AN3350 datasheet(98/185 Pages) STMICROELECTRONICS

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C compiler

AN3350

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Doc ID 018512 Rev 2

12.3.2

Initializing automatic arrays and structures

When the GCC extensions setting is on, array and structure variables that are local to a

function and have the automatic storage class may be initialized with values that do not

need to be constant. Example 38 shows an example.

Example 38 Initializing arrays and structures with non-constant values

void f(int i)

{

int j = i * 10; /* Always OK. */

/* These initializations are only accepted when GCC extensions

* are on. */

struct { int x, y; } s = { i + 1, i + 2 };

int a[2] = { i, i + 2 };

}

12.3.3

The sizeof() operator

When the GCC extensions setting is on, the sizeof() operator computes the size of

function and void types. In both cases, the sizeof() operator evaluates to 1. The ISO/IEC

9899-1990 C Standard (“C90”) does not specify the size of the void type and functions.

Example 39 shows an example.

Example 39 Using the sizeof() operator with void and function types

int f(int a)

{

return a * 10;

}

void g(void)

{

size_t voidsize = sizeof(void); /* voidsize contains 1 */

size_t funcsize = sizeof(f); /* funcsize contains 1 */

}

12.3.4

Statements in expressions

When the GCC extensions setting is on, expressions in function bodies may contain

statements and definitions. To use a statement or declaration in an expression, enclose it

within braces. The last item in the brace-enclosed expression gives the expression its value.

Example 40 shows an example.

Example 40 Using statements and definitions in expressions

Table 16.

Controlling GCC extensions to the C language

To control this option from here...

use this setting

Source code

#pragma gcc_extensions

Command line

-gcc_extensions

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