Why are constants in C-header files of libraries always defined as hexadecimal?

Question

No matter which C-compatible library I use, when I look at the header defined constants, they are always defined as hexadecimal values. Here, for instance, in GL/gl.h:

#define GL_POINTS                               0x0000
#define GL_LINES                                0x0001
#define GL_LINE_LOOP                            0x0002
#define GL_LINE_STRIP                           0x0003
#define GL_TRIANGLES                            0x0004
#define GL_TRIANGLE_STRIP                       0x0005
#define GL_TRIANGLE_FAN                         0x0006
#define GL_QUADS                                0x0007
#define GL_QUAD_STRIP                           0x0008
#define GL_POLYGON                              0x0009

Is there any particular reason for this convention, why not simply use decimal values instead?

Answer 1

There are a number of possible reasons:

1) Bit flags are much easier to express as hex, since each hex digit represents exactly 4 bits.

2) Even for values which aren't explicitly bit flags, there are often intentional bit patterns that are more evident when written as hex.

For instance, all the AlphaFunctions start with 0x02 and differ in only a single byte:

#define GL_NEVER                          0x0200
#define GL_LESS                           0x0201
#define GL_EQUAL                          0x0202
#define GL_LEQUAL                         0x0203
#define GL_GREATER                        0x0204
#define GL_NOTEQUAL                       0x0205
#define GL_GEQUAL                         0x0206
#define GL_ALWAYS                         0x0207

3) Hex values are allowed to have leading zeroes, so it is easier to line up the values. This can make reading (and proof-reading) easier. You might be surprised that leading zeroes are allowed in hex and octal literals but not decimal, but the C++ spec says quite explicitly

A decimal integer literal (base ten) begins with a digit other than 0 and consists of a sequence of decimal digits.