qecvt / qecvt_r / qfcvt / qfcvt_r / qgcvt (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 <stdlib.h>

char *qecvt(long double value, int ndigit, int *restrict decpt,
            int *restrict sign);
char *qfcvt(long double value, int ndigit, int *restrict decpt,
            int *restrict sign);
char *qgcvt(long double value, int ndigit, char *buf);
int   qecvt_r(long double value, int ndigit, int *restrict decpt,
              int *restrict sign, char *restrict buf, size_t len);
int   qfcvt_r(long double value, int ndigit, int *restrict decpt,
              int *restrict sign, char *restrict buf, size_t len);

qecvt/qfcvt/qgcvt and all _r forms share the #ifdef __USE_MISC block of /usr/include/stdlib.h (gate at line 1029; qecvt 1030, qfcvt 1033, qgcvt 1036, qecvt_r 1049, qfcvt_r 1053). Feature macros per qecvt(3): _DEFAULT_SOURCE since glibc 2.19, _SVID_SOURCE before. The long double twins of ecvt family; the historical q referred to quad-precision.

Semantics

Each function is equivalent to its double counterpart except that the first parameter is long double, and ndigit is restricted by the precision of a long double rather than a double (manual, arith.texi lines 3253-3292). qecvt produces significant-digit strings with *decpt/*sign side effects and a static return buffer; qfcvt counts digits after the decimal point; qgcvt is the "%*g"-style conversion into a caller buffer, returning buf. The _r forms write into a user buffer of length len and return -1 on error, 0 otherwise (ecvt_r(3), which documents all four _r names).

Gotchas and portability

• The static-buffer hazard is identical to ecvt/fcvt: consecutive calls from different threads or interleaved calls overwrite the result; use the _r forms under an external buffer.
• These names are pure glibc AT&T lineage – POSIX removed even the double versions in 2008, so nothing here is portable.
• On x86-64 long double is 80-bit extended; the digit limit differs from the double trio.
• musl 1.2.5: none of the five names is declared (musl include/stdlib.h has no qecvt/qfcvt/qgcvt/ecvt_r matches – verified absent).

References

• glibc manual: arith.texi, node Old-fashioned System V number-to-string functions (lines 3183-3338)
• man3: qecvt(3) (covers qfcvt, qgcvt), ecvt_r(3) (covers qecvt_r, qfcvt_r)
• headers: /usr/include/stdlib.h
• symbols: nm -D --with-symbol-versions libc.so.6