#include <stdlib.h>
unsigned long strtoul(const char *s, char **ep, int b);
unsigned long long strtoull(const char *s, char **ep, int b);
long strtol(const char *s, char **ep, int b);
long long strtoll(const char *s, char **ep, int b);

#include <inttypes.h>
uintmax_t strtoumax(const char *s, char **ep, int b);
intmax_t strtoimax(const char *s, char **ep, int b);

#include <wchar.h>
unsigned long wcstoul(const wchar_t *s, wchar_t **ep, int b);
unsigned long long wcstoull(const wchar_t *s, wchar_t **ep, int b);
long wcstol(const wchar_t *s, wchar_t **ep, int b);
long long wcstoll(const wchar_t *s, wchar_t **ep, int b);

#include <inttypes.h>
uintmax_t wcstoumax(const wchar_t *s, wchar_t **ep, int b);
intmax_t wcstoimax(const wchar_t *s, wchar_t **ep, int b);

These functions parse an integer in base b. After skipping initial white space, they accept an optional sign + or -, an optional type prefix 0x or 0X if b is 16, and then one or more digits from the first b characters of the following list, or their upper-case equivalents:

0123456789abcdefghijklmnopqrstuvwxyz

b must be in the range 2 to 36, or the special value 0. The special value indicates that the base is to be chosen by the type prefix. If it is 0x or 0X, the base is taken to be 16. If it is 0, the base is taken to be 8. Otherwise, the base is taken to be 10.

The functions return the converted number if it has the expected form. If not, they return zero, and *ep is set to s if ep is not NULL. The *tou* functions convert the result to an unsigned value first. If the result overflows, each function sets errno to ERANGE, and returns the largest value of its return type, LONG_MAX, ULONG_MAX, LLONG_MAX or ULLONG_MAX.

#include <stdlib.h>
int atoi(const char *s);
long atol(const char *s);
long long atoll(const char *s);

atoi(s) is equivalent to (int) strtol(s, NULL, 10).

atol(s) is equivalent to strtol(s, NULL, 10).

atoll(s) is equivalent to strtoll(s, NULL, 10).

#include <stdlib.h>
float strtof(const char *s, char **ep);
double strtod(const char *s, char **ep);
long double strtold(const char *s, char **ep);

#include <wchar.h>
float wcstof(const wchar_t *s, wchar_t **ep);
double wcstod(const wchar_t *s, wchar_t **ep);
long double wcstold(const wchar_t *s, wchar_t **ep);

These function parse a real number in decimal or hexadecimal. After skipping initial white space, they accept an optional sign + or -, an optional type prefix 0x or 0X indicating that the format is hexadecimal, at least one digit of the right base, with at most one decimal point, and an optional exponent. The exponent begins with e or E for decimal, and p or P for hexadecimal. It is followed by an optional sign + or -, and at least one decimal digit, even for hexadecimal numbers.

The functions return the converted number if it has the expected form. If not, they return zero, and *ep is set to s if ep is not NULL. If the result overflows, each function sets errno to ERANGE, and returns one of HUGE_VALF, HUGE_VAL or HUGE_VALL according to its return type. If the result underflows, errno is set to ERANGE, and a very small value is returned, but with the correct sign.

These functions also accept INF or INFINITY as a representation of infinity, and return INFINITY if the type supports it, or behave as if the value overflows. They also accept NAN and NAN(n-char-sequence_opt) as a quiet NaN, if supported. The enclosed character sequence must consist only of the following characters:

0 1 2 3 4 5 6 7 8 9 _
a b c d e f g h i j k l m
n o p w r s t u v w x y z
A B C D E F G H I J K L M
N O P W R S T U V W X Y Z

n-char-sequence

n-char

n-char-sequence n-char

n-char

any of the characters 0–9

any of the characters A–Z

any of the characters a–z

_

underscore

#include <stdlib.h>
double atof(const char *s, char **ep);

atof(s) is equivalent to strtod(s, NULL, 10).

#include <math.h>
float nanf(const char *str);
double nan(const char *str);
long double nanl(const char *str);

These functions take a string and treat it as a parameter for generating a quiet NaN, if supported. A valid string consists only of characters from n-char-sequence.

Passing an empty string "" indicates that the parameter is empty. nanf("") is equivalent to strtof("NAN()", NULL). nan("") is equivalent to strtod("NAN()", NULL). nanl("") is equivalent to strtold("NAN()", NULL). Passing an invalid string indicates that no parameter shall be used to generate the NaN.