gosub.c

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/* GOSUB support */

#include "fb.h"
#include <setjmp.h>

/* slow but easy to manage dynamic GOSUB call-stack */
typedef struct gosubnode {
        jmp_buf buf;
        struct gosubnode *next;
} GOSUBNODE;

/* the gosub context allocated in auto-local storage by the compiler */
typedef struct {
        GOSUBNODE *top;
} GOSUBCTX;

/*
        NOTES:
                On the compiler side, GOSUBCTX is an ANY PTR.  To extend 
                GOSUBCTX, the compiler must allocate additional space for 
                the GOSUBCTX pseudo-object.

                GOSUBCTX does not have a constructor, but it is expected that
                the compiler will initialize the variable to zero.

                See ast-gosub.bas::astGosubAddInit()
*/


/*:::::*/
FBCALL void * fb_GosubPush( GOSUBCTX * ctx )
{
        GOSUBNODE *node = malloc( sizeof( GOSUBNODE ) );
        node->next = ctx->top;
        ctx->top = node;

        /* returns address of ctx->top->buf     */
        return &(ctx->top->buf);
}

/*:::::*/
FBCALL int fb_GosubPop( GOSUBCTX * ctx )
{
        if( ctx && ctx->top )
        {
                GOSUBNODE *node = ctx->top->next;
                free(ctx->top);
                ctx->top = node;

                /* return success */
                return fb_ErrorSetNum( FB_RTERROR_OK );
        }

        /* don't know where to go next so return an error */
        return fb_ErrorSetNum( FB_RTERROR_RETURNWITHOUTGOSUB );
}

/*:::::*/
FBCALL int fb_GosubReturn( GOSUBCTX * ctx )
{
        if( ctx && ctx->top )
        {
                GOSUBNODE *node = ctx->top->next;
                
                /* TODO: with a different stack allocation strategy, this
                 * temporary copy won't be needed */
                jmp_buf buf;
                FB_MEMCPY( buf, ctx->top->buf, sizeof(jmp_buf));

                free(ctx->top);
                ctx->top = node;

                longjmp( buf, -1 );
        }

        /* don't know where to go next so return an error */
        return fb_ErrorSetNum( FB_RTERROR_RETURNWITHOUTGOSUB );
}

/*:::::*/
FBCALL void fb_GosubExit( GOSUBCTX * ctx )
{
        if( ctx )
        {
                while( ctx->top )
                {
                        GOSUBNODE *node = ctx->top->next;
                        free(ctx->top);
                        ctx->top = node;
                }
        }
}