mask_addr.c

[詳解]

/*++
/* NAME
/*  mask_addr 3
/* SUMMARY
/*  address bit banging
/* SYNOPSIS
/*  #include <mask_addr.h>
/*
/*  void    mask_addr(addr_bytes, addr_byte_count, network_bits)
/*  unsigned char *addr_bytes;
/*  unsigned addr_byte_count;
/*  unsigned network_bits;
/* DESCRIPTION
/*  mask_addr() clears all the host bits in the specified
/*  address.  The code can handle addresses of any length,
/*  and bytes of any width.
/*
/*  Arguments:
/* .IP addr_bytes
/*  The network address in network byte order.
/* .IP addr_byte_count
/*  The network address length in bytes.
/* .IP network_bits
/*  The number of initial bits that will not be cleared.
/* DIAGNOSTICS
/*  Fatal errors: the number of network bits exceeds the address size.
/* LICENSE
/* .ad
/* .fi
/*  The Secure Mailer license must be distributed with this software.
/* AUTHOR(S)
/*  Wietse Venema
/*  IBM T.J. Watson Research
/*  P.O. Box 704
/*  Yorktown Heights, NY 10598, USA
/*--*/

/* System library. */

#include <sys_defs.h>
#include <limits.h>         /* CHAR_BIT */

/* Utility library. */

#include <msg.h>
#include <mask_addr.h>

/* mask_addr - mask off a variable-length address */

void    mask_addr(unsigned char *addr_bytes,
                  unsigned addr_byte_count,
                  unsigned network_bits)
{
    unsigned char *p;

    if (network_bits > addr_byte_count * CHAR_BIT)
    msg_panic("mask_addr: address byte count %d too small for bit count %d",
          addr_byte_count, network_bits);

    p = addr_bytes + network_bits / CHAR_BIT;
    network_bits %= CHAR_BIT;

    if (network_bits != 0)
    *p++ &= ~0U << (CHAR_BIT - network_bits);

    while (p < addr_bytes + addr_byte_count)
    *p++ = 0;
}