Why are C character literals ints instead of chars?

In C++, sizeof('a') == sizeof(char) == 1. This makes intuitive sense, since 'a' is a character literal, and sizeof(char) == 1 as defined by the standard.

In C however, sizeof('a') == sizeof(int). That is, it appears that C character literals are actually integers. Does anyone know why? I can find plenty of mentions of this C quirk but no explanation for why it exists.

discussion on same subject

> "More specifically the integral promotions. In K&R C it was virtually (?) impossible to use a character value without it being promoted to int first, so making character constant int in the first place eliminated that step. There were and still are multi character constants such as 'abcd' or however many will fit in an int."

The original question is "why?"

The reason is that the definition of a literal character has evolved and changed, while trying to remain backwards compatible with existing code.

In the dark days of early C there were no types at all. By the time I first learnt to program in C, types had been introduced, but functions didn't have prototypes to tell the caller what the argument types were. Instead it was standardised that everything passed as a parameter would either be the size of an int (this included all pointers) or it would be a double.

This meant that when you were writing the function, all the parameters that weren't double were stored on the stack as ints, no matter how you declared them, and the compiler put code in the function to handle this for you.

This made things somewhat inconsistent, so when K&R wrote their famous book, they put in the rule that a character literal would always be promoted to an int in any expression, not just a function parameter.

When the ANSI committee first standardised C, they changed this rule so that a character literal would simply be an int, since this seemed a simpler way of achieving the same thing.

When C++ was being designed, all functions were required to have full prototypes (this is still not required in C, although it is universally accepted as good practice). Because of this, it was decided that a character literal could be stored in a char. The advantage of this in C++ is that a function with a char parameter and a function with an int parameter have different signatures. This advantage is not the case in C.

This is why they are different. Evolution...

I don't know the specific reasons why a character literal in C is of type int. But in C++, there is a good reason not to go that way. Consider this:

void print(int);
void print(char);

print('a');

You would expect that the call to print selects the second version taking a char. Having a character literal being an int would make that impossible. Note that in C++ literals having more than one character still have type int, although their value is implementation defined. So, 'ab' has type int, while 'a' has type char.

using gcc on my MacBook, I try:

#include <stdio.h>
#define test(A) do{printf(#A":\t%i\n",sizeof(A));}while(0)
int main(void){
  test('a');
  test("a");
  test("");
  test(char);
  test(short);
  test(int);
  test(long);
  test((char)0x0);
  test((short)0x0);
  test((int)0x0);
  test((long)0x0);
  return 0;
};

which when run gives:

'a':    4
"a":    2
"":     1
char:   1
short:  2
int:    4
long:   4
(char)0x0:      1
(short)0x0:     2
(int)0x0:       4
(long)0x0:      4

which suggests that a character is 8 bits, like you suspect, but a character literal is an int.

Back when C was being written, the PDP-11's MACRO-11 assembly language had:

MOV #'A, R0      // 8-bit character encoding for 'A' into 16 bit register

This kind of thing's quite common in assembly language - the low 8 bits will hold the character code, other bits cleared to 0. PDP-11 even had:

MOV #"AB, R0     // 16-bit character encoding for 'A' (low byte) and 'B'

This provided a convenient way to load two characters into the low and high bytes of the 16 bit register. You might then write those elsewhere, updating some textual data or screen memory.

So, the idea of characters being promoted to register size is quite normal and desirable. But, let's say you need to get 'A' into a register not as part of the hard-coded opcode, but from somewhere in main memory containing:

address: value
20: 'X'
21: 'A'
22: 'A'
23: 'X'
24: 0
25: 'A'
26: 'A'
27: 0
28: 'A'

If you want to read just an 'A' from this main memory into a register, which one would you read?

• Some CPUs may only directly support reading a 16 bit value into a 16 bit register, which would mean a read at 20 or 22 would then require the bits from 'X' be cleared out, and depending on the endianness of the CPU one or other would need shifting into the low order byte.

• Some CPUs may require a memory-aligned read, which means that the lowest address involved must be a multiple of the data size: you might be able to read from addresses 24 and 25, but not 27 and 28.

