Question 214228 in Ubuntu Question posted in
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Ubuntu – I have an unexpected infinite loop using `for` in C

I’m working on a simple (simple minded?) implementation of AES for learning purposes. I have made a small utility for generating the SBox table, but am having an odd infinite loop, which keeps spitting out the table ad infinitum. Below is the code in C that I’m trying to use. I just can’t seem to get it under control! I expected this to spit out 256 SBox values; it’s the same basic way that I (successfully) generated the log and anti-log tables.
The actual sbox() function is implemented in an external file and is prototyped in the included header file. Note that I have successfully tested sbox() to ensure that it returns the correct value for any given input; ok up to that point. Note that I am using unsigned char for in index variable type, as sbox() expects that type.
I know I’m missing something simple, and will be embarrassed, but grateful once someone has shown me the error of my ways. 😉 Thanks in advance!

//mk_sbox.c
//output the sbox table

#include "gfmath.h" 

int main() {
    
    uchar i,e;

    //display table
    printf("sbox table:n");
    for (i=0; i<256; i++) {
        e=sbox(i);
        printf("%hhu",e);
        printf("%c",',');
    }
    printf("n");

    return 0;
}

I’ve tried using while instead of for, but I get the same result. In case it matters here is the version of GCC:

$ gcc --version
gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0

I’m working in a GitHub Codespace.

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2

Answers


  1. 0

    Change i to unsigned int. uchar has the range 0..255 so it will never reach 256.

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  2. 0

    The range of unsigned char type is [0, 255], when i is 255, incrementing i (i++) result in its value wrap around and its value becomes 0. In next iteration the loop condition i < 256 result in true and loop continues.

    To solve this, either change the type of i to unsigned int (as suggested in other post) or use do..while loop, like this:

        unsigned char i = 0;
    do {
        // loop body
    } while (i++ < 255);

    Additional –

    You can combine these two statements

            printf("%hhu",e);
        printf("%c",',');

    in one

            printf("%hhu,",e);
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