The strtol() function converts the initial portion of the string pointed to by str to a type long int representation.

The strtoll() function converts the initial portion of the string pointed to by str to a type long long representation.

Both functions first decompose the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by isspace(3C)); a subject sequence interpreted as an integer represented in some radix determined by the value of base; and a final string of one or more unrecognized characters, including the terminating null byte of the input string. They then attempt to convert the subject sequence to an integer and return the result.

If the value of base is 0, the expected form of the subject sequence is that of a decimal constant, octal constant or hexadecimal constant, any of which may be preceded by a + or - sign. A decimal constant begins with a non-zero digit, and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 to 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and letters a (or A) to f (or F) with values 10 to 15 respectively.

If the value of base is between 2 and 36, the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a + or - sign. The letters from a (or A) to z (or Z) inclusive are ascribed the values 10 to 35; only letters whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white-space characters, or if the first non-white-space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is 0, the sequence of characters starting with the first digit is interpreted as an integer constant. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated. A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the POSIX locale, additional implementation-dependent subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of str is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Except for behavior on error, atol() is equivalent to: strtol(str, (char **)NULL, 10).

Except for behavior on error, atoll() is equivalent to: strtoll(str, (char **)NULL, 10).

Except for behavior on error, atoi() is equivalent to: (int) strtol(str, (char **)NULL, 10).

The lltostr() function returns a pointer to the string represented by the long long value. The endptr argument is assumed to point to the byte following a storage area into which the decimal representation of value is to be placed as a string. The lltostr() function converts value to decimal and produces the string, and returns a pointer to the beginning of the string. No leading zeros are produced, and no terminating null is produced. The low-order digit of the result always occupies memory position endptr-1. The behavior of lltostr() is undefined if value is negative. A single zero digit is produced if value is 0.

The ulltostr() function is similar to lltostr() except that value is an unsigned long long.