long int atol(const char *str);

The string pointed to by the argument str is converted to a long integer (type long int). Any initial whitespace characters are skipped (space, tab, carriage return, new line, vertical tab, or formfeed). The number may consist of an optional sign and a string of digits. Conversion stops when the first unrecognized character is reached.

On success the converted number is returned. If the number cannot be converted, then 0 is returned.