Page 747 in Software

Paint QR in Software Page 747

The prescan( ) function works like this: Each time an opening curly brace is encountered, brace is incremented Whenever a closing curly brace is read, brace is decremented Therefore, whenever brace is greater than zero, the current token is being read from within a function However, if brace equals zero when a variable is found, the prescanner knows that it must be a global variable By the same method, if a function name is encountered when brace equals zero, it must be that function's definition (Remember, Little C does not support function prototypes) Global variables are stored in a global variable table called global_vars by decl_global( ), shown here:

/* An array of these structures will hold the info associated with global variables */ struct var_type { char var__name[ID_LEN]; int v_type; int value; } global_vars[NUM_GLOBAL_VARS] ; int gvar_index; /* index into global variable table */ /* Declare a global variable */ void decl_global(void) { int vartype; get_token(); /* get type */

vartype = tok; /* save var type */ do { /* process comma-separated list */ global_vars[gvar_index]v_type = vartype; global_vars[gvar_index]value = 0; /* init to 0 */ get_token(); /* get name */ strcpy(global_vars[gvar_index]var_name, token); get_token(); gvar_index++; } while(*token == ','); if(*token != ';') sntx_err(SEMI_EXPECTED); }

The location of each user-defined function is put into the func_table array, shown here:

struct func_type { char func_name[ID_LEN]; int ret_type; char *loc; /* location of entry point in file */ } func_table[NUM_FUNC]; int func_index; /* index into function table */

The func_index variable holds the index of the next free location in the table The main( ) Function The main( ) function to the Little C interpreter, shown here, loads the source code, initializes the global variables, calls prescan( ), ''primes" the interpreter for the call to main( ), and then executes call( ), which begins execution of the program The operation of the call( ) function will be discussed shortly

int main(int argc, char *argv[]) { if(argc != 2) { printf("Usage: littlec <filename>\n"); exit(1); } /* allocate memory for the program */ if((p_buf = (char *) malloc(PROG_SIZE))==NULL) { printf("Allocation Failure"); exit(1); } /* load the program to execute */ if(!load_program(p_buf, argv[1])) exit(1); if(setjmp(e_buf)) exit(1); /* initialize long jump buffer */ gvar_index = 0; /* initialize global variable index */ /* set program pointer to start of program buffer */ prog = p_buf; prescan(); /* find the location of all functions

Page 749 and global variables in the program */ lvartos = 0; functos = 0; /* initialize local variable stack index */ /* initialize the CALL stack index */

/* setup call to main() */ prog = find_func(''main"); /* find program starting point */ if(!prog) { /* incorrect or missing main() function in program */ printf("main() not found\n"); exit(1); } prog--; /* back up to opening ( */ strcpy(token, "main"); call(); /* call main() to start interpreting */ return 0; }

The interp_block( ) Function The interp_block( ) function is the heart of the interpreter It is the function that decides what action to take based upon the next token in the input stream The function is designed to interpret one block of code and then return If the "block" consists of a single statement, that statement is interpreted and the function returns By default, interp_block( ) interprets one statement and returns However, if an opening curly brace is read, the flag block is set to 1 and the function continues to interpret statements until a closing curly brace is read The interp_block( ) function is shown here:

/* Interpret a single statement or block of code When interp_block() returns from its initial call, the final brace (or a return) in main() has been encountered */ void interp_block(void) int value; char block = 0; do {

Page 750 token_type = get_token(); /* If interpreting single statement, return on first semicolon */ /* see what kind of token is up */ if(token_type == IDENTIFIER) { /* Not a keyword, so process expression */ putback(); /* restore token to input stream for further processing by eval_exp() */ eval_exp(&value); /* process the expression */ if(*token!=';') sntx_err(SEMI_EXPECTED); } else if(token_type==BLOCK) { /* if block delimiter */ if(*token == '{') /* is a block */ block = 1; /* interpreting block, not statement */ else return; /* is a }, so return */ } else /* is keyword */ switch(tok) { case CHAR: case INT: /* declare local variables */ putback(); decl_local(); break; case RETURN: /* return from function call */ func_ret(); return; case IF: /* process an if statement */ exec_if(); break; case ELSE: /* process an else statement */ find_eob(); /* find end of else block and continue execution */ break; case WHILE: /* process a while loop */ exec_while(); break; case DO: /* process a do-while loop */ exec_do(); break;