Confused with C string weird behaviour...

[syazhani]

Here is my little simple function before fixing:

Code

GeSHi (c):
void charStrToHexStr(char* t, int size, char* hexStrEncrypted)
{
    int i; //Index of input
 
    char c[2]; //Temporary to hold hex value
 
    hexStrEncrypted[0] = '\0';
    for(i = 0; i < size; i++)  //Iterate through input
    {
        sprintf(c, "%02X", t[i]&0xFF); //Convert input into hex and put into c[2].
                                    //1 char will have 2 hex digit max so c[0] holds higher order
                                    //hex (0xF0) and c[1] holds lower order hex (0x0F)
        strcat(hexStrEncrypted, c); //Append c to hexStrEncrypted
    }
    hexStrEncrypted[i*2+1] = '\0';  //Terminate output with null
 
}
 
Created by GeSHI 1.0.7.18

The purpose is just to convert a char into it's hex equivalent but as a string. So char 'a' will be converted into string "41".

The problem is if input string t is longer than 1, I get into infinite loop with counter i gets reset to 0 on every iteration. What a weird behaviour!

The problem is fixed buy changing size of char c[] to 3 instead of 2. I can't think WHY. c[2] should be enough to hold the hex digits which will be at most 2 per char. c[0] = first hex, c[1] = second hex, c[2] = null.

Why?

[C-Man]

well you are forgetting that a C string isn't just the chars but also the null terminator
what happens is your sprintf writes the two digits then the 3rd null terminator goes out ofrange and overwrites the counter which is right next to c in memory

the C syntax for arrays

type array[number_of_elements_it_contains]

so c[N] will have N chars and the indexes will be valid only 0 trough N-1 not 0 trough N

[syazhani]

I see. All these time I thought the valid index is up to N, but is reserved for the null character.