Question

I’m new to programming in C and I have the following situation where I know what needs to be done, I’m just not sure of the right way to do. I’ll try to keep it as simple as possible, and please let me know if I’m not providing enough info.

There’s a .h file with this line:

#define NS_CONN_ENTITYTOOLARGE    0x2000

Then in the C code we have the following flag being set:

connPtr->flags |= NS_CONN_ENTITYTOOLARGE;

Then later on this test:

if (connPtr->flags & NS_CONN_ENTITYTOOLARGE) {
}

Now what I want to do is basically “undo” the flag NS_CONN_ENTITYTOOLARGE. After some Googling I discovered that |= is a “bitwise OR”, but I don’t feel a lot more enlightened after learning that. Do I need to do a bitwise XOR, a NOT or what?

Here are all the possible values for flags that I can find in the header file.

#define NS_CONN_CLOSED        0x1
#define NS_CONN_SKIPHDRS      0x2
#define NS_CONN_SKIPBODY      0x4
#define NS_CONN_READHDRS      0x8
#define NS_CONN_SENTHDRS      0x10
#define NS_CONN_KEEPALIVE     0x20
#define NS_CONN_WRITE_ENCODED     0x40
#define NS_CONN_FILECONTENT       0x80
#define NS_CONN_RUNNING           0x100
#define NS_CONN_OVERFLOW      0x200
#define NS_CONN_TIMEOUT           0x400
#define NS_CONN_GZIP          0x800
#define NS_CONN_CHUNK         0x1000
#define NS_CONN_ENTITYTOOLARGE    0x2000

Answer 1

connPtr->flags &= ~NS_CONN_ENTITYTOOLARGE;

This doesn’t exactly “undo” |= because it may have been a no-op for some or all of the bits in NS_CONN_ENTITYTOOLARGE, if they were already in the variable. In these case, those bits will be cleared too regardless of whether they were there in the first place.