c - Initializing floating point variable with large literal -

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#include <stdio.h>  int main(void) {     double x = 0.12345678901234567890123456789;     printf("%0.16f\n", x);     return 0; };

in code above i'm initializing x literal large represented ieee 754 double. on pc gcc 4.9.2 works well. literal rounded nearest value fits double. i'm wondering happens behind scene (on compiler level) in case? behaviour depend on platform? legal?

when write double x = 0.1;, decimal number have written rounded nearest double. happens when write 0.12345678901234567890123456789 not fundamentally different.

the behavior implementation-defined, compilers use nearest representable double in place of constant. c standard specifies has either double above or 1 below.


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