GCC Quad-Precision Math Library example

There is small PDF on GCC website with libquadmath documentation. It contains explanation and example how to convert string to __float128 and how to convert __float128 back to string. It took me some googling to find out how to compile example and to figure out what is width parameter. To compile it we need -lquadmath linker switch and width, fourth argument to quadmath_snprintf, is overall width of output, right anchored. Here is my version of mentioned example:

#include <quadmath.h> #include <stdlib.h> #include <stdio.h> int main(){ __float128 pi = strtoflt128 ("3.1415926535897932384626433832795029", NULL); __float128 dpi = 3.1415926535897932384626433832795; __float128 qpi = 3.1415926535897932384626433832795029Q; char buf[128]; int n = quadmath_snprintf (buf, sizeof buf, "%*.34Qf",10, pi); if ((size_t) n < sizeof buf) printf ("%s\n", buf); n = quadmath_snprintf (buf, sizeof buf, "%*.34Qf", 40, dpi); if ((size_t) n < sizeof buf) printf ("%s\n", buf); n = quadmath_snprintf (buf, sizeof buf, "%*.34Qf", 45, qpi); if ((size_t) n < sizeof buf) printf ("%s\n", buf); return 0; }

If we try to assign number without Q suffix then we will have ordinary double inside quad storage. The fourth argument to quadmath_snprintf will be ignored if it is smaller than length of string representation, no spaces will be added in front of number.
If we need to examine layout in memory, we can use modified example from last article. We do not need here -lquadmath linker switch to compile.

#include <stdio.h> #include <quadmath.h> #include <stdlib.h> typedef struct quad_storage { unsigned long mant_low:64; unsigned long mant_high:48; unsigned int exponent:15; unsigned int sign:1; } Quad; typedef union quad_helper { __float128 number; Quad storage; } Helper; int main(){ Helper quad = {3.1415926535897932384626433832795029Q}; printf("sign = %u, exponent = %u, mantissa_h = %lu, mantissa_l = %lu\n", quad.storage.sign, quad.storage.exponent, quad.storage.mant_high, quad.storage.mant_low); printf("sign = %X, exponent = %X, mantissa_h = %lX, mantissa_l = %lX\n", quad.storage.sign, quad.storage.exponent, quad.storage.mant_high, quad.storage.mant_low); return 0; }

Interesting part from GDB session:

(gdb) print &pi $1 = (__float128 *) 0x7fffffffe650 (gdb) x/4 0x7fffffffe650 0x7fffffffe650: 0xc51701b8 0x8469898c 0xb54442d1 0x4000921f

We compare it with output of program to see what we are looking at.
Examining tests from GCC source, we find out that all operators which are supported for other floats are supported on __float128, so no funny function calls to add or compare two numbers. Comfort of normal syntax and 34 significant digits. For example we do square root round trip:

dpi = 2.0; qpi = sqrtq(dpi); n = quadmath_snprintf (buf, sizeof buf, "%*.34Qe", 45, dpi-qpi*qpi); if ((size_t) n < sizeof buf) printf ("%s\n", buf);

That code produced -3.8518598887744717061119558851698546e-34 error on my box.