# V2625. MISRA. Identifiers that define objects or functions with external linkage shall be unique.

The diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) software development guidelines.

The rule is relevant only for C. An identifier with external linkage should be unique in a program. The name should not be used by other identifiers that have a different linkage type (no linkage, internal linkage) within functions or other translation units. The rule applies to both objects and functions.

Note. To search for non-unique identifiers in different translation units, enable the intermodular analysis mode.

Here is an example:

    int var;  // external linkage

    void foo()
    {
      short var;  // no linkage
    }

In the code fragment above, the 'var' identifier with external linkage is hidden by a local variable in the 'foo' function. The fixed code looks like this:

    int var;  // external linkage

    void foo()
    {
      short temp;  // no linkage
    }

Let's look at another example, but this one is based on the contents of two files from the same project:

    // file1.c
    int x;             // external linkage
    static int y;      // internal linkage

    static void bar(); // internal linkage

    // file2.c
    void bar()  // external linkage
    {
      int y;    // no linkage
    }

    void foo()  // external linkage
    {
      int x;    // no linkage
    }

The 'x' identifiers from 'file1.c' and 'bar' from 'file2.c' have external linkage and are not unique, so they violate the rule. The 'y' identifier is not unique, either. However, since it has internal linkage in 'file1.c' and no linkage in 'file2.c', the rule is not violated for this name.

Here is the fixed code:

    // file1.c
    static int x;        // internal linkage
    static int y;        // internal linkage

    static void func();  // internal linkage

    // file2.c
    void bar()  // external linkage
    {
      int y;    // no linkage
    }

    void foo()  // external linkage
    {
      int x;    // no linkage
    }

This diagnostic is classified as:

MISRA-C-2012-5.8 MISRA-C-2023-5.8