C Write Bit in Memory

If I have an 8 bit number like so, 00101001,

• how would I replace one of the bits with a 1 (setting the bit)?


• how would I replace one of the bits with a 0?

Currently in my code I have two functions as shown:

void ADDSET(unsigned char *signalmask, int SIGNUM) {
    *signalmask = *signalmask | (SIGNUM);
}

void DELSET(unsigned char *signalmask, int SIGNUM) {
    *signalmask = *signalmask &~ SIGNUM;
}

With a function call like so:

int main() {
    unsigned char signalmask = 41;
    int SIGNUM = 7; // signal 1-8

    ADDSET(&signalmask, SIGNUM);

    return 0;
}

For some reason when I output my signalmask's memory address it either doesn't replace/remove the bit, or it does the wrong one. I think this is because the SIGNUM is starting at 0 when it should be starting at 1.

I output using:

for (i = 0; i < 8; i++) {
    printf("%d", !!((signalmask << i) & 0x80));
}

Can anybody help me?

Note: My INSET function works as desired:

int INSET(unsigned char signalmask, int SIGNUM){
    unsigned char bitMask = 1 << (SIGNUM - 1);
    if ((signalmask & bitMask) == 0)
        return 0;
    else
        return 1;
}

My problem is I don't know how to port this code over to the ADDSET and DELSET functions.

Thanks in advance.