Purpose of using a void cast char* as a statement

[cyqotiq]

I have some legacy code in a project I'm working. We have a line of code that looks like the following:

int init_tbl (char *tblname) {
    int ret = -1;
 
    /* some more code ... */
 
    setBatchSize(getBatchSize());
 
    (void) tblname;
 
    return ( ret );
}
 

What is the purpose of the line:

(void) tblname;

[donbock]

That line does nothing; and yet, it serves a purpose. Presumably, argument tblname is not otherwise accessed within the function. Although not required to do so, some compilers (or static checkers like lint) will generate a warning if a function argument is not used. The line in question fools the compiler into thinking tblname is used, so the warning is not generated.

I would classify this as tricky code that ought to be well commented. Some static checkers allow you to embed directives in comments that tune what generates an warning. A directive in a comment is better because it is obvious that comments have no effect on program execution.

[donbock]

Strictly speaking that line directs the compiler to read the current value of tblname and then to throw the value away. The compiler may well optimize out this access to tblname.

Behavior is similar in the following code snippet, but the purpose is different.

void function(volatile char *p) {
    (void) *p;
}

In this case, the value at the location referenced by p is read and then discarded. However, the volatile keyword prevents this access from being optimized out. There are real situations where a memory-mapped I/O port needs to be read from, but the value you get out of it is irrelevant.