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

Question

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    amp 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

User · Answer

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:

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;

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;

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;
```
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;
```
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"

User · Answer

Combining R   and PickBoy answers for brevity  long strtol  const char  String  char   EndPointer  int Base      examples strtol s  NULL  10   strtol s   amp s  10

User · Answer

You have 3 choices    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    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    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  lt 10  s         x 10 x   s- 0      Or you can use  for robustness    if  isdigit  s       x strtol s   amp s  10   else    error

User · Answer

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

User · Answer

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

User · Answer

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

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

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