char s[]="999";
int i=atoi(s);

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num;
    char *mystring;

    mystring = "9999";
    num = atoi(mystring);
    num = 2 * num;
    printf("2 times %s = %d\n", mystring, num);
    return 0;
}

string_number = "1984"
power = len(string_number)
sum = 0
for digit in string_number
{
   switch(digit)
   {
      case '0':
         value = 0;
         break;
      case '1':
         value = 1;
         break;
      ...
   }
   value *= (pow(10, power));
   sum += value;
}

man -k convert | grep string | less

STRTOL(3)                                            Linux Programmer's Manual                                            STRTOL(3)



NAME
       strtol, strtoll, strtoq - convert a string to a long integer

SYNOPSIS
       #include <stdlib.h>

       long int
       strtol(const char *nptr, char **endptr, int base);

       long long int
       strtoll(const char *nptr, char **endptr, int base);

DESCRIPTION
       The  strtol()  function converts the initial part of the string in nptr to a long integer value according to the given base,
       which must be between 2 and 36 inclusive, or be the special value 0.

       The string must begin with an arbitrary amount of white space (as determined by isspace(3)) followed by  a  single  optional
       `+'  or `-' sign.  If base is zero or 16, the string may then include a `0x' prefix, and the number will be read in base 16;
       otherwise, a zero base is taken as 10 (decimal) unless the next character is `0', in which case it is taken as 8 (octal).

       The remainder of the string is converted to a long int value in the obvious manner, stopping at the first character which is
       not  a  valid  digit in the given base.  (In bases above 10, the letter `A' in either upper or lower case represents 10, `B'
       represents 11, and so forth, with `Z' representing 35.)

       If endptr is not NULL, strtol() stores the address of the first invalid character in *endptr.  If there were  no  digits  at
       all,  strtol()  stores  the  original  value  of  nptr  in *endptr (and returns 0).  In particular, if *nptr is not `\0' but
       **endptr is `\0' on return, the entire string is valid.

       The strtoll() function works just like the strtol() function but returns a long long integer value.

RETURN VALUE
       The strtol() function returns the result of the conversion, unless the value would underflow or overflow.  If  an  underflow
       occurs,  strtol()  returns  LONG_MIN.   If  an  overflow  occurs, strtol() returns LONG_MAX.  In both cases, errno is set to
       ERANGE.  Precisely the same holds for strtoll() (with LLONG_MIN and LLONG_MAX instead of LONG_MIN and LONG_MAX).

ERRORS
       EINVAL (not in C99) The given base contains an unsupported value.

       ERANGE The resulting value was out of range.

       The implementation may also set errno to EINVAL in case no conversion was performed (no digits seen, and 0 returned).

NOTES
       In locales other than the "C" locale, also other strings may be accepted.  (For example, the thousands separator of the cur-
       rent locale may be supported.)

       BSD also has

           quad_t
           strtoq(const char *nptr, char **endptr, int base);

       with completely analogous definition.  Depending on the wordsize of the current architecture, this may be equivalent to str-
       toll() or to strtol().

CONFORMING TO
       strtol() conforms to SVID 3, 4.3BSD, ISO 9899 (C99) and POSIX, and strtoll() to ISO 9899 (C99) and POSIX 1003.1-2001.

SEE ALSO
       atof(3), atoi(3), atol(3), strtod(3), strtoul(3)



GNU                                                          2002-05-30                                                   STRTOL(3)