigned	tmp, numbytes;
	register u_char	*bp;
	
	bp = buffp;
	tmp = val;
	if (A1_SIGNED(a1type) && val < 0)
		tmp = -val;
	if (tmp < (unsigned) CHAR_HIBIT)
		*bp++ = numbytes = (u_char) 1;
	else if (tmp < (unsigned) BYTE2_HIBIT)
		*bp++ = numbytes = (u_char) 2;
	else if (tmp < (unsigned) BYTE3_HIBIT)
		*bp++ = numbytes = (u_char) 3;
	else if (tmp < (unsigned) BYTE4_HIBIT)
		*bp++ = numbytes = (u_char) 4;
	else {	/* 5 bytes for unsigned with high bit set */
		*bp++ = (u_char) 5;	/* length */
		*bp++ = (u_char) 0;
		numbytes = 4;
	}
	/* works for big-endian & little-endian */
	switch (numbytes) {
	  case 4: *bp++ = (u_char)(0xff&((val&0xff000000)>>24)); /* fall thru */
	  case 3: *bp++ = (u_char)(0xff&((val&0x00ff0000)>>16)); /* fall thru */
	  case 2: *bp++ = (u_char)(0xff&((val&0x0000ff00)>>8));	 /* fall thru */
	  case 1: *bp++ = (u_char)(0xff&((val&0x000000ff)));
	}

	return bp - buffp;
}
#endif	/* SNMP */