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Sequence points in programming define moments when a variable’s value is fully calculated with no pending changes. They exist before code execution, at the end of control expressions, and at the end of complete expressions. Violating the main rule of a sequence point can lead to unpredictable behavior. Sequence points ensure consistent and predictable execution of instructions by a compiler.
A sequence point in computer programming is a point in program execution when the value of a variable has been fully calculated, with no pending changes from previous operations, and no calculations have yet been made in a future expression . There are a number of places where sequence points exist, mostly defined in the C language standard, such as before code is executed within a function, at the end of control expressions in statements such as “for” and “if” and at the end of any complete expression, such as a simple line of C code. Some reasons for defining a sequence point are to avoid situations that are ambiguous, that lead to undefined behavior, or that might confuse the compiler and generate unpredictable code. In many cases, programmers don’t explicitly care about a sequence point, although, in building a compiler, the concept is very important to ensure that the code executes correctly.
An example of a sequence point in the C programming language is in the statement A = A + B;. In this expression, the semicolon is the sequence point; once the expression is complete, the value of A will be evaluated and no residual calculations will be performed on it until the start of the next expression. The equal sign is not a sequence point, because the value of A could be changed by the compiler in any order during the expression.
The main rule of a sequence point is that no variable will be accessed more than once between points for any purpose other than calculating a change in its value. A violation of this rule is best expressed when assigning a value to an array. If there is a variable A and an array called I, then grammatically in C it is possible to write the expression I( A ) = A++. Here, the variable is accessed more than once for a purpose other than evaluating its current value; that is, it is used as an index into the array I. This means that the compiler may increment A either before it is used as an index or after it has been used, creating unpredictable behavior that cannot be relied upon in your program.
A sequence point basically can be seen as a way to ensure that instructions can and will be consistently evaluated and executed by a compiler. This also allows a compiler to use optimization strategies, since the defined behavior is predictable. Within the C language standard, there are three main instances of sequence points, namely when a function begins execution, at the point of logical operators and commas, and at the end of a complete expression that ends with a period semicolon, as most C statements do.
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