In practice, the difference is in the location where the preprocessor searches for the included file.
For #include <filename>
the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.
For #include "filename"
the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename>
form. This method is normally used to include programmer-defined header files.
A more complete description is available in the GCC documentation on search paths.
Setting a bit
Use the bitwise OR operator (|
) to set a bit.
number |= 1UL << n;
That will set the n
th bit of number
. n
should be zero, if you want to set the 1
st bit and so on upto n-1
, if you want to set the n
th bit.
Use 1ULL
if number
is wider than unsigned long
; promotion of 1UL << n
doesn't happen until after evaluating 1UL << n
where it's undefined behaviour to shift by more than the width of a long
. The same applies to all the rest of the examples.
Clearing a bit
Use the bitwise AND operator (&
) to clear a bit.
number &= ~(1UL << n);
That will clear the n
th bit of number
. You must invert the bit string with the bitwise NOT operator (~
), then AND it.
Toggling a bit
The XOR operator (^
) can be used to toggle a bit.
number ^= 1UL << n;
That will toggle the n
th bit of number
.
Checking a bit
You didn't ask for this, but I might as well add it.
To check a bit, shift the number n to the right, then bitwise AND it:
bit = (number >> n) & 1U;
That will put the value of the n
th bit of number
into the variable bit
.
Changing the nth bit to x
Setting the n
th bit to either 1
or 0
can be achieved with the following on a 2's complement C++ implementation:
number ^= (-x ^ number) & (1UL << n);
Bit n
will be set if x
is 1
, and cleared if x
is 0
. If x
has some other value, you get garbage. x = !!x
will booleanize it to 0 or 1.
To make this independent of 2's complement negation behaviour (where -1
has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.
number ^= (-(unsigned long)x ^ number) & (1UL << n);
or
unsigned long newbit = !!x; // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);
It's generally a good idea to use unsigned types for portable bit manipulation.
or
number = (number & ~(1UL << n)) | (x << n);
(number & ~(1UL << n))
will clear the n
th bit and (x << n)
will set the n
th bit to x
.
It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.
Best Answer
calloc()
gives you a zero-initialized buffer, whilemalloc()
leaves the memory uninitialized.For large allocations, most
calloc
implementations under mainstream OSes will get known-zeroed pages from the OS (e.g. via POSIXmmap(MAP_ANONYMOUS)
or WindowsVirtualAlloc
) so it doesn't need to write them in user-space. This is how normalmalloc
gets more pages from the OS as well;calloc
just takes advantage of the OS's guarantee.This means
calloc
memory can still be "clean" and lazily-allocated, and copy-on-write mapped to a system-wide shared physical page of zeros. (Assuming a system with virtual memory.)Some compilers even can optimize malloc + memset(0) into calloc for you, but you should use calloc explicitly if you want the memory to read as
0
.If you aren't going to ever read memory before writing it, use
malloc
so it can (potentially) give you dirty memory from its internal free list instead of getting new pages from the OS. (Or instead of zeroing a block of memory on the free list for a small allocation).Embedded implementations of
calloc
may leave it up tocalloc
itself to zero memory if there's no OS, or it's not a fancy multi-user OS that zeros pages to stop information leaks between processes.On embedded Linux, malloc could
mmap(MAP_UNINITIALIZED|MAP_ANONYMOUS)
, which is only enabled for some embedded kernels because it's insecure on a multi-user system.