1. Rudra Dutta CSC 230 - Spring 2007, Section 001
Sequence Points
Rudra Dutta
CSC Spring 2007, Section 001

2. Sequence of Instructions
Program consists of series of instructions
Source code is specification of not only the instructions to execute but the sequence of execution
However, “single” instructions of source code become sets of multiple actual instructions
Even more so for a compact language such as C
There is a limit to the power of specification of instruction sequence
Formalized in the concept of “sequence points”
Certain points in the execution sequence where
all side effects of previous evaluations shall be complete,
and no side effects of subsequent evaluations shall have taken place
Copyright Rudra Dutta, NCSU, Spring, 2007

3. Sequence Points
The call to a function, after the arguments have been evaluated
The end of the first operand of the following operators:
logical AND &&
logical OR ||
conditional ?
comma ,
The end of a full declarator
A declarator is the part of a declaration (definition or initialization) that is not the type
Copyright Rudra Dutta, NCSU, Spring, 2007

4. Sequence Points The end of a full expression:
an initializer
the expression in an expression statement
the controlling expression of a selection statement (if or switch)
the controlling expression of a while or do statement
each of the expressions of a for statement
the expression in a return statement
Immediately before a library function returns
After the actions associated with each formatted input/output function conversion specifier
Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call
Copyright Rudra Dutta, NCSU, Spring, 2007

5. Example of Sequence Point Issue
#include <stdio.h>
#include <stdlib.h>
int j;
void func_1 (int i, int k) {
for (j=0; j<i; j++) {
printf ("%d ", j); }
printf ("%d\n", k);
int main () {
int j;
for (j=0; j<10; j++) {
func_1 (j, j); }
Copyright Rudra Dutta, NCSU, Spring, 2007

6. Example of Sequence Point Issue
#include <stdio.h>
#include <stdlib.h>
int j;
void func_1 (int i, int k) {
for (j=0; j<i; j++) {
printf ("%d ", j); }
printf ("%d\n", k);
int main () {
/* int j; */
for (j=0; j<10; j++) {
func_1 (j, j); }
Copyright Rudra Dutta, NCSU, Spring, 2007

7. Example of Sequence Point Issue
#include <stdio.h>
#include <stdlib.h>
int j;
void func_1 (int i, int k) {
/* int j; */
for (j=0; j<i; j++) {
printf ("%d ", j); }
printf ("%d\n", k);
int main () {
/* int j; */
for (j=0; j<10; j++) {
func_1 (j, j); }
Copyright Rudra Dutta, NCSU, Spring, 2007

8. Example of Sequence Point Issue
#include <stdio.h>
#include <stdlib.h>
int j;
void func_1 (int i, int k) {
/* int j; */
for (j=0; j<i; j++) {
printf ("%d ", j); }
printf ("%d\n", k);
int main () {
/* int j; */
for (j=0; j<10; j++) {
func_1 (++j, j); }
Copyright Rudra Dutta, NCSU, Spring, 2007

9. Example of Sequence Point Issue
#include <stdio.h>
#include <stdlib.h>
int j;
void func_1 (int i, int k) {
/* int j; */
for (j=0; j<i; j++) {
printf ("%d ", j); }
printf ("%d\n", k);
int main () {
/* int j; */
for (j=0; j<10; j++) {
func_1 (j, ++j); }
Copyright Rudra Dutta, NCSU, Spring, 2007

10. Expressions and Sequence Points
Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.
i = i + 1;
a[i] = i;
i = ++i + 1;
a[i++] = i;
Copyright Rudra Dutta, NCSU, Spring, 2007

11. Function Calls and Sequence Points
The order of evaluation of the
function designator,
the actual arguments, and
subexpressions within the actual arguments
is unspecified
BUT there is a sequence point before the actual call
Copyright Rudra Dutta, NCSU, Spring, 2007