QA·C Help - 3408

	3408	'%s' has external linkage and is being defined without any previous declaration.

Declarations and Definitions

An object or function is being defined with external linkage but no previous declaration has been encountered.

When an item has external linkage, it can usually be assumed that it will be accessed in more than one translation unit. In order to ensure consistency between definition and usage, it is conventional to implement a single declaration located in a header file and for that header file to be included in the translation unit where the item is defined and every translation unit where it is used.

Message 3408 is generated whenever an object or function with external linkage is defined in the absence of a previous declaration.
Message 3447 is generated whenever an object or function with external.linkage is declared, but the declaration is not located in a header file.

For example:


/*PRQA S 2017,3211,3603 ++*/

extern int gv1 = 0;                      /* Message 3408 */

extern int gv2;                          /* Message 3447 */
int gv2 = 0;

extern void foo1(void)                   /* Message 3408 */
{
}

extern void foo2(void);                  /* Message 3447 */

extern void foo2(void)
{
}

MISRA-C:2004 Rules applicable to message 3408:

Rule 8.8 (Required)

An external object or function shall be declared in one and only one file.