c语言中的符号"|="是什么意思？

bit_file->rack |=bit_file->mask;

补充：上面语句中的 |= 是什么意思？

完整源代码：

/*************开始BITIO.C**********/
/*
*该文件包含了所有的存取BIT的函数，适用于ANSI或者K&R C
*本书的每个程序都必须引用它
*/
#include <stdio.h>
#include <stdlib.h>
#include “bitio.h”
#include “errhand.h”
BIT_FILE *OpenOutputBitFile( name )
char *name;
{
 BIT_FILE *bit_file;
 bit_file = (BIT_FILE *) calloc(1,sizeof( BIT_FILE ));
 if ( bit_file == NULL )
  return( bit_file );
 bit_file->file=fopen( name,”rb”  );
 bit_file->rack = 0;
 bit_file->mask = 0×80;
 bit_file->pacifier_counter = 0;
 return( bit_file );
}
BIT_FILE *OpenInputBitFile( name )
char *name;
{
 BIT_FILE *bit_file;
 bit_file=(BIT_FILE *)calloc(1,sizeof( BIT_FILE ));
 if( bit_file == NULL )
  return( bit_file );
 bit_file->file=fopen( name,”rb” );
 bit_file->rack=0;
 bit_file->mask=0×80;
 bit_file->pacifier_counter=0;
 return( bit_file );
}
void CloseOutputBitFile( bit_file )
BIT_FILE *bit_file;
{
 if( bit_file->mask != 0×80 )
  if( putc( bit_file->rack, bit_file->file )!=bit_file->rack )
   fatal_error( “关闭Bit文件时发生致命错误！\n” );
 fclose( bit_file->file );
 free((char *)bit_file);
}
void CloseInputBitFile( bit_file )
BIT_FILE *bit_file;
{
 fclose( bit_file->file );
 free((char *)bit_file );
}
void OutputBit( bit_file,bit )
BIT_FILE *bit_file;
int bit;
{
 if(bit)
  bit_file->rack |=bit_file->mask;
 bit_file->mask>>=1;
 if( bit_file->mask==0){
  if(putc(bit_file->rack,bit_file->file )!=bit_file->rack)
   fatal_error(“在OutputBit处发生致命错误！\n”);
  else
   if((bit_file->pacifier_counter++ & 4095 )==0)
    putc(‘.’,stdout);
  bit_file->rack=0;
  but_file->mask=0×80;
 }
}
int InputBit( bit_file )
BIT_FILE *bit_file;
{
 int value;
 if( bit_file->mask==0×80 )
 {
  bit_file->rack=getc( bit_file->file );
  if(bit_file->rack==EOF)
   fatal_error(“在InputBit里发生致命错误！\n”);
  if(( bit_file->pacifier_counter++&2047)==0)
   putc(‘.’,stdout); 
 }
 value=bit_file->rack & bit_file->mask;
 bit_file->mask>>=1;
 if( bit_file->mask=0)
  bit_file->mask=0×80;
 return (value?1:0);

}
unsigned long InputBits( bit_file,bit_count )
BIT_FILE *bit_file;
int bit_count;
{
 unsigned long mask;
 unsigned long return_value;
 mask=1L<<(bit_count-1);
 return_value=0;
 while(mask!=0)
 {
  if(bit_file->mask==0×80)
  {
   bit_file->rack=getc( bit_file->file);
   if(bit_file->rack==EOF)
    fatal_error(“在InputBit里发生致命错误！\n”);
  if((bit_file->pacifier_counter++ & 2047)==0)
   putc(‘.’,stdout);  
  }
  if(bit_file->rack&bit_file->mask)
   return_value |=mask;
  mask>>=1;
  bit_file->mask>>=1;
  if(bit_file->mask=00)
   bit_file->mask=0×80;
 }
 return (return_value);
}
void FilePrintBinary( file,code,bits )
FILE *file;
unsigned int code;
int bits;
{
 unsigned int mask;
 mask=1<<(bits-1);
 while(mask!=0)
 {
  if(code&mask)
   fputc(’1′,file);
  else
   fputc(’0′,file);
  mask>>=1;
 }
}
/********************结束 BITIO.C************/