So, a compiler generating code to get an 'A' into the register may prefer to waste a little extra memory and encode the value as 0 'A' or 'A' 0 - depending on endianness, and also ensuring it is aligned properly (i.e. not at an odd memory address).

My guess is that C's simply carried this level of CPU-centric behaviour over, thinking of character constants occupying register sizes of memory, bearing out the common assessment of C as a "high level assembler".

(See 6.3.3 on page 6-25 of http://www.dmv.net/dec/pdf/macro.pdf)

I remember reading K&R and seeing a code snippet that would read a character at a time until it hit EOF. Since all characters are valid characters to be in a file/input stream, this means that EOF cannot be any char value. What the code did was to put the read character into an int, then test for EOF, then convert to a char if it wasn't.

I realize this doesn't exactly answer your question, but it would make some sense for the rest of the character literals to be sizeof(int) if the EOF literal was.

int r;
char buffer[1024], *p; // don't use in production - buffer overflow likely
p = buffer;

while ((r = getc(file)) != EOF)
{
  *(p++) = (char) r;
}

I haven't seen a rationale for it (C char literals being int types), but here's something Stroustrup had to say about it (from Design and Evolution 11.2.1 - Fine-Grain Resolution):

> In C, the type of a character literal such as 'a' is int. > Surprisingly, giving 'a' type char in C++ doesn't cause any compatibility problems. > Except for the pathological example sizeof('a'), every construct that can be expressed > in both C and C++ gives the same result.

So for the most part, it should cause no problems.

The historical reason for this is that C, and its predecessor B, were originally developed on various models of DEC PDP minicomputers with various word sizes, which supported 8-bit ASCII but could only perform arithmetic on registers. (Not the PDP-11, however; that came later.) Early versions of C defined int to be the native word size of the machine, and any value smaller than an int needed to be widened to int in order to be passed to or from a function, or used in a bitwise, logical or arithmetic expression, because that was how the underlying hardware worked.

That is also why the integer promotion rules still say that any data type smaller than an int is promoted to int. C implementations are also allowed to use one’s-complement math instead of two’s-complement for similar historical reasons. The reason that octal character escapes and octal constants are first-class citizens compared to hex is likewise that those early DEC minicomputers had word sizes divisible into three-byte chunks but not four-byte nibbles.

This is the correct behavior, called "integral promotion". It can happen in other cases too (mainly binary operators, if I remember correctly).

EDIT: Just to be sure, I checked my copy of Expert C Programming: Deep Secrets, and I confirmed that a char literal does not start with a type int. It is initially of type char but when it is used in an expression, it is promoted to an int. The following is quoted from the book:

> Character literals have type int and > they get there by following the rules > for promotion from type char. This is > too briefly covered in K&R 1, on page > 39 where it says: > > Every char in an expression is > converted into an int....Notice that > all float's in an expression are > converted to double....Since a > function argument is an expression, > type conversions also take place when > arguments are passed to functions: in > particular, char and short become int, > float becomes double.

I don't know, but I'm going to guess it was easier to implement it that way and it didn't really matter. It wasn't until C++ when the type could determine which function would get called that it needed to be fixed.

I didn't know this indeed. Before prototypes existed, anything narrower than an int was converted to an int when using it as a function argument. That may be part of the explanation.

This is only tangential to the language spec, but in hardware the CPU usually only has one register size -- 32 bits, let's say -- and so whenever it actually works on a char (by adding, subtracting, or comparing it) there is an implicit conversion to int when it is loaded into the register. The compiler takes care of properly masking and shifting the number after each operation so that if you add, say, 2 to (unsigned char) 254, it'll wrap around to 0 instead of 256, but inside the silicon it is really an int until you save it back to memory.

It's sort of an academic point because the language could have specified an 8-bit literal type anyway, but in this case the language spec happens to reflect more closely what the CPU is really doing.

(x86 wonks may note that there is eg a native addh op that adds the short-wide registers in one step, but inside the RISC core this translates to two steps: add the numbers, then extend sign, like an add/extsh pair on the PowerPC)