The bitwise Or operation, signified by the
| operator, takes two values (normally
integers) as operands and returns a value of the
same type. It performs an or operation on corresponding
bits of the two operands. If a particular bit is
on in either of the two operands, the corresponding
bit in the result is on, otherwise it is off.
Here is an example
Operand 1 00000000 00000000 00000000 00110101
Operand 2 00000000 00000000 00000000 00011000
-------- -------- -------- --------
Result 00000000 00000000 00000000 00111101
This is often used to set flags for a function. For
example, when a file is opened, there are a number of
different flags which can be set. Typically each flag
is given a corresponding symbolic name and refers to
a single bit. This is done in a header file.
For example, a header file might contain code like this.
#define O_RDONLY 0x0 /*00000000 00000000 00000000 00000000*/ #define O_WRONLY 0x1 /*00000000 00000000 00000000 00000001*/ #define O_RDWR 0x2 /*00000000 00000000 00000000 00000010*/ #deinfe O_NDELAY 0x4 /*00000000 00000000 00000000 00000100*/ #define O_APPEND 0x8 /*00000000 00000000 00000000 00001000*/ #define O_CREAT 0x10 /*00000000 00000000 00000000 00010000*/ #define O_TRUNC 0x20 /*00000000 00000000 00000000 00100000*/ #define O_EXCL 0x40 /*00000000 00000000 00000000 01000000*/ #define O_NOCTTY 0x80 /*00000000 00000000 00000000 10000000*/Each of these sets a single bit of a 32 bit word on. Thus, if open is called like this:
fd = open("myfile",O_WRONLY | O_APPEND | O_CREAT)
The second argument will have three bits set. It would look like this in binary
00000000 00000000 00000000 00011001