In a general context, we can view a sequence point as a specific moment during the execution of a computer program where it ensures that all side effects from the preceding code evaluation have been completed. Simultaneously, it also guarantees that none of the modifications or side effects from the following evaluations have taken place yet. Put simply, in imperative programming, a sequence point marks a juncture in the program's execution where it is confirmed that all side effects from the previous evaluation have been executed, and no side effects from subsequent evaluations have occurred. The term "side effect" might seem perplexing initially, but let's delve into its precise meaning.
Side Effect
The concept of a "Side Effect" refers to alterations made by a function or expression that modify the state of an entity.
To gain a deeper understanding of sequence points, let's examine the examples provided below:
Example 1
#include <stdio.h>
int f1() { printf ("hi,there"); return 1;}
int f2() { printf ("hello, there"); return 1;}
int main()
{
int p = f1() + f2();
return 0;
}
The result produced by the previously mentioned code may be indeterminate or unpredictable as it can vary depending on the specific machine or compiler being used. This uncertainty arises because the behavior is akin to querying the value of an uninitialized automatic variable. The primary cause of this unpredictability in the mentioned code is the absence of a specified standard sequence of evaluation for the operands of the "+" operator. Consequently, it is impossible to determine whether the functions "f1" or "f2" will be executed first. Nevertheless, there exist various other operators similar to the "+" operator, including '-', '/', '*', Bitwise AND &, Bitwise OR |, and more.
Example 2
#include <stdio.h>
int x = 20;
int f1() { x = x+10; return x;}
int f2() { x = x-5; return x;}
int main()
{
int p = f1() + f2();
printf ("p = %d", p);
return 0;
}
The resulting outcome from the aforementioned code is indeterminate as the evaluation of the expression itself can lead to side effects. For instance, in the given code snippet, the definitive value of variable p remains uncertain due to the sequence followed for evaluating expressions. Consequently, if the function "f1;" is the initial one to be executed, the variable p will hold the value 55; otherwise, it will be 40.
Sequence points
These are some of the basic sequence points available in C Language
- logical AND &&
- logical OR ||
- conditional ?
- comma ,
- logical AND &&
In the scenario of the logical AND && operator, the left operand undergoes full execution initially, ensuring all side effects are completed before proceeding. Nonetheless, if the left operand results in false, the execution halts, preventing the other operand from executing at all.
- logical OR ||
In the situation of the logical OR || operator, the execution begins with evaluating the left operand entirely along with all its side effects before moving forward. Nonetheless, if the left operand results in being false (or nonzero), the second operand will not be processed at all.
Is this a Conditional Expression?
When dealing with the ternary operator, the initial operand undergoes evaluation before any further actions, ensuring that all side effects are executed before proceeding.
- comma,
The left side of the comma operator must undergo full evaluation initially, ensuring all its side effects are executed before moving forward. Nevertheless, both sides of the comma operator are invariably assessed.