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What is the type of a constant in C?

开发技术 开发技术 2022-11-17 次浏览

https://jameshfisher.com/2017/01/23/c-type-of-constants/#:~:text=The%20C%20Programming%20Language%20says%3A%20An%20integer%20constant,the%20suffix%20ul%20or%20UL%20indicates%20unsigned%20long.

 

When we write expressions like this in C:

bool b = 1234567890 > 09876;

What are the types of those constants? The number 1234567890 - what is its type? How does C represent it when compiling it? The C Programming Language says:

An integer constant like 1234 is an int. A long constant is written with a terminal l (ell) or L, as in 123456789L; an integer constant too big to fit into an int will also be taken as a long. Unsigned constants are written with a terminal u or U, and the suffix ul or UL indicates unsigned long.

Floating-point constants contain a decimal point (123.4) or an exponent (1e-2) or both; their type is double, unless suffixed. The suffixes f or F indicate a float constant; l or L indicate a long double.

Here are some examples:

0                       // int
0l                      // long
1234                    // int
1234L                   // long
0ul                     // unsigned long
0u                      // unsigned int
2147483647              // int (just)
2147483648              // long
2147483647u             // unsigned int
2147483648u             // unsigned long (but could have fitted into an unsigned int)
0x0101010101010101ULL   // unsigned long long


Basic types

Here's a table containing commonly used types in C programming for quick access.

TypeSize (bytes)Format Specifier
int at least 2, usually 4 %d, %i
char 1 %c
float 4 %f
double 8 %lf
short int 2 usually %hd
unsigned int at least 2, usually 4 %u
long int at least 4, usually 8 %ld, %li
long long int at least 8 %lld, %lli
unsigned long int at least 4 %lu
unsigned long long int at least 8 %llu
signed char 1 %c
unsigned char 1 %c
long double at least 10, usually 12 or 16 %Lf

 

1 << 31 and related issues

There are a few cases in FreeBSD where the expression (1 << 31) is used.

However this produces undefined behavior as '1' is of type int. (see
6.4.4p5: The type of an unmarked integer constant is the first of the
following list in which its value can be represented: int, long int,
long long int). The shift of 31 is illegal (see 6.5.7p4) if the size
of an int is 32. The same issue arises with 2 << 30 and 3 << 30.

I have been working on fixing this issue in a few different projects.
The correct fix here is to use 1U as the literal instead. adrian@ has
advocated for the macros BIT32(bit) and BIT64(bit) which I would also
support.

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