Atoi of a long string error

Question

I have this code:

char kbits[k]; long int bits;

The kbits string is always filled with '1', for example:

If k = 5, kbits = "11111"

If k = 9, kbits = "111111111"

To do this I use this code:

for(i = 0; i < k; i++) kbits[i] = '1';

And then I run:

bits = atoi(kbits);

So I have the integer of kbits, for example, if kbits = "1111", bits = 1111;

For k <= 10, it runs perfectly fine.

For k > 10 it puts a 7 in last position of kbits (for example, if k = 11, kbits = 11111111117) and bits = 2147483647 (this value is for any value of k, I think it is random?)

Answer 1

atoi interprets its input as a decimal number, not a binary number. Your longer string is causing the value to overflow, in which case the return value of atoi is undefined (although returning INT_MAX is common, which is equal to the value you're seeing).

To interpret the input as a binary number you can use strtol like this:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <limits.h>

int main()
{
    char line[256];

    while (printf("\nInput: "),
           fgets(line, sizeof line, stdin))
    {
        // skip initial whitespace and break if input is nothing but whitespace
        char *p = line;
        while (isspace(*p)) ++p;
        if (*p == '\0') break;

        // interpret input as a binary number    
        long n = strtol(line, &p, 2);

        if (p == line)
            printf("** Can't convert any characters. **\n");
        else if (n == LONG_MAX || n == LONG_MIN)
            printf("** Range error **\n");
        else
            printf("Value: %ld\n", n); // prints value in decimal

        while (isspace(*p)) ++p; // skip whitespace after the value
        if (*p != '\0')
            printf("** Excess characters in input **\n");
    }

    return 0;
}

Answer 2

Your input is too large for the return type. Per the docs:

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

An int is a signed type of at least 16-bits. On your platform it may be 32-bits and, in that case, its max value is 2^31-1. Too small for your input of ~11 billion. Use strtol, which returns a long int, instead.

Also, make sure you're terminating that string.

Answer 3

2147483647 (this value is for any value of k, I think it is random?)

No, it is not random; it is the largest value a signed 32-bit integer can hold.

Your inputs are too big.

When this happens, the return value of atoi is undefined (per the documentation); the result you're seeing is common in practice.

Answer 4

Define kbits to be one byte longer then the number of 1s you want to store and do initialise it to all 0s.

#define K (5)
char kbits[K + 1] = ""; /* One more for the `0`temrination neseccary to terminate a C-"string". */