int atoi (const char *string)
string is the string, represented by an array of characters. When atoi encounters a string with no numerical sequence, it returns a 0 (zero). Also, if the string holds a valid sequence of digits that represents the number 0, it would also return a 0, making it impossible to tell from the return value alone whether the string held a valid number or not. The newer function strtol does not have this deficiency.
Variants of the atoi function, atol, atof, and atoll (the latter formerly known as atoq), are used to convert a string into a long, float, or long long integer, respectively:
long atol (const char *string)
double atof(const char *string)
long long atoll (const char *string)(C99)