daddl / dsubl / dmull / ddivl / dsqrtl / dfmal (3) - glibc

Split from the chapter glibc: arithmetic functions into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <math.h>

double daddl(long double x, long double y);
double dsubl(long double x, long double y);
double dmull(long double x, long double y);
double ddivl(long double x, long double y);
double dsqrtl(long double x);
double dfmal(long double x, long double y, long double z);

The d##name##l block sits in the same #if __GLIBC_USE (IEC_60559_BFP_EXT_C23) region of /usr/include/math.h: _Mret_ = double, _Marg_ = long double, __MATHCALL_NAME(name) = d ## name ## l (lines 569-585), drawing the six prototypes from bits/mathcalls-narrow.h lines 23-39. The gate is set by _GNU_SOURCE, __STDC_WANT_IEC_60559_BFP_EXT__ or C23 mode (bits/libc-header-start.h lines 67-77). See fadd family for the float-returning counterparts.

Semantics

Per the manual (node Misc FP Arithmetic, arith.texi lines 2318-2432): daddl returns x + y rounded once to the return type of the function without any intermediate rounding to the type of the arguments; the same single-rounding contract holds for dsubl (x - y), dmull (x * y), ddivl (x / y), dsqrtl (sqrt(x)) and dfmal ((x * y) + z, with the product itself unrounded). Here the wide computation is done in long double and the single rounding lands in double – the mirror image of the f...l names, which round into float.

Gotchas and portability

• The names read as “double result, long double arguments”: prefix d selects the return type, trailing l the argument type.
• dfmal is the narrow fused multiply-add; the plain fma family (ISO C99, same argument and result type) and the TS 18661-3 fMfmafN group are different ledger rows and different symbols.
• tgmath.h exposes generic narrowing macros under the same gate (/usr/include/tgmath.h lines 1123-1127).
• No man page exists for these names in the cached man-pages set.
• musl 1.2.5: absent from musl include/math.h (verified, 0 matches).

References

• glibc manual: arith.texi, node Misc FP Arithmetic (lines 2091-2432)
• headers: /usr/include/math.h, bits/mathcalls-narrow.h, bits/libc-header-start.h
• symbols: nm -D --with-symbol-versions libm.so.6