Here's a direct quote from the Book (K&R, 2nd ed, p. 35):
"At least the first 31 characters of an internal name are significant. For function names and external variables, the number may be less than 31, because external names may be used by assemblers and loaders over which the language has no control. For external names, the standard guarantees only for 6 characters and a single case."
And in C99 there is no length limitation on its internal names, but only the first 63 are guaranteed to be significant (§5.2.4.1 Translation Limits).
My question is why are these limits specifically 31 or 63? Why this number specifically? Why not 19, 24 or any other number? If it's an implementation issue, is there a benefit from making it 31 or 63?
The compiler/linker writers associated with a language design committee usually impose some limits so that they can make assumptions in the implementation of the toolchain or in the accompanying binary file formats (e.g., ELF, COFF, etc). The numbers 31 and 63 were probably chosen simply because they are 2n-1 and programmers like 2n for some silly reason. The -1 is usually to account for either a leading _ or a trailing NUL character in the name.
来源：https://stackoverflow.com/questions/10141422/at-least-the-first-31-or-63-characters-of-an-internal-name-are-significant