The strtol function converts the initial portion of the string pointed to by nptr to long int representation. First it decomposes the input string into three parts: an initial, possibly empty, sequence of white space characters (as specified by the The isspace function The isspace function), a subject sequence resembling 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 character of the input string. Then it attempts to convert the subject sequence to an integer, and returns the result.
The strtol function returns the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, LONG_MAX or LONG_MIN is returned (according to the sign of the value), and the value of the macro ERANGE is is stored in errno.