Question

To determine the endianness of a system, I plan to store a multi-byte integer value in a variable and access the first byte via an unsigned char wrapped in a union; for example:

union{
    unsigned int val;
    unsigned char first_byte;
} test;

test.val = 1; /* stored in little-endian system as "0x01 0x00 0x00 0x00" */

if(test.first_byte == 1){
    printf("Little-endian system!");
}else{
    printf("Big-endian system!");
}

I want to make this test portable across platforms, but I’m not sure if the C99 standard guarantees that the unsigned int data type will be greater than one byte in size. Furthermore, since a “C byte” does not technically have to be 8-bits in size, I cannot use exact width integer types (e.g. uint8_t, uint16_t, etc.).

Are there any C data types guaranteed by the C99 standard to be at least two bytes in size?

P.S. Assuming an unsigned int is in fact greater than one byte, would my union behave as I’m expecting (with the variable first_byte accessing the first byte in variable val) across all C99 compatible platforms?

Answer 1

Since int must have a range of at least 16 bits, int will meet your criterion on most practical systems. So would short (and long, and long long). If you want exactly 16 bits, you have to look to see whether int16_t and uint16_t are declared in <stdint.h>.

If you are worried about systems where CHAR_BIT is greater than 8, then you have to work harder. If CHAR_BIT is 32, then only long long is guaranteed to hold two characters.

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What the C standard says about sizes of integer types

In a comment, Richard J Ross III says:

The standard says absolutely nothing about the size of an int except that it must be larger than or equal to short, so, for example, it could be 10 bits on some systems I’ve worked on.

On the contrary, the C standard has specifications on the lower bounds on the ranges that must be supported by different types, and a system with 10-bit int would not be conformant C.

Specifically, in ISO/IEC 9899:2011 §5.2.4.2.1 Sizes of integer types <limits.h>, it says:

¶1 The values given below shall be replaced by constant expressions suitable for use in #if
preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the
following shall be replaced by expressions that have the same type as would an
expression that is an object of the corresponding type converted according to the integer
promotions. Their implementation-defined values shall be equal or greater in magnitude
(absolute value) to those shown, with the same sign.

— number of bits for smallest object that is not a bit-field (byte)
CHAR_BIT 8

[…]

— minimum value for an object of type short int

SHRT_MIN -32767 // −(2¹⁵ − 1)

— maximum value for an object of type short int

SHRT_MAX +32767 // 2¹⁵ − 1

— maximum value for an object of type unsigned short int

USHRT_MAX 65535 // 2¹⁶ − 1

— minimum value for an object of type int

INT_MIN -32767 // −(2¹⁵ − 1)

— maximum value for an object of type int

INT_MAX +32767 // 2¹⁵ − 1

— maximum value for an object of type unsigned int

UINT_MAX 65535 // 2¹⁶ − 1