AN3350 datasheet(131/185 Pages) STMICROELECTRONICS

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AN3350

Pragmas for standard C conformance

Doc ID 018512 Rev 2

131/185

caused by referring to a function before you define it. For example, without a function

prototype, you might pass data of the wrong type. As a result, your code might not work as

you expect even though it compiles without error.

In Example 80, function main() calls PrintNum() with an integer argument even though

PrintNum()

takes an argument of type float.

Example 80 Unnoticed type-mismatch

#include <stdio.h>

void main(void)

{

PrintNum(1);

/* PrintNum() tries to interpret the

integer as a float. Prints 0.000000. */

}

void PrintNum(float x)

{

printf("%f\n", x);

}

When you run this program, you could get this result:

0.000000

Although the compiler does not complain about the type mismatch, the function does not

give the result you intended. Since PrintNum() does not have a prototype, the compiler

does not know to generate instructions to convert the integer to a floating-point number

before calling PrintNum(). Consequently, the function interprets the bits it received as a

floating-point number and prints nonsense.

A prototype for PrintNum(), as in Example 81, gives the compiler sufficient information

about the function to generate instructions to properly convert its argument to a floating-

point number. The function prints what you expected.

Example 81 Using a prototype to avoid type-mismatch

#include <stdio.h>

void PrintNum(float x); /* Function prototype. */

void main(void)

{

PrintNum(1);

Compiler converts int to float.

}

Prints 1.000000. */

void PrintNum(float x)

{

printf("%f\n", x);

}

In other situations where automatic conversion is not possible, the compiler generates an

error message if an argument does not match the data type required by a function

prototype. Such a mismatched data type error is easier to locate at compile time than at

runtime.

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