return Statement (C)

A return statement ends the execution of a function, and returns control to the calling function. Execution resumes in the calling function at the point immediately following the call. A return statement can return a value to the calling function. For more information, see Return type.

Syntax

jump-statement:
 return expression_opt ;

The value of expression, if present, is returned to the calling function. If expression is omitted, the return value of the function is undefined. The expression, if present, is evaluated and then converted to the type returned by the function. When a return statement contains an expression in functions that have a void return type, the compiler generates a warning, and the expression isn't evaluated.

If no return statement appears in a function definition, control automatically returns to the calling function after the last statement of the called function is executed. In this case, the return value of the called function is undefined. If the function has a return type other than void, it's a serious bug, and the compiler prints a warning diagnostic message. If the function has a void return type, this behavior is okay, but may be considered poor style. Use a plain return statement to make your intent clear.

As a good engineering practice, always specify a return type for your functions. If a return value isn't required, declare the function to have void return type. If a return type isn't specified, the C compiler assumes a default return type of int.

Many programmers use parentheses to enclose the expression argument of the return statement. However, C doesn't require the parentheses.

The compiler may issue a warning diagnostic message about unreachable code if it finds any statements placed after the return statement.

In a main function, the return statement and expression are optional. What happens to the returned value, if one is specified, depends on the implementation. Microsoft-specific: The Microsoft C implementation returns the expression value to the process that invoked the program, such as cmd.exe. If no return expression is supplied, the Microsoft C runtime returns a value that indicates success (0) or failure (a non-zero value).

Example

This example is one program in several parts. It demonstrates the return statement, and how it's used both to end function execution, and optionally, to return a value.

// C_return_statement.c
// Compile using: cl /W4 C_return_statement.c
#include <limits.h>      // for INT_MAX
#include <stdio.h>       // for printf

long long square( int value )
{
    // Cast one operand to long long to force the
    // expression to be evaluated as type long long.
    // Note that parentheses around the return expression
    // are allowed, but not required here.
    return ( value * (long long) value );
}

The square function returns the square of its argument, in a wider type to prevent an arithmetic error. Microsoft-specific: In the Microsoft C implementation, the long long type is large enough to hold the product of two int values without overflow.

The parentheses around the return expression in square are evaluated as part of the expression, and aren't required by the return statement.

double ratio( int numerator, int denominator )
{
    // Cast one operand to double to force floating-point
    // division. Otherwise, integer division is used,
    // then the result is converted to the return type.
    return numerator / (double) denominator;
}

The ratio function returns the ratio of its two int arguments as a floating-point double value. The return expression is forced to use a floating-point operation by casting one of the operands to double. Otherwise, the integer division operator would be used, and the fractional part would be lost.

void report_square( void )
{
    int value = INT_MAX;
    long long squared = 0LL;
    squared = square( value );
    printf( "value = %d, squared = %lld\n", value, squared );
    return; // Use an empty expression to return void.
}

The report_square function calls square with a parameter value of INT_MAX, the largest signed integer value that fits in an int. The long long result is stored in squared, then printed. The report_square function has a void return type, so it doesn't have an expression in its return statement.

void report_ratio( int top, int bottom )
{
    double fraction = ratio( top, bottom );
    printf( "%d / %d = %.16f\n", top, bottom, fraction );
    // It's okay to have no return statement for functions
    // that have void return types.
}

The report_ratio function calls ratio with parameter values of 1 and INT_MAX. The double result is stored in fraction, then printed. The report_ratio function has a void return type, so it doesn't need to explicitly return a value. Execution of report_ratio "falls off the bottom" and returns no value to the caller.

int main()
{
    int n = 1;
    int x = INT_MAX;

    report_square();
    report_ratio( n, x );

    return 0;
}

The main function calls two functions: report_square and report_ratio. As report_square takes no parameters and returns void, we don't assign its result to a variable. Likewise, report_ratio returns void, so we don't save its return value, either. After each of these function calls, execution continues at the next statement. Then main returns a value of 0 (typically used to report success) to end the program.

To compile the example, create a source code file named C_return_statement.c. Then, copy all the example code, in the order shown. Save the file, and compile it in a Developer command prompt window by using the command:

cl /W4 C_return_statement.c

Then, to run the example code, enter C_return_statement.exe at the command prompt. The output of the example looks like this:

value = 2147483647, squared = 4611686014132420609
1 / 2147483647 = 0.0000000004656613

See also

Statements