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.
Combining R.. and PickBoy answers for brevity
To @R.. I think it's not enough to check
errno
for error detection instrtol
call.You'll also need to check
EndPointer
for errors.*scanf()
family of functions return the number of values converted. So you should check to make suresscanf()
returns 1 in your case.EOF
is returned for "input failure", which means thatssacnf()
will never returnEOF
.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()
orstrtoul()
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 usestrtol()
, and then check the returned value to see if it lies betweenINT_MIN
andINT_MAX
.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, likeatoi
, behaviour on integer overflow is unspecified.strtol
and familyRobust error reporting, provided you set
errno
to 0 before making the call. Return values are specified on overflow anderrno
will be set. You can choose any number base from 2 to 36, or specify 0 as the base to auto-interpret leading0x
and0
as hex and octal, respectively. Choices of type to convert to are signed/unsigned versions oflong/long long/intmax_t
.If you need a smaller type you can always store the result in a temporary
long
orunsigned 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.,
Or you can use (for robustness):
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()
, norsscanf()
. This good answer all ready details the lack of error checking withatoi()
and some error checking withsscanf()
.strtol()
is the most stringent function in converting a string toint
. 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.This is the like
atoi()
and neglects to use the error detection features ofstrtol()
.To fully use
strtol()
, there are various features to consider:Detection of no conversion: Examples:
"xyz"
, or""
or"--0"
? In these cases,endptr
will matchnptr
.Should the whole string convert or just the leading portion: Is
"123xyz"
OK?Detect if value was so big, the the result is not representable as a
long
like"999999999999999999999999999999"
.Detect if the value was outside the range of than
int
, but notlong
. Ifint
andlong
have the same range, this test is not needed.Some implementations go beyond the C standard and set
errno
for additional reasons such as errno to EINVAL in case no conversion was performed orEINVAL
The value of the Base parameter is not valid.. The best time to test for theseerrno
values is implementation dependent.Putting this all together: (Adjust to your needs)
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"
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