strfromd / strfromf / strfromfN / strfromfNx / strfroml (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>

int strfromd(char *restrict str, size_t n,
             const char *restrict format, double fp);
int strfromf(char *restrict str, size_t n,
             const char *restrict format, float fp);
int strfroml(char *restrict str, size_t n,
             const char *restrict format, long double fp);

strfromd/strfromf/strfroml are inside #if __GLIBC_USE (IEC_60559_BFP_EXT_C23) at /usr/include/stdlib.h lines 277-290 (gate comment line 276). The _FloatN/_FloatNx forms (strfromf16/f32/f64/f128 and strfromf32x/f64x/f128x, lines 291-331) need __GLIBC_USE (IEC_60559_TYPES_EXT) plus the __HAVE_FLOAT* macros. The BFP gate requires __STDC_WANT_IEC_60559_BFP_EXT__, _GNU_SOURCE or C23 mode (bits/libc-header-start.h lines 67-77); the man page synopsis lists __STDC_WANT_IEC_60559_BFP_EXT__.

Semantics

Equivalent to snprintf(str, n, format, fp) except for the restricted format string (strfromd(3) DESCRIPTION): format must start with %, optionally followed by a precision .N (precision 0 if the period is bare), then exactly one of a A e E f F g G. No length modifier and no * are allowed; a malformed format is undefined behavior. The conversion is typed by the function suffix, so no length modifier is needed. Returns the number of characters that would have been written excluding the terminating null; the null is written if and only if the return is less than n (strfromd(3) RETURN VALUE). NaN and infinity print as nan/inf (or NAN/INF with uppercase specifiers). The manual adds the same contract for strfromfN/strfromfNx (arith.texi lines 3176-3182).

Gotchas and portability

• A return value of n or greater means the output was truncated (strfromd(3) RETURN VALUE); the man page defines no negative error return.
• Respects LC_NUMERIC for the decimal point (strfromd(3) NOTES); glibc 2.25 and later (strfromd(3) VERSIONS).
• strfromfN/strfromfNx are family notation; exported names are strfromf32, strfromf64, strfromf32x, strfromf64x, strfromf128.
• musl 1.2.5: no strfrom* in musl include/ or musl src/ (verified absent).

References

• glibc manual: arith.texi, node Printing of Floats (lines 3132-3182)
• man3: strfromd(3) (covers strfromf, strfroml)
• headers: /usr/include/stdlib.h
• symbols: nm -D --with-symbol-versions libc.so.6