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The definitions above ensure that, at the time they call your function, the data they pass in has values which are consistent with the behaviour specified by the abstract machine, and any data returned by your function has a state which is also consistent with the abstract machine. This includes data accessible via pointers i. The above is a slight simplification, since compilers exist that perform whole-program optimisation at link time. Importantly however, although they might perform optimisations, the visible side effects of the program must be the same as if they were produced by the abstract machine.
The C standards both C89 and C99 both forbid this construct in conforming programs. Not allowed in a conforming program; modifies x twice before argument evaluation is complete. Allowed; the function bar takes one argument the value 2 is passed here , and there is a sequence point at the comma operator.
Allowed; there is a sequence point at the end of the controlling expression of the if 3. Allowed; there is a sequence point before the?
Not allowed; the object at p is being modified twice before the evaluation of the arguments to bar is complete. The fact that this is done once via p and once via q is irrelevant, since they both point to the same object.
Suppose the code actually looks like this:. Is this code allowed in a standard-conforming program? Although the expression in foo modifies a twice, this is not a problem. Since f returns a value other than void, it must contain a return statement. Therefore, there is a sequence point at the end of the return expression. That comes between the modification to a that f makes and the evaluation of the left operand.
First, a is incremented. Then the arguments to f are evaluated there are zero of them. Then there is a sequence point before f is actually called. So, we see that our program is standard-conforming. Notice that the above argument does not actually depend on the details of the body of the function f.
It only depends on the function containing something ending in a sequence point — in our example this is a return statement, but an expression statement or a full declarator would do just as well. If the left-hand operand is evaluated first, foo returns 6. Otherwise, it returns The C standard does not specify in which order the operands should be evaluated, and also does not require an implementation either to document the order or even to stick to one order. The effect of this code is unspecified , meaning that one of several specific things will happen, but the C standards do not say which.
When a signal is received, this will happen between sequence points. Side effects on volatile objects prior to the previous sequence point will have occurred, but other updates may not have occurred yet. The C standard is quite restrictive about what data access can occur within a signal handler. The POSIX standard also allows a small number of library functions to be called from a signal handler. These functions are referred to as the set of async-signal-safe functions. If your program is intended to run on a POSIX system but not on other systems, you can safely call these from your signal handler too.
You write statements to cause actions and to control flow within your programs. You can also write statements that do not do anything at all, or do things that are uselessly trivial.
You can use labels to identify a section of source code for use with a later goto see The goto Statement.
A label consists of an identifier such as those used for variable names followed by a colon. GCC will compile code that does not meet this requirement, but be aware that if you violate it, your code may have portability issues. You can turn any expression into a statement by adding a semicolon to the end of the expression.
In each of those, all that happens is that each expression is evaluated. However, they are useless because they do not store a value anywhere, nor do they actually do anything, other than the evaluation itself. The compiler is free to ignore such statements. Expression statements are only useful when they have some kind of side effect, such as storing a value, calling a function, or this is esoteric causing a fault in the program.
Here are some more useful examples:. You can use the if statement to conditionally execute part of your program, based on the truth value of a given expression. Here is the general form of an if statement:. If test evaluates to true, then then-statement is executed and else-statement is not.
If test evaluates to false, then else-statement is executed and then-statement is not. The else clause is optional. You can use the switch statement to compare one expression with others, and then execute a series of sub-statements based on the result of the comparisons.
Here is the general form of a switch statement:. The switch statement compares test to each of the compare expressions, until it finds one that is equal to test.
Then, the statements following the successful case are executed. All of the expressions compared must be of an integer type, and the compare-N expressions must be of a constant integer type e.
Optionally, you can specify a default case. Notice the usage of the break statement in each of the cases. This is because, once a matching case is found, not only are its statements executed, but so are the statements for all following cases:.
This is often not desired. Including a break statement at the end of each case redirects program flow to after the switch statement. As a GNU C extension, you can also specify a range of consecutive integer values in a single case label, like this:.
