利用mmap實現文件拷貝功能

/* 
 * gcc -Wall -O3 -o copy_mmap copy_mmap.c
 */ 
#include < stdio.h > 
#include < stdlib.h > 
#include < string .h > /* for memcpy */ 
#include < strings.h > 
#include < sys / mman.h > 
#include < sys / types.h > 
#include < sys / stat.h > 
#include < fcntl.h > 
#include < unistd.h > 

#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 );
 }
  /* 為了完成復制，必須包含讀打開，否則mmap()失敗 */ 
  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 );
 }
  /* 
  * 參看前面man手冊中的說明，mmap()不能用于擴展文件長度。所以這里必須事
  * 先擴大目標文件長度，準備一個空架子等待復制。
  */ 
  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 );
 } 
 
  /* 讀的時候指定 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 );
 }
  /* 這里必須指定 MAP_SHARED 才可能真正改變靜態(tài)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 );
  /* 
  * 可以不要這行代碼
  *
  * msync( dm, ( size_t )statbuf.st_size, MS_SYNC );
  */ 
  return ( EXIT_SUCCESS );
}