The return value for main
indicates how the program exited. Normal exit is represented by a 0 return value from main
. Abnormal exit is signaled by a non-zero return, but there is no standard for how non-zero codes are interpreted. As noted by others, void main()
is prohibited by the C++ standard and should not be used. The valid C++ main
signatures are:
int main()
and
int main(int argc, char* argv[])
which is equivalent to
int main(int argc, char** argv)
It is also worth noting that in C++, int main()
can be left without a return-statement, at which point it defaults to returning 0. This is also true with a C99 program. Whether return 0;
should be omitted or not is open to debate. The range of valid C program main signatures is much greater.
Efficiency is not an issue with the main
function. It can only be entered and left once (marking the program's start and termination) according to the C++ standard. For C, re-entering main()
is allowed, but should be avoided.
The C++ standard does not specify the size of integral types in bytes, but it specifies minimum ranges they must be able to hold. You can infer minimum size in bits from the required range. You can infer minimum size in bytes from that and the value of the CHAR_BIT
macro that defines the number of bits in a byte. In all but the most obscure platforms it's 8, and it can't be less than 8.
One additional constraint for char
is that its size is always 1 byte, or CHAR_BIT
bits (hence the name). This is stated explicitly in the standard.
The C standard is a normative reference for the C++ standard, so even though it doesn't state these requirements explicitly, C++ requires the minimum ranges required by the C standard (page 22), which are the same as those from Data Type Ranges on MSDN:
signed char
: -127 to 127 (note, not -128 to 127; this accommodates 1's-complement and sign-and-magnitude platforms)
unsigned char
: 0 to 255
- "plain"
char
: same range as signed char
or unsigned char
, implementation-defined
signed short
: -32767 to 32767
unsigned short
: 0 to 65535
signed int
: -32767 to 32767
unsigned int
: 0 to 65535
signed long
: -2147483647 to 2147483647
unsigned long
: 0 to 4294967295
signed long long
: -9223372036854775807 to 9223372036854775807
unsigned long long
: 0 to 18446744073709551615
A C++ (or C) implementation can define the size of a type in bytes sizeof(type)
to any value, as long as
- the expression
sizeof(type) * CHAR_BIT
evaluates to a number of bits high enough to contain required ranges, and
- the ordering of type is still valid (e.g.
sizeof(int) <= sizeof(long)
).
Putting this all together, we are guaranteed that:
char
, signed char
, and unsigned char
are at least 8 bits
signed short
, unsigned short
, signed int
, and unsigned int
are at least 16 bits
signed long
and unsigned long
are at least 32 bits
signed long long
and unsigned long long
are at least 64 bits
No guarantee is made about the size of float
or double
except that double
provides at least as much precision as float
.
The actual implementation-specific ranges can be found in <limits.h>
header in C, or <climits>
in C++ (or even better, templated std::numeric_limits
in <limits>
header).
For example, this is how you will find maximum range for int
:
C:
#include <limits.h>
const int min_int = INT_MIN;
const int max_int = INT_MAX;
C++:
#include <limits>
const int min_int = std::numeric_limits<int>::min();
const int max_int = std::numeric_limits<int>::max();
Best Answer
This is because of padding added to satisfy alignment constraints. Data structure alignment impacts both performance and correctness of programs:
SIGBUS
).Here's an example using typical settings for an x86 processor (all used 32 and 64 bit modes):
One can minimize the size of structures by sorting members by alignment (sorting by size suffices for that in basic types) (like structure
Z
in the example above).IMPORTANT NOTE: Both the C and C++ standards state that structure alignment is implementation-defined. Therefore each compiler may choose to align data differently, resulting in different and incompatible data layouts. For this reason, when dealing with libraries that will be used by different compilers, it is important to understand how the compilers align data. Some compilers have command-line settings and/or special
#pragma
statements to change the structure alignment settings.