[C] File Locking @ Windows & FreeBSD

使用上儘管對第三者進行 locking ，或是避免使用 BufferI /O，不然容易出現資料不同步的現象，且跟 OS 的相依性會提高，對於要移值的程式就容易頭大啦！

開發與環境

Windows XP with SP3 + Dev-C++ v4.9.9.2 (gcc 3.4.2 )
FreeBSD 6.2-RELEASE-p1 (gcc 3.4.6)

程式碼：

#ifdef _WORK_FOR_WIN32_

#include <windows.h>
#include <io.h>

#endif

void File_LockByPtr( FILE * fd )
{
#ifndef _WORK_FOR_WIN32_
        struct flock fl;
        fl.l_type = F_WRLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;
        fl.l_pid = getpid();
        if( fcntl( fileno( fd )  , F_SETLKW , &fl ) < 0 )
        {
                perror("Lock File Pointer Error @ LockFilePtr");
                exit(1);
        }
#else
        HANDLE hFile = (HANDLE)_get_osfhandle(_fileno( fd ));
        OVERLAPPED ovl;
        memset(&ovl, 0, sizeof(ovl));
        if( LockFileEx(hFile, LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ovl) == 0 )
        {
                perror("Lock File Pointer Error @ LockFilePtr");
                exit(1);
        }
#endif

}
void File_UnlockByPtr( FILE * fd )
{
#ifndef _WORK_FOR_WIN32_
        struct flock fl;

#ifndef SYS_OPT_LOCKING_FILE_PONITER_WITHOUT_FFLUSH
        fflush(fd);
#endif

        fl.l_type = F_UNLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;
        fl.l_pid = getpid();

        if( fcntl( fileno( fd )  , F_SETLK , &fl ) < 0 )
        {
                perror("Unlock File Pointer Error @ UnlockFilePtr");
                exit(1);
        }
#else
        HANDLE hFile = (HANDLE)_get_osfhandle(_fileno( fd ));
        OVERLAPPED ovl;
        memset(&ovl, 0, sizeof(ovl));

#ifndef SYS_OPT_LOCKING_FILE_PONITER_WITHOUT_FFLUSH
        fflush( fd );
#endif

        if( UnlockFileEx(hFile, 0, 1, 0, &ovl) == 0 )
        {
                perror("Unlock File Pointer Error @ UnlockFileByPtr");
                exit(1);
        }
#endif
}
void File_LockByFD( int fd )
{
#ifndef WIN32
        struct flock fl;
        fl.l_type = F_WRLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;
        fl.l_pid = getpid();
        if( fcntl( fd , F_SETLKW , &fl ) < 0 )
        {
                fprintf( stderr , "Lock File Pointer Error @ LockFilePtr\n");
                exit(1);
        }
#else
        HANDLE hFile = (HANDLE)_get_osfhandle( fd );
        OVERLAPPED ovl;
        memset(&ovl, 0, sizeof(ovl));
        if( ( lock_status = LockFileEx(hFile, LOCKFILE_EXCLUSIVE_LOCK, 0, 1, 0, &ovl) ) == 0 )
        {
                fprintf( stderr , "Lock File Pointer Error @ LockFileFD\n");
                exit(1);
        }
#endif

}
void File_UnlockByFD( int fd )
{
#ifndef WIN32
        struct flock fl;

        fl.l_type = F_UNLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;
        fl.l_pid = getpid();

        if( fcntl( fd , F_SETLK , &fl ) < 0 )
        {
                fprintf( stderr , "Unlock File Pointer Error @ UnlockFilePtr\n");
                exit(1);
        }
#else
        HANDLE hFile = (HANDLE)_get_osfhandle( fd );
        OVERLAPPED ovl;
        memset(&ovl, 0, sizeof(ovl));

        if( UnlockFileEx(hFile, 0, 1, 0, &ovl) == 0 )
        {
                fprintf( stderr , "Unlock File Pointer Error @ UnlockFileByFD\n");
                exit(1);
        }
#endif
}

用法：

if( ( opt->dblock = fopen( lock_file , "w" ) )  == NULL )
{
         fprintf( stderr , "Cannot Open Lock File @ DB_Lock\n" );
         exit(1);
}
File_LockByPtr( opt->dblock );

...

File_UnlockByPtr( opt->dblock );
fclose(opt->dblock);

或

#ifndef WIN32
        struct flock fl;
        if( ( db_lock_fd = open( db_lock_file ,  O_RDWR | O_CREAT ) )  == -1 )
#else
        HANDLE hFile;
        OVERLAPPED ovl;
        if( ( db_lock_fd = _open( db_lock_file ,  _O_RDWR | _O_CREAT , _S_IREAD| _S_IWRITE ) )  == -1 )
#endif
        {
                switch( errno )
                {
                case EACCES:
                        printf("Tried to open read-only file for writing, file's sharing mode does not allow specified operations, or given path is directory.\n");
                        break;
                case EEXIST:
                        printf("_O_CREAT and _O_EXCL flags specified, but filename already exists.\n");
                        break;
                case EINVAL:
                        printf("Invalid oflag or pmode argument.\n");
                        break;
                case EMFILE:
                        printf("No more file descriptors available (too many open files).\n");
                        break;
                case ENOENT:
                        printf("File or path not found.\n");
                        break;
                }
                fprintf( stderr , "Cannot Open Lock File @ db_lock ,%d, %s\n" , errno , db_lock_file );
                exit(1);
        }
        File_LockByFD( db_lock_fd );

        ...

        File_UnlockByFD( db_lock_fd );