atof / atoi / atol / atoll / strtod / strtof / strtold (3) - glibc

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

Declaration and gates

#include <stdlib.h>

double atof(const char *nptr);
int    atoi(const char *nptr);
long   atol(const char *nptr);
long long atoll(const char *nptr);   /* _ISOC99_SOURCE */

double strtod(const char *restrict nptr, char **restrict endptr);
float  strtof(const char *restrict nptr, char **restrict endptr);
long double strtold(const char *restrict nptr, char **restrict endptr);

strtof/strtold are gated __USE_ISOC99 in the installed header (/usr/include/stdlib.h lines 118-131); glibc additionally ships GNU *_l twins (strtod_l, strtof_l, strtold_l, plus C23 float twins) under __USE_GNU (lines 415-475).

Semantics

The ato* functions are documented wrappers with no error reporting: atoi(nptr) behaves exactly like (int) strtol(nptr, NULL, 10), atol like strtol, atoll like strtoll, and atof like strtod(nptr, NULL) (atoi(3), atof(3); the same expansions appear as extern-inline definitions in /usr/include/stdlib.h lines 479-498). The strto* family parses optional whitespace, sign, then a decimal number (digits with optional locale radix point and e exponent), a hexadecimal number (0x...p+/-exp), INF/INFINITY, or NAN optionally followed by an n-char-sequence (strtod(3)). On success the value is returned and *endptr points past the converted text; with no conversion, 0 is returned and *endptr is set to nptr. Overflow returns +/-HUGE_VAL/HUGE_VALF/HUGE_VALL and sets errno = ERANGE; underflow returns a value no larger in magnitude than DBL_MIN/FLT_MIN/LDBL_MIN with ERANGE (strtod(3) RETURN VALUE/ERRORS).

Gotchas and portability

• The ato* family cannot report failure: atoi(3) BUGS lists no errno, no overflow/underflow check, base-10 only, and 0 returned for garbage; atoi("123abc") is 123, atoi("") is 0, indistinguishable from real zeros. New code should use strtol/strtod with the errno=0 discipline.
• errno=0 discipline applies to strtod too, because 0 is a legitimate result on both success and failure (strtod(3) NOTES).
• NAN payloads: glibc interprets the NAN(n-char-sequence) payload as an integer (decimal, or 0/0x prefixed) placed into the mantissa (strtod(3) NOTES).
• Locale: the radix character is locale-dependent, and MT-Safe locale marking follows from that (strtod(3) ATTRIBUTES); in non-C locales strtod may accept additional implementation-defined forms.
• musl 1.2.5 also returns 0 on error for atoi and ships the same wrapper semantics (atoi(3) NOTES names glibc, musl and uClibc).

References

• glibc manual: arith.texi, subsection Parsing of Floats (and Parsing of Integers for the ato* wrappers)
• man3: atoi(3) (covers atol, atoll), atof(3), strtod(3) (covers strtof, strtold)
• header: /usr/include/stdlib.h
• symbols: nm -D --with-symbol-versions libc.so.6