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value = 0; /* get return value, if any */ eval_exp(&value);
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If the function being called contains a return statement, then interp_block( ) calls func_ret( ) prior to returning to call( ) This function processes any return value It is shown here:
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Assigning Values to Variables Let's return briefly to the expression parser When an assignment statement is encountered, the value of the right side of the expression is computed, and this value is assigned to the variable on the left using a call to assign_var( ) However, as you know, the C language is structured and supports global and local variables Thus, given a program such as this
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int count; int main() { int count, i; count = 100; i = f(); return 0; } int f() { int count; count = 99; return count; }
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how does the assign_var( ) function know which variable is being assigned a value in each assignment The answer is simple: First, in C, local variables take priority over global variables of the same name; second, local variables are not known outside their own function To see how we can use these rules to resolve the above assignments, examine the assign_var( ) function, shown here:
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/* Assign a value to a variable */ void assign_var(char *var_name, int value) { register int i; /* first, see if it's a local variable */ for(i=lvartos-1; i >= call_stack[functos-1]; i--) { if(!strcmp(local_var_stack[i]var_name, var_name)) {
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Page 772 local_var_stack[i]value = value; return; } } if(i < call_stack[functos-1]) /* if not local, try global var table */ for(i=0; i < NUM_GLOBAL_VARS; i++) if(!strcmp(global_vars[i]var_name, var_name)) { global_vars[i]value = value; return; } sntx_err(NOT_VAR); /* variable not found */ }
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As explained in the previous section, each time a function is called, the current value of the local variable stack index (lvartos) is pushed onto the function call stack This means that any local variables (or parameters) defined by the function will be pushed onto the stack above that point Therefore, the assign_var( ) function first searches local_var_stack, beginning with the current topof-stack value and stopping when the index reaches that value saved by the latest function call This mechanism ensures that only those variables local to the function are examined (It also helps support recursive functions because the current value of lvartos is saved each time a function is invoked) Therefore, the line ''count = 100;" in main( ) causes assign_var( ) to find the local variable count inside main( ) In f( ), assign_var( ) finds its own count and does not find the one in main( ) If no local variable matches the name of a variable, the global variable list is searched Executing an if Statement Now that the basic structure of the Little C interpreter is in place, it is time to add some control statements Each time a keyword statement is encountered inside interp_block( ), an appropriate function is called, which processes that statement One of the easiest is the if The if statement is processed by exec_if( ), shown here:
/* Execute an if statement */ void exec_if(void) { int cond; eval_exp(&cond); /* get if expression */
Page 773 if(cond) { /* is true so process target of IF */ interp_block(); } else { /* otherwise skip around IF block and process the ELSE, if present */ find_eob(); /* find start of next line */ get_token(); if(tok != ELSE) { putback(); /* restore token if no ELSE is present */ return; } interp_block(); } }
Let's look closely at this function The first thing the function does is to compute the value of the conditional expression by calling eval_exp( ) If the condition (cond) is true (nonzero), the function calls interp_block( ) recursively, allowing the if block to execute If cond is false, the function find_eob( ) is called, which advances the program pointer to the location immediately after the end of the if block If an else is present, the else is processed by exec_if( ), and the else block is executed Otherwise, execution simply begins with the next line of code If the if block executes and there is an else block present, there must be some way for the else block to be bypassed This is accomplished in interp_block( ) by simply calling find_eob( ) to bypass the block when an else is encountered Remember, the only time an else will be processed by interp_block( ) (in a syntactically correct program) is after an if block has been executed When an else block executes, the else is processed by exec_if( ) Processing a while Loop A while loop, like the if, is quite easy to interpret The function that actually performs this task, exec_while( ), is shown here:
/* Execute a while loop */ void exec_while(void) { int cond; char *temp;
Page 774 putback(); temp = prog; /* save location of top of while loop */ get_token(); eval_exp(&cond); /* check the conditional expression */ if(cond) interp_block(); /* if true, interpret */ else { /* otherwise, skip around loop */ find_eob(); return; } prog = temp; /* loop back to top */ }
The exec_while( ) works like this: First, the while token is put back into the input stream, and the location of the while is saved into temp This address will be used to allow the interpreter to loop back to the top of the while Next, the while is reread to remove it from the input stream, and eval_exp( ) is called to compute the value of the while's conditional expression If the conditional expression is true, then interp_block( ) is called recursively to interpret the while block When interp_block( ) returns, prog (the program pointer) is loaded with the location of the start of the while loop, and control returns to interp_block( ), where the entire process repeats If the conditional expression is false, the end of the while block is found, and the function returns Processing a do-while Loop A do-while loop is processed much like the while When interp_block( ) encounters a do statement, it calls exec_do( ), shown here:
/* Execute a do loop */ void exec_do(void) { int cond; char *temp; putback(); temp = prog; /* save location of top of do loop */ get_token(); /* get start of loop */ interp_block(); /* interpret loop */ get_token(); if(tok != WHILE) sntx_err(WHILE_EXPECTED);
Page 775 eval_exp(&cond); /* check the loop condition */ if(cond) prog = temp; /* if true loop; otherwise, continue on */ }
The main difference between the do-while and the while loops is that the do-while always executes its block of code at least once because the conditional expression is at the bottom of the loop Therefore, exec_do( ) first saves the location of the top of the loop into temp and then calls interp_block( ) recursively to interpret the block of code associated with the loop When interp_block( ) returns, the corresponding while is retrieved, and the conditional expression is evaluated If the condition is true, prog is reset to the top of the loop; otherwise, execution will continue on The for Loop The interpretation of the for loop poses a more difficult challenge than the other constructs Part of the reason for this is that the structure of the C for is definitely designed with compilation in mind The main trouble is that the conditional expression of the for must be checked at the top of the loop, but the increment portion occurs at the bottom of the loop Therefore, even though these two pieces of the for loop occur next to each other in the source code, their interpretation is separated by the block of code being iterated However, with a little work, the for can be correctly interpreted When interp_block( ) encounters a for statement, exec_for( ) is called This function is shown here:
/* Execute a for loop */ void exec_for(void) { int cond; char *temp, *temp2; int brace ; get_token(); eval_exp(&cond); /* initialization expression */ if(*token != ';') sntx_err(SEMI_EXPECTED); prog++; /* get past the ; */ temp = prog; for(;;) { eval_exp(&cond); /* check the condition */ if(*token != ';') sntx_err(SEMI_EXPECTED); prog++; /* get past the ; */
Page 776 temp2 = prog; /* find the start of the for block */ brace = 1; while(brace) { get_token(); if(*token == '(') brace++; if(*token == ')') brace--; } if(cond) interp_block(); /* if true, interpret */ else { /* otherwise, skip around loop */ find_eob(); return; } prog = temp2; eval_exp(&cond); /* do the increment */ prog = temp; /* loop back to top */ } }
This function begins by processing the initialization expression in the for The initialization portion of the for is executed only once and does not form part of the loop Next, the program pointer is advanced to a point immediately after the semicolon that ends the initialization statement, and its value is assigned to temp A loop is then established, which checks the conditional portion of the for loop and assigns temp2 a pointer to the start of the increment portion The beginning of the loop code is found, and, finally, if the conditional expression is true, the loop block is interpreted (Otherwise, the end of the block is found, and execution continues on after the for loop) When the recursive call to interp_block( ) returns, the increment portion of the loop is executed, and the process repeats The Little C Library Functions Because the C programs executed by Little C are never compiled and linked, any library routines they use must be handled directly by Little C The best way to do this is to create an interface function that Little C calls when a library function is encountered This interface function sets up the call to the library function and handles any return values Because of space limitations, Little C contains only five ''library" functions: getche( ), putch( ), puts( ), print( ), and getnum( ) Of these, only puts( ), which outputs a string
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