Use of double underscore in C -

i studying c programming modern approach book. found question in it:

why not idea identifier contain more 1 adjacent underscore (as in current__balance example)?

can explain me why so?

identifiers begin 2 underscores or underscore , capital letter reserved c standard , should not used in own code, cf. iso 9899:2011 §7.1.3 ¶1 #1:

7.1.3 reserved identifiers

1 each header declares or defines identifiers listed in associated subclause, , optionally declares or defines identifiers listed in associated future library directions subclause , identifiers reserved either use or use file scope identifiers.

• all identifiers begin underscore , either uppercase letter or underscore reserved use.
• all identifiers begin underscore reserved use identifiers file scope in both ordinary , tag name spaces.
• each macro name in of following subclauses (including future library directions) reserved use specified if of associated headers included; unless explicitly stated otherwise (see 7.1.4).
• all identifiers external linkage in of following subclauses (including future library directions) , errno reserved use identifiers external linkage.¹⁸⁴⁾
• each identifier file scope listed in of following subclauses (including future library directions) reserved use macro name , identifier file scope in same name space if of associated headers included.

2 no other identifiers reserved. if program declares or defines identifier in context in reserved (other allowed 7.1.4), or defines reserved identifier macro name, behavior undefined.

3 if program removes (with #undef) macro definition of identifier in first group listed above, behavior undefined.

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184) the list of reserved identifiers external linkage includes math_errhandling, setjm, va_copy, , va_end.

for double underscores inside names: these hard tell apart single underscores in many typefaces , lead confusion. recommend avoid doing that.