mdist.c

一个C编写网络广播的程序

/* mdist.c -- core buffer-ring functions, client/server
 *  
 * This file is part of 'netcast' program, released under BSD License. 
 * (c) 2001-2002 Stanis砤w Pa秌o <[e-mail]>. All rights reserved.   
 */

#include <stdlib.h>
#include "mdist.h"

struct mdmsg *mdbuf[ROUND];
struct mdctl mdhead;
struct mdinit mdini;
struct mdmsg empty;

//! initialize buffer ring.
void md_init() {
  int i;  
  for (i=0;i<ROUND;i++) {
    mdbuf[i]=(struct mdmsg *) malloc(MMSIZE+4);
    mdbuf[i]->siz=0;
    mdbuf[i]->seq=0; 
  }
  mdhead.blk=0;
  mdhead.base=0;
  mdhead.pkt=0;
  empty.siz=0;
}

//! calculate buffer checksum
uint32_t md_sum(uint8_t *data,int len) {
  uint32_t *item,sum=0;
  int i=0;
  while (len%4) { data[len]=0; len++; }
  while (i<len) {
    item=(uint32_t *) ((char *)data+i);
    sum+=*item;
    i+=4;
  }
  return sum;
} 

//! fill buffers from file descriptor
long md_fill(int fd) {
  int s=0,r=MMDSIZ; 
  mdhead.blk++;
  mdhead.base+=mdhead.pkt;
  mdhead.siz=0;
  while (r>0 && s<ROUND) {
    if ((r=read(fd,mdbuf[s]->data,MMDSIZ))<0) 
      MSG(1,"Error reading data");
    if (r>0) {
      mdhead.siz+=r;
      mdbuf[s]->siz=r;
      mdbuf[s]->seq=mdhead.base+s;
      mdbuf[s]->crc=md_sum(mdbuf[s]->data,r);
      s++;
    }    
  } 
  MSG(6,"input %d bytes of data in %d packets",mdhead.siz,s);
  mdhead.pkt=s;
  return s;
}

//! flush data to file descriptor 
long md_flush(int fd) {
  int s=0;
  while (mdhead.pkt--) {
    write(fd,mdbuf[s]->data,mdbuf[s]->siz);
    mdhead.siz-=mdbuf[s]->siz;
    mdbuf[s]->siz=0;
    mdbuf[s]->seq=0;
    mdbuf[s]->crc=0;
    s++;
  }
  return s;
}

//! returns previous buffer cell - used by the reader for no-copy operation 
struct mdmsg *md_put(struct mdmsg *src) {
  struct mdmsg *tmp;
  uint32_t i;
  if (src->seq<mdhead.base || src->seq-mdhead.base>=mdhead.pkt) return 0;
  if (src->siz == 0) return 0;
  i = md_sum(src->data,src->siz);
  /* refuse packets with wrong checksum */
  if (src->crc != i) return 0;
  i = src->seq-mdhead.base;
  tmp = mdbuf[i];
  mdbuf[i] = src;
  return tmp;
}

//! get some data [retransmission ->> tcp server]
struct mdmsg *md_get(uint32_t seq) {
  empty.seq=seq;
  if (seq<mdhead.base || seq-mdhead.base>=mdhead.pkt) return &empty;
  if (mdbuf[seq-mdhead.base]->siz==0) return &empty;
  return mdbuf[seq-mdhead.base];
}