External and tentative definitions
At the top level of a translation unit (that is, a source file with all the #includes after the preprocessor), every C program is a sequence of declarations, which declare functions and objects with external linkage. These declarations are known as external declarations because they appear outside of any function.
extern int n; // external declaration with external linkage int b = 1; // external definition with external linkage static const char *c = "abc"; // external definition with internal linkage int f(void) { // external definition with external linkage int a = 1; // non-external return b; } static void x(void) { // external definition with internal linkage }
Objects declared with an external declaration have static storage duration, and as such cannot use auto or register specifiers. The identifiers introduced by external declarations have file scope.
Tentative definitions
A tentative definition is an external declaration without an initializer, and
either without a storage-class specifier or with the specifier static.
A tentative definition is a declaration that may or may not act as a definition. If an actual external definition is found earlier or later in the same translation unit, then the tentative definition just acts as a declaration.
int i1 = 1; // definition, external linkage int i1; // tentative definition, acts as declaration because i1 is defined extern int i1; // declaration, refers to the earlier definition extern int i2 = 3; // definition, external linkage int i2; // tentative definition, acts as declaration because i2 is defined extern int i2; // declaration, refers to the external linkage definition
If there are no definitions in the same translation unit, then the tentative definition acts as an actual definition with the initializer = 0 (or, for array, structure, and union types, = {0}).
int i3; // tentative definition, external linkage int i3; // tentative definition, external linkage extern int i3; // declaration, external linkage // in this translation unit, i3 is defined as if by "int i3 = 0;"
Unlike the extern declarations, which don't change the linkage of an identifier if a previous declaration established it, tentative definitions may disagree in linkage with another declaration of the same identifier. If two declarations for the same identifier are in scope and have different linkage, the behavior is undefined:
static int i4 = 2; // definition, internal linkage int i4; // Undefined behavior: linkage disagreement with previous line extern int i4; // declaration, refers to the internal linkage definition static int i5; // tentative definition, internal linkage int i5; // Undefined behavior: linkage disagreement with previous line extern int i5; // refers to previous, whose linkage is internal
A tentative definition with internal linkage must have complete type.
static int i[]; // Error, incomplete type in a static tentative definition int i[]; // OK, equivalent to int i[1] = {0}; unless redeclared later in this file
One definition rule
Each translation unit may have zero or one external definition of every identifier with internal linkage (a static global).
If an identifier with internal linkage is used in any expression other than a non-VLA (since C99), sizeof, or alignof (since C11), there must be one and only one external definition for that identifier in the translation unit.
The entire program may have zero or one external definition of every identifier (other than an inline function) (since C99) with external linkage.
If an identifier with external linkage is used in any expression other than a non-VLA (since C99), sizeof, or alignof (since C11), there must be one and only one external definition for that identifier somewhere in the entire program.
Notes
See inline for the details on the inline function definitions.
See storage duration and linkage for the meaning of the keyword extern with declarations at file scope
See definitions for the distinction between declarations and definitions.
Tentative definitions were invented to standardize various pre-C89 approaches to forward declaring identifiers with internal linkage.
References
- C11 standard (ISO/IEC 9899:2011):
- 6.9 External definitions (p: 155-159)
- C99 standard (ISO/IEC 9899:1999):
- 6.9 External definitions (p: 140-144)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.7 EXTERNAL DEFINITIONS