What is the difference between sscanf or atoi to convert a string to an integer?

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You have 3 choices:

  1. atoi

This is probably the fastest if you’re using it in performance-critical code, but it does no error reporting. If the string does not begin with an integer, it will return 0. If the string contains junk after the integer, it will convert the initial part and ignore the rest. If the number is too big to fit in int, the behaviour is unspecified.

  1. sscanf

Some error reporting, and you have a lot of flexibility for what type to store (signed/unsigned versions of char/short/int/long/long long/size_t/ptrdiff_t/intmax_t).

The return value is the number of conversions that succeed, so scanning for "%d" will return 0 if the string does not begin with an integer. You can use "%d%n" to store the index of the first character after the integer that’s read in another variable, and thereby check to see if the entire string was converted or if there’s junk afterwards. However, like atoi, behaviour on integer overflow is unspecified.

  1. strtol and family

Robust error reporting, provided you set errno to 0 before making the call. Return values are specified on overflow and errno will be set. You can choose any number base from 2 to 36, or specify 0 as the base to auto-interpret leading 0x and 0 as hex and octal, respectively. Choices of type to convert to are signed/unsigned versions of long/long long/intmax_t.

If you need a smaller type you can always store the result in a temporary long or unsigned long variable and check for overflow yourself.

Since these functions take a pointer to pointer argument, you also get a pointer to the first character following the converted integer, for free, so you can tell if the entire string was an integer or parse subsequent data in the string if needed.

Personally, I would recommend the strtol family for most purposes. If you’re doing something quick-and-dirty, atoi might meet your needs.

As an aside, sometimes I find I need to parse numbers where leading whitespace, sign, etc. are not supposed to be accepted. In this case it’s pretty damn easy to roll your own for loop, eg., 

for (x=0; (unsigned)*s-'0'<10; s++) 
    x=10*x+(*s-'0');

Or you can use (for robustness):

if (isdigit(*s))
    x=strtol(s, &s, 10);
else /* error */

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