tas.c.template

关系型数据库 Postgresql 6.5.2

/*
 * To generate tas.s using this template:
 *	1. cc +O2 -S -c tas.c
 *	2. edit tas.s:
 *		- replace the LDW with LDCWX
 * For details about the LDCWX instruction, see the "Precision
 * Architecture and Instruction Reference Manual" (09740-90014 of June
 * 1987), p. 5-38.
 */

int
tas(lock)
    int *lock;	/* LDCWX is a word instruction */
{
    /*
     * LDCWX requires that we align the "semaphore" to a 16-byte
     * boundary.  The actual datum is a single word (4 bytes).
     */
    lock = ((long) lock + 15) & ~15;

    /*
     * The LDCWX instruction atomically clears the target word and
     * returns the previous value.  Hence, if the instruction returns
     * 0, someone else has already acquired the lock before we tested
     * it (i.e., we have failed).
     *
     * Notice that this means that we actually clear the word to set
     * the lock and set the word to clear the lock.  This is the
     * opposite behavior from the SPARC LDSTUB instruction.  For some
     * reason everything that H-P does is rather baroque...
     */
    if (*lock) {	/* this generates the LDW */
	return(0);	/* success */
    }
    return(1);    	/* failure */
}