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

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

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

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

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

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

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   amp 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   amp 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   amp 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   amp endptr  10   if  L  lt  INT MIN    L  gt  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  lt errno h gt   include  lt stdlib h gt   int strtoi const char  nptr  int  error code      char  endptr    errno   0    long i   strtol nptr   amp endptr  10       if LONG MIN  lt  INT MIN    LONG MAX  gt  INT MAX   if  errno    ERANGE    i  gt  INT MAX    i  lt  INT MIN        errno   ERANGE      i   i  gt  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

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

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