利用mmap实现文件拷贝功能

/* 
 * gcc -Wall -O3 -o copy_mmap copy_mmap.c
 */ 
#include  
#include  
#include  /* for memcpy */ 
#include  
#include  
#include  
#include  
#include  
#include  

#define PERMS 0600 

int main ( int argc, char * argv[] )
{
  int   src, dst;
  void   * sm, * dm; 
  struct stat statbuf;

  if ( argc != 3 )
 {
  fprintf( stderr, " Usage: %s \n " , argv[ 0 ] );
  exit( EXIT_FAILURE );
 }
  if ( ( src = open( argv[ 1 ], O_RDONLY ) ) <0 )
 {
  perror( " open source " );
  exit( EXIT_FAILURE );
 }
  /* 为了完成复制，必须包含读打开，否则mmap()失败 */ 
  if ( ( dst = open( argv[ 2 ], O_RDWR | O_CREAT | O_TRUNC, PERMS ) ) <0 )
 {
  perror( " open target " );
  exit( EXIT_FAILURE );
 }
  if ( fstat( src, & statbuf ) <0 )
 {
  perror( " fstat source " );
  exit( EXIT_FAILURE );
 }
  /* 
  * 参看前面man手册中的说明，mmap()不能用于扩展文件长度。所以这里必须事
  * 先扩大目标文件长度，准备一个空架子等待复制。
  */ 
  if ( lseek( dst, statbuf.st_size - 1 , SEEK_SET ) <0 )
 {
  perror( " lseek target " );
  exit( EXIT_FAILURE ); 
 } 
  if ( write( dst, & statbuf, 1 ) != 1 )
 {
  perror( " write target " );
  exit( EXIT_FAILURE );
 } 
 
  /* 读的时候指定 MAP_PRIVATE 即可 */ 
 sm = mmap( 0 , ( size_t )statbuf.st_size, PROT_READ,
    MAP_PRIVATE | MAP_NORESERVE, src, 0 );
  if ( MAP_FAILED == sm )
 {
  perror( " mmap source " );
  exit( EXIT_FAILURE );
 }
  /* 这里必须指定 MAP_SHARED 才可能真正改变静态文件 */ 
 dm = mmap( 0 , ( size_t )statbuf.st_size, PROT_WRITE,
    MAP_SHARED, dst, 0 );
  if ( MAP_FAILED == dm )
 {
  perror( " mmap target " );
  exit( EXIT_FAILURE );
 }
 memcpy( dm, sm, ( size_t )statbuf.st_size );
  /* 
  * 可以不要这行代码
  *
  * msync( dm, ( size_t )statbuf.st_size, MS_SYNC );
  */ 
  return ( EXIT_SUCCESS );
}