defining unused parameters in C

Question

I need to use pthreat but I dont need to pass any argument to the function. Therefore, I pass NULL to the function on pthread_create. I have 7 pthreads, so gcc compiler warns me that I have 7 unsued parameters. How can I define these 7 parameters as unused in C programming? If I do not define these parameters as unused, would it cause any problem? Thank you in advance for the responses.

void *timer1_function(void * parameter1){
//<statement>
}

int main(int argc,char *argv[]){
  int thread_check1;
  pthread_t timer1;
  thread_check1 = pthread_create( &timer1, NULL, timer1_function,  NULL);
    if(thread_check1 !=0){
        perror("thread creation failed");
        exit(EXIT_FAILURE);
    }
while(1){}
return 0;
}

Solution 1

You can cast the parameter to void like this:

void *timer1_function(void * parameter1) {
  (void) parameter1; // Suppress the warning.
  // <statement>
}

Solution 2

GCC has an "attributes" facility that can be used to mark unused parameters. Use

void *timer1_function(__attribute__((unused))void *parameter1)

Solution 3

Two commonly used techniques:

1) Omit the name of the unused parameter:

void *timer1_function(void *) { ... }

2) Comment out the parameter name:

void *timer1_function(void * /*parameter1*/) { ... }

-- Chris

Solution 4

By default, GCC does not produce this warning, not even with -Wall. I think the workaround shown in other question might be needed when you have no control over the environment, but if you do, just remove the flag (-Wunused-parameter).

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Comments

[Flexo]

stackoverflow.com/a/4851173/168175 has an alternative form that works better for volatile apparently

[R.. GitHub STOP HELPING ICE]

+1 this is the best fix. This warning is fundamentally idiotic. Whenever the address of a function is taken, GCC should turn off "unused parameter" warnings for it, because whether they're used internally or not, they're used as part of the required interface for the function. Personally I would say the same thing should apply to all external functions too...

[Kyle Jones]

@R I caught a bug in my code a few days ago thanks to this warning. I was doing a refactor of some functions and typed 0 instead of the identifier for a bitmask that came in as a function parameter. Enabling -Wextra allowed me to immediately fix a subtle bug that had been introduced days ago.

[Étienne]

-1: I regularly find bugs thanks to this warning being activated.

[ideasman42]

For code that is many years old and modified... over time sometimes args are added and end up being unused later on, so this warning can be very useful, especially when it becomes a hassle to pass some argument to a function (lookup or if it has to be calculated), only to find its not even used.

[Duncan Jones]

I get parameter name omitted if I do that (in C11). Pretty sure this is invalid for C (rather than C++).