atoi — how to identify the difference between zero and error?

c atoi
Nahum picture Nahum · Jan 15, 2012 · Viewed 45k times · Source

http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/

Return Value

On success, the function returns the converted integral number as an int value. If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX or INT_MIN is returned.

So how I differ between atoi("poop") and atoi("0") and atoi("0000000")

Yes I can loop and check for all zeroes in case I get 0 result, but isn't there a better way?

Notice: I use ANSI C89

Answer

cnicutar picture cnicutar · Jan 15, 2012

That's one of the reasons atoi is sometimes considered unsafe. Use strtol / strtoul instead. And if you have it use strtonum.

The function atoi is more dangerous than you might think. The POSIX standard says:

If the value cannot be represented, the behavior is undefined.

The C99 standard says this also:

7.20.1

The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno on an error. If the value of the result cannot be represented, the behavior is undefined.

Related questions

How to convert a string to integer in C?

I am trying to find out if there is an alternative way of converting string to integer in C. I regularly pattern the following in my code. char s[] = "45"; int num = atoi(s); So, is there a better way or …

Read More
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, "…

Read More
atoi() from hex representation string

Need to make int from hex representation string like "0xFA" or better "FA". Need something like atoi("FA"). Is there are any standard solutions for that?

Read More