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

gcc 4.4.4 c89

What is better to convert a string to an integer value.

I have tried 2 different methods atoi and sscanf. Both work as expected.

char digits[3] = "34";
int device_num = 0;

if(sscanf(digits, "%d", &device_num) == EOF) {
    fprintf(stderr, "WARNING: Incorrect value for device\n");
    return FALSE;
}

or using atoi

device_num = atoi(digits);

I was thinking that the sscanf would be better as you can check for errors. However, atoi doesn't doing any checking.

If user enters 34abc and you pass them to atoi it will return 34. If you want to validate the value entered then you have to use isdigit on the entered string iteratively

To @R.. I think it's not enough to check errno for error detection in strtol call.

long strtol (const char *String, char **EndPointer, int Base)

You'll also need to check EndPointer for errors.

When there is no concern about invalid string input or range issues, use the simplest: atoi()

Otherwise, the method with best error/range detection is neither atoi(), nor sscanf(). This good answer all ready details the lack of error checking with atoi() and some error checking with sscanf().

strtol() is the most stringent function in converting a string to int. Yet it is only a start. Below are detailed examples to show proper usage and so the reason for this answer after the accepted one.

// Over-simplified use
int strtoi(const char *nptr) {
  int i = (int) strtol(nptr, (char **)NULL, 10);
  return i; 
}

This is the like atoi() and neglects to use the error detection features of strtol().

To fully use strtol(), there are various features to consider:

1. Detection of no conversion: Examples: "xyz", or "" or "--0"? In these cases, endptr will match nptr.

   char *endptr;
   int i = (int)strtol(nptr, &endptr, 10);
   if (nptr == endptr) return FAIL_NO_CONVERT;
   

2. Should the whole string convert or just the leading portion: Is "123xyz" OK?

   char *endptr;
   int i = (int)strtol(nptr, &endptr, 10);
   if (*endptr != '\0') return FAIL_EXTRA_JUNK;
   

3. Detect if value was so big, the the result is not representable as a long like "999999999999999999999999999999".

   errno = 0;
   long L = strtol(nptr, &endptr, 10);
   if (errno == ERANGE) return FAIL_OVERFLOW;
   

4. Detect if the value was outside the range of than int, but not long. If int and long have the same range, this test is not needed.

   long L = strtol(nptr, &endptr, 10);
   if (L < INT_MIN || L > INT_MAX) return FAIL_INT_OVERFLOW;
   

5. Some implementations go beyond the C standard and set errno for additional reasons such as errno to EINVAL in case no conversion was performed or EINVAL The value of the Base parameter is not valid.. The best time to test for these errno values is implementation dependent.

Putting this all together: (Adjust to your needs)

#include <errno.h>
#include <stdlib.h>

int strtoi(const char *nptr, int *error_code) {
  char *endptr;
  errno = 0;
  long i = strtol(nptr, &endptr, 10);

  #if LONG_MIN < INT_MIN || LONG_MAX > INT_MAX
  if (errno == ERANGE || i > INT_MAX || i < INT_MIN) {
    errno = ERANGE;
    i = i > 0 : INT_MAX : INT_MIN;
    *error_code = FAIL_INT_OVERFLOW;
  }
  #else
  if (errno == ERANGE) {
    *error_code = FAIL_OVERFLOW;
  }
  #endif

  else if (endptr == nptr) {
    *error_code = FAIL_NO_CONVERT;
  } else if (*endptr != '\0') {
    *error_code = FAIL_EXTRA_JUNK;
  } else if (errno) {
    *error_code = FAIL_IMPLEMENTATION_REASON;
  }
  return (int) i;
}

Note: All functions mentioned allow leading spaces, an optional leading sign character and are affected by locale change. Additional code is required for a more restrictive conversion.

Note: Non-OP title change skewed emphasis. This answer applies better to original title "convert string to integer sscanf or atoi"

*scanf() family of functions return the number of values converted. So you should check to make sure sscanf() returns 1 in your case. EOF is returned for "input failure", which means that ssacnf() will never return EOF.

For sscanf(), the function has to parse the format string, and then decode an integer. atoi() doesn't have that overhead. Both suffer from the problem that out-of-range values result in undefined behavior.

You should use strtol() or strtoul() functions, which provide much better error-detection and checking. They also let you know if the whole string was consumed.

If you want an int, you can always use strtol(), and then check the returned value to see if it lies between INT_MIN and INT_MAX.

Combining R.. and PickBoy answers for brevity

long strtol (const char *String, char **EndPointer, int Base)

// examples
strtol(s, NULL, 10);
strtol(s, &s, 10);