strtol / strtoul / strtoll / strtoull (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>

long strtol(const char *restrict nptr, char **restrict endptr, int base);
unsigned long strtoul(const char *restrict nptr, char **restrict endptr, int base);
long long strtoll(const char *restrict nptr, char **restrict endptr, int base);
unsigned long long strtoull(const char *restrict nptr, char **restrict endptr, int base);

strtoll/strtoull are gated __USE_ISOC99 in the installed header (/usr/include/stdlib.h lines 201-206). When C23 features are enabled, glibc redirects the four names to __isoc23_strtol and friends, which additionally accept 0b/0B binary literals (header lines 215-272; manual arith.texi Parsing of Integers: “only supported when C23 features are enabled”). strtouq aliases strtoull under the same gate family.

Semantics

Each function skips whitespace (per isspace), takes an optional sign, and parses digits in base: 0 means auto-detect (0x -> 16, leading 0 -> 8, else 10), otherwise 2..36, with 0x allowed when base is 16 (strtol(3), manual Parsing of Integers). On success the value is returned and, if endptr is non-NULL, *endptr points at the first unconverted character. If no digits were seen, *endptr is set to the original nptr and 0 returned. On overflow the clamped extreme (LONG_MAX/LONG_MIN, ULONG_MAX) is returned and errno is set to ERANGE; an out-of-range base yields EINVAL (“not in C99”, strtol(3) ERRORS).

Gotchas and portability

• The errno=0 discipline: valid inputs can return 0, LONG_MAX or LONG_MIN, so callers must set errno = 0 before the call and test it afterwards, and check endptr == nptr for “no digits” (strtol(3) NOTES; manual Parsing of Integers states the same three checks).
• strtoul accepts a leading - and negates: strtoul("-1", ...) is ULONG_MAX, and a value more negative than LONG_MIN yields (ULONG_MAX + 1) / 2 on 64-bit (strtol(3)-family manual, strtoul entry; strtoul(3) RETURN VALUE).
• Locale: outside “C”/“POSIX” the functions may accept implementation-defined additional numeric forms (strtol(3) NOTES); MT-Safe locale marking covers that dependence.
• EINVAL for bad base is glibc/BSD behavior, not C99; C99 leaves the behavior undefined for base outside 0..36 (strtol(3) ERRORS).
• The full-check idiom from strtol(3) EXAMPLES: errno = 0; val = strtol(str, &endptr, base); then test errno, endptr == str, and finally *endptr == '\0' for whole-string parses.
• musl 1.2.5 has the same surface and errno contract (musl src/stdlib/strtol.c family); it has no C23 redirect layer.

References

• glibc manual: arith.texi, subsection Parsing of Integers
• man3: strtol(3) (covers strtoll, strtoq), strtoul(3) (covers strtoull, strtouq)
• header: /usr/include/stdlib.h
• symbols: nm -D --with-symbol-versions libc.so.6