This has the same effect as the corresponding number of individual case labels, one for each integer value from low to high , inclusive. Be careful to include spaces around the For example, write this:. It is common to use a switch statement to handle various possible values of errno. The while statement is a loop statement with an exit test at the beginning of the loop. Here is the general form of the while statement:. The while statement first evaluates test.
If test evaluates to true, statement is executed, and then test is evaluated again. The do statement is a loop statement with an exit test at the end of the loop. Here is the general form of the do statement:. The do statement first executes statement.
After that, it evaluates test. If test is true, then statement is executed again. The for statement is a loop statement whose structure allows easy variable initialization, expression testing, and variable modification. It is very convenient for making counter-controlled loops. Here is the general form of the for statement:. The for statement first evaluates the expression initialize. Then it evaluates the expression test. If test is false, then the loop ends and program control resumes after statement.
Otherwise, if test is true, then statement is executed. Finally, step is evaluated, and the next iteration of the loop begins with evaluating test again. Here is another example that prints the integers from zero through nine:. First, it evaluates initialize , which assigns x the value 0. Then, as long as x is less than 10, the value of x is printed in the body of the loop. Then x is incremented in the step clause and the test re-evaluated. All three of the expressions in a for statement are optional, and any combination of the three is valid.
Since the first expression is evaluated only once, it is perhaps the most commonly omitted expression. You could also write the above example as:. In this example, x receives its value prior to the beginning of the for statement. If you leave out the test expression, then the for statement is an infinite loop unless you put a break or goto statement somewhere in statement.
This is like using 1 as test ; it is never false. This for statement starts printing numbers at 1 and then continues indefinitely, always printing x incremented by If you leave out the step expression, then no progress is made toward completing the loop—at least not as is normally expected with a for statement. Perhaps confusingly, you cannot use the comma operator see The Comma Operator for monitoring multiple variables in a for statement, because as usual the comma operator discards the result of its left operand.
This loop:. Here is an example of a function that computes the summation of squares, given a starting integer to square and an ending integer to square:. A block is a set of zero or more statements enclosed in braces. Blocks are also known as compound statements. Often, a block is used as the body of an if statement or a loop statement, to group statements together. You can declare variables inside a block; such variables are local to that block. In C89, declarations must occur before other statements, and so sometimes it is useful to introduce a block simply for this purpose:.
A null statement does not do anything. It does not store a value anywhere. It does not cause time to pass during the execution of your program. Most often, a null statement is used as the body of a loop statement, or as one or more of the expressions in a for statement. Here is an example of a for statement that uses the null statement as the body of the loop and also calculates the integer square root of n , just for fun :.
Here is another example that uses the null statement as the body of a for loop and also produces output:. A null statement is also sometimes used to follow a label that would otherwise be the last thing in a block. You can use the goto statement to unconditionally jump to a different place in the program. Here is the general form of a goto statement:. You have to specify a label to jump to; when the goto statement is executed, program control jumps to that label.
See Labels. The label can be anywhere in the same function as the goto statement that jumps to it, but a goto statement cannot jump to a label in a different function. You can use goto statements to simulate loop statements, but we do not recommend it—it makes the program harder to read, and GCC cannot optimize it as well. You should use for , while , and do statements instead of goto statements, when possible. Here is a contrived example:.
You can use the break statement to terminate a while , do , for , or switch statement. That example prints numbers from 1 to 7. If you put a break statement inside of a loop or switch statement which itself is inside of a loop or switch statement, the break only terminates the innermost loop or switch statement.
You can use the continue statement in loops to terminate an iteration of the loop and begin the next iteration. If you put a continue statement inside a loop which itself is inside a loop, then it affects only the innermost loop. You can use the return statement to end the execution of a function and return program control to the function that called it. Here is the general form of the return statement:. You can, however, use the return statement without a return value.
In that case, the function cosine was called in a context that required a return value, so the value could be assigned to x. Even in contexts where a return value is not required, it is a bad idea for a non- void function to omit the return value. With GCC, you can use the command line option -Wreturn -type to issue a warning if you omit the return value in such functions.
Here are some examples of using the return statement, in both a void and non- void function:. You can use the typedef statement to create new names for data types. Here is the general form of the typedef statement:. Creating this new name for the type does not cause the old name to cease to exist. In the case of custom data types, you can use typedef to make a new name for the type while defining the type:.
To make a type definition of an array, you first provide the type of the element, and then establish the number of elements at the end of the type definition:. You can write functions to separate parts of your program into distinct subprocedures. To write a function, you must at least create a function definition.
Every program requires at least one function, called main. A function declaration ends with a semicolon. Here is the general form:. A typical parameter consists of a data type and an optional name for the parameter.
You can also declare a function that has a variable number of parameters see Variable Length Parameter Lists , or no parameters using void. Leaving out parameter-list entirely also indicates no parameters, but it is better to specify it explicitly with void. The parameter names in the declaration need not match the names in the definition. You should write the function declaration above the first use of the function. You can put it in a header file and use the include directive to include that function declaration in any source code files that use the function.
You write a function definition to specify what a function actually does. The function body is a series of statements enclosed in braces; in fact it is simply a block see Blocks. Here is an simple example of a function definition—it takes two integers as its parameters and returns the sum of them as its return value:.
For compatibility with the original design of C, you can also specify the type of the function parameters after the closing parenthesis of the parameter list, like this:.
However, we strongly discourage this style of coding; it can cause subtle problems with type casting, among other problems. You can call a function by using its name and supplying any needed parameters.
Here is the general form of a function call:. A function call can make up an entire statement, or it can be used as a subexpression.
Here is an example of a standalone function call:. Function parameters can be any expression—a literal value, a value stored in variable, an address in memory, or a more complex expression built by combining these.
Within the function body, the parameter is a local copy of the value passed into the function; you cannot change the value passed in by changing the local copy. If you wish to use the function to change the original value of x , then you would have to incorporate the function call into an assignment statement:.
If the value that you pass to a function is a memory address that is, a pointer , then you can access and change the data stored at the memory address. This achieves an effect similar to pass-by-reference in other languages, but is not the same: the memory address is simply a value, just like any other value, and cannot itself be changed. The difference between passing a pointer and passing an integer lies in what you can do using the value within the function.
The formal parameter for the function is of type pointer-to- int , and we call the function by passing it the address of a variable of type int.
By dereferencing the pointer within the function body, we can both see and change the value stored in the address.
In this case, unless you are working on a computer with very large memory addresses, it will take less memory to pass a pointer to the structure than to pass an instance of the structure. In this example, calling the function foo with the parameter a does not copy the entire array into a new local parameter within foo ; rather, it passes x as a pointer to the first element in x.
Be careful, though: within the function, you cannot use sizeof to determine the size of the array x — sizeof instead tells you the size of the pointer x. Indeed, the above code is equivalent to:. Explicitly specifying the length of the array in the parameter declaration will not help. If you really need to pass an array by value, you can wrap it in a struct , though doing this will rarely be useful passing a const -qualified pointer is normally sufficient to indicate that the caller should not modify the array.
You can write a function that takes a variable number of arguments; these are called variadic functions. To do this, the function needs to have at least one parameter of a known data type, but the remaining parameters are optional, and can vary in both quantity and data type.
Here is an example function prototype:. To use optional parameters, you need to have a way to know how many there are. So, we might call the function like this:. However, you might want to include it to maximize compatibility with other compilers. You can also call a function identified by a pointer. This is where the program begins executing.
You do not need to write a declaration or prototype for main , but you do need to define it. The return type for main is always int. You do not have to specify the return type for main , but you can. However, you cannot specify that it has a return type other than int. Otherwise, the significance of the value returned is implementation-defined.
In C89, the effect of this is undefined, but in C99 the effect is equivalent to return 0;. You can write your main function to have no parameters that is, as int main void , or to accept parameters from the command line. Here is a very simple main function with no parameters:. You can change the names of those parameters, but they must have those data types— int and array of pointers to char.
Here is an example main function that accepts command line parameters, and prints out what those parameters are:. You can write a function that is recursive—a function that calls itself. Grewal Book Free Download April Bansal Book Free October Verma Book Free Download February March Trending on EasyEngineering.
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