modifying already initialised array of strings

Question

In the code below if i remove both strncpy its compiling and running without seg fault. But with strcpy its throwing seg fault. IN both cases i am trying to modify a read only address right ?Then why the unexpected behaviour ..

#include<stdio.h> 
#include<string.h>
int main()
{
unsigned char* newPrompt="# ";
static unsigned char* au1CLIPromptStrings [] =
{
 "",
 "Login: ",
 "Password: ",
  "0123456789012345678901234",
  "0123456789012345678901234",
};
/* here am trying to moodify the read only address */
au1CLIPromptStrings[3] = "# \0";
au1CLIPromptStrings[4]  = "# \0";
/* removed two strncpy second time */
printf("a1 = %s and a2 = %s\n",au1CLIPromptStrings[3],au1CLIPromptStrings[4]);

/* here using strcpy am trying to modify */
strncpy(au1CLIPromptStrings[3],newPrompt,strlen(au1CLIPromptStrings[3])) ;
strncpy(au1CLIPromptStrings[4],newPrompt,strlen(au1CLIPromptStrings[4])) ;
}

thanks in advance..

There is one problem i am facing now. I need to pass the values of au1CLIPromptStrings to another double pointer which is bieng used in many places .So if i assign the address of au1CLIPromptStrings to that doublepointer of type unsigned char which is in a structure elemet.Now i am not able to retrive the elements its getting NULL evenif address is coming proper.I cannot use the same au1CLIPromptStrings varibale everywhere , Here is a proto of waht i told,

 unsigned char **newPrompt1 =NULL;
 newPrompt1 = au1CLIPromptStrings;
 printf("a1 = %s and a2 = %s\n", newPrompt1[3],newPrompt1[4]); 


#include<stdio.h>
#include<string.h>
int main()
{
unsigned char *newPrompt="# ";
unsigned char **newPrompt1 =NULL;
unsigned char au1CLIPromptStrings [7][30] =
{
"",
"Login: ",
"Password: ",
" 0123456789012345678901234",
" 0123456789012345678901234",
};
newPrompt1 = au1CLIPromptStrings; // here am assigning the address
printf("b1 = %u and b2 = %u\n",newPrompt1,au1CLIPromptStrings);
printf("a1 = %s and a2 = %s\n",newPrompt1[3],newPrompt1[4]);
}

I'm not sure what you're asking. Please show clearly the code that leads to behavior you don't expect, and why you don't expect it. — StilesCrisis
– StilesCrisis, Commented Mar 19, 2013 at 5:34
From my point of view this should print the elements in au1CLIPromptStrings. So you mean it prints NULL for newPrompt1[3] and newPrompt1[4]? — user1149862
– user1149862, Commented Mar 19, 2013 at 9:19
au1CLIPromptStrings values we can index and its proper also ,but for newPrompt1 values indexing is giving null values — kelvin
– kelvin, Commented Mar 19, 2013 at 9:38
I tested your code under Cygwin with gcc, it prints the same value as au1CLIPromptStrings as expected. Which OS and which compiler are you using? — user1149862
– user1149862, Commented Mar 19, 2013 at 9:44
using Redhat linux 6, may be i will check it outin cygwin also .. — kelvin
– kelvin, Commented Mar 19, 2013 at 9:52

Barath Ravikumar · Accepted Answer · 2013-03-19 07:07:40Z

2

Your own program , modified to show how using [][] , can help reduce confusion instead of using *[]

#include<stdio.h> 
#include<string.h>
int main()
{
unsigned char newPrompt[10]="# ";
unsigned char au1CLIPromptStrings [5][30] =
{
 "",
 "Login: ",
 "Password: ",
  " 0123456789012345678901234",
  " 0123456789012345678901234",
};

printf("a1 = %s and a2 = %s\n",au1CLIPromptStrings[3],au1CLIPromptStrings[4]);

strcpy(au1CLIPromptStrings[3],"# \0");
strcpy(au1CLIPromptStrings[4],"# \0");

printf("a1 = %s and a2 = %s\n",au1CLIPromptStrings[3],au1CLIPromