fcntl 的学习过程

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "ourhdr.h"
#include "error.c"
#include "f10.17.c"
static void lockabyte(const char *, int, off_t );

int lock_reg( int fd, int cmd, int type, off_t offset, int whence, off_t len )
{
        struct flock lock;

        lock.l_type = type;     /* F_RDLCK, F_WRLCK, F_UNLCK */
        lock.l_start = offset;  /* byte offset, relative to l_whence */
        lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
        lock.l_len = len;       /* bytes ( 0 means to EOF )*/

        return ( fcntl( fd, cmd, &lock) );
}

int main( void )
{
        int fd;
        pid_t pid;

        /* create a file and write two bytes to it */
        if ( ( fd = creat( "templock", FILE_MODE ) ) < 0 )
                err_sys( "create error" );

        if ( write( fd, "ab", 2 ) != 2 )
                err_sys( "write error" );

        TELL_WAIT();
        if ( ( pid = fork() ) < 0 )
                err_sys( "fork error" );
        else if ( pid == 0 ) {  /* child */
                lockabyte( "child", fd, 0 );
                TELL_PARENT( getppid() );
                WAIT_PARENT();
                lockabyte( "child", fd, 1 );
        } else {
                lockabyte( "parent", fd, 1 );
                TELL_CHILD( pid );
                WAIT_CHILD();
                lockabyte( "parent", fd, 0 );
        }
        exit( 0 );
}

static void lockabyte( const char *name, int fd, off_t offset )
{
        if ( writew_lock( fd, offset, SEEK_SET, 1 ) < 0 )
                err_sys( "%s: writew_lock error", name );

        printf( "%s: got the lock, byte %d\n", name, offset );
}