Unexpected Readings on Normal Compiling, But Everything working fine with GDB

Question

I've been trying my program in GDB during the making of my own program but when I run the compiled without -g it gives unexpected results, the problem results from the 2nd Command Onwards that have less or equal to 4 letters

"Help" ( This case persists with every 4 letter command like stop & exit )

Expected : With GDB Received : Without GDB Note: This is how Letters are Read

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ncurses.h>

int curs; /*curs Used for Cursor*/
int Nullpos;
char *cmd;

void ShowCMD()
{
    int Strlen = strlen(cmd) ;
    printw("\nFull Command: %s\n NullPosition = %d\n", cmd, Nullpos);
    for(int i = 0 ; i < Strlen ; i++ )
    {
        printw("Char %d : %u %c [] \n",i, cmd[i], cmd[i]);
    }
        printw("Char %d : %u [] \n",Strlen, cmd[Strlen]);
}
int AddNUllChar(char *Str)
{
    int nullpos= 0;
    for(int i = 0; (Str[i] <= 127 && Str[i] >= 0) && i < strlen(Str); i++)
        nullpos = i;
    Str[nullpos + 1 ] = (char)0;
    return nullpos;     
}
int IsInAllowedChars(char c)
{
    if (c >= 32 && c <= 57)
        return 1;
    if (c >= 63 && c <= 90)
        return 1;
    if (c >= 95 && c <= 122)
        return 1;

    return 0;
}

void srvr_cmd()
{
e:;
    curs = 0;
    cmd = malloc(1024 * sizeof(char));
    char ch;
    while (1)
    {
        if((ch = getch()) == ERR)
            continue;

        if(ch > 127 || ch < 0 || ch == 10)
        {
            break;
        }
        if (IsInAllowedChars((ch)))
        {
            printw("%c", ch,ch);
            cmd[curs] = ch;
            curs++;
        }
    }
    Nullpos = AddNUllChar(cmd);
cm:;
    ShowCMD();          
    free(cmd);
    goto e;
}
int main(int argc, char *argv[])
{
    clear();
    initscr();
    cbreak();
    noecho();
    keypad(stdscr, TRUE);
    srvr_cmd();
    return 0;
}

Answer 1

The main problem is AddNUllChar(). I must assume that it is supposed to add the terminating null character to the character array. So when the function is entered, the array is not yet null-terminated. If so, you cannot call strlen() because it depends on the null character.

AddNUllChar() will have undefined behavior. It all depends on the bytes in cmd, which is undefined as malloc() does not clear it.

As it stands, AddNUllChar() cannot be fixed. Instead, remove it and replace:

Nullpos = AddNUllChar(cmd);

with:

cmd[curs] = 0;
Nullpos = curs;

Note that Nullpos now has a value that is offset by 1 compared to your original version. It now actually points at the array element with the null character.