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Do not define a modifiable object with static or thread storage duration in an external inline function, or reference an identifier with internal linkage.  The C Standard, subclause 6.7.4 paragraph 3, says:

An inline definition of a function with external linkage shall not contain a definition of a modifiable object with static or thread storage duration, and shall not contain a reference to an identifier with internal linkage.

Noncompliant Code Example (Internal Linkage)

This code refers to a file scope static variable, which has internal linkage, inside an external inline function:

static int I = 12;
extern inline void func(int a) {
  int b = a * I;
  /* ... */
}

Compliant Solution (Internal Linkage)

This compliant solution does not declare the variable at file scope to be static and so the variable omits the static qualifier;  consequently,  the variable I has external linkage by default.

int I = 12;
extern inline void func(int a) {
  int b = a * I;
  /* ... */
}

Noncompliant Code Example (Modifiable Static)

This noncompliant code example defines a modifiable static variable within an extern inline function.

extern inline void func(void) {
  static int I = 12;
  /* Perform calculations which may modify I */
}

Compliant Solution (Modifiable Static)

It is invalid to define a static or thread-local value within an extern inline function. This compliant solution removes the static keyword from the local variable definition. if If the modifications to I must be retained between invocations of func(), it must be declared at file scope so that it will be defined with external linkage.

extern inline void func(void) {
  int I = 12;
  /* Perform calculations which may modify I */
}

Risk Assessment

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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