ecvt(3)

✨ ecvt(3) — Library Functions Manual ✨

okay bestie, gather round, because today we're talking about ecvt and fcvt — two little float-to-string converters from the Standard C library (libc, link with -lc) that are, and I cannot stress this enough, deprecated 💀. They still exist, they still work, but the vibes are very "your cool aunt who still uses LimeWire." Let's get into it 🫶

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🌸 NAME 🌸

ecvt, fcvt — convert a floating-point number to a string 💅

📚 LIBRARY 📚

Standard C library (libc, -lc) — the classic, the icon, the one you link against without even thinking about it.

🎀 SYNOPSIS 🎀

#include <stdlib.h>

[[deprecated]] char *ecvt(double number, int ndigits,
                          int *restrict decpt, int *restrict sign);
[[deprecated]] char *fcvt(double number, int ndigits,
                          int *restrict decpt, int *restrict sign);

not the [[deprecated]] attribute just sitting right there in the signature 😭 the compiler is literally telling you to your face. anyway.

🦋 Feature Test Macro Requirements for glibc 🦋

(peep feature_test_macros(7) if you're like "what is a feature test macro bestie")

ecvt(), fcvt():

• Since glibc 2.17:

(_XOPEN_SOURCE >= 500 && ! (_POSIX_C_SOURCE >= 200809L))
    || /* glibc >= 2.20 */ _DEFAULT_SOURCE
    || /* glibc <= 2.19 */ _SVID_SOURCE

• glibc 2.12 to glibc 2.16:

(_XOPEN_SOURCE >= 500 && ! (_POSIX_C_SOURCE >= 200112L))
    || _SVID_SOURCE

• Before glibc 2.12:

_SVID_SOURCE || _XOPEN_SOURCE >= 500

copy that exactly, do not freestyle it, it's giving very specific 💀

💖 DESCRIPTION 💖

so ecvt() takes your number and turns it into a null-terminated string of ndigits digits — except, plot twist, ndigits gets reduced down to a system-specific limit that's decided by the precision of a double. and then it hands you back a pointer to that string. ✨

some deets you need to know, no cap:

• the high-order digit is nonzero, unless number is zero (in which case, fair enough).
• the low order digit is rounded 🔥
• the string does not contain a decimal point — like, at all. the decimal point is simply not there.
• instead, where the decimal point would be, relative to the start of the string, gets stashed in *decpt.
• if *decpt is negative, that means the decimal point is to the left of the start of the string (so like, very smol number energy).
• if the sign of number is negative, *sign gets set to a nonzero value. otherwise it's set to 0.
• if number is zero, it is unspecified whether *decpt is 0 or 1. unspecified bestie. don't count on it. 🙈

as for fcvt()? it's identical to ecvt(), except ndigits means the number of digits after the decimal point. that's the whole difference. that's it. slay.

👑 RETURN VALUE 👑

both ecvt() and fcvt() return a pointer to a static string containing the ASCII representation of number.

and here's the part where you go 😭: the static string is overwritten by each call to ecvt() or fcvt(). like, every single call. it's the same string. it's always the same string. copy it out before you call again, bestie, i am begging.

🧚 ATTRIBUTES 🧚

for the deets on what all this terminology means, go read attributes(7) — that's the source, i'm just the messenger 🫶

• ecvt() — Thread safety: MT-Unsafe race:ecvt
• fcvt() — Thread safety: MT-Unsafe race:fcvt

translation: these are not thread safe, and it's not even a subtle kind of not-thread-safe. it's a whole race. if two threads call these at the same time and you expected sensible output, that's on you, not on them 💀

🌈 STANDARDS 🌈

None. that's it. that's the whole section. none. zero. nothing. she's unstandardized, she's free, she's living her truth 🦋

⭐ HISTORY ⭐

• SVr2 was where she was born 🎀
• marked as LEGACY in POSIX.1-2001
• POSIX.1-2008 straight up removes the specifications of ecvt() and fcvt(), and instead recommends you use sprintf(3) (though snprintf(3) may be preferable — and honestly yeah, use the bounds-checked one, we've all been there) 🙈

💜 NOTES 💜

not all locales use a point as the radix character ("decimal point") 🌍 — so don't assume it's always gonna be a . out there in the wild. check your locale. (that's setlocale(3), see below 👇)

🔥 SEE ALSO 🔥

• ecvt_r(3)
• gcvt(3)
• qecvt(3)
• setlocale(3)
• sprintf(3)

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and that's the whole page, bestie 💅 this was Linux man-pages 6.20, dated 2026-02-08 — go forth, convert your floats, but maybe use snprintf(3) instead, i'm just saying 🫶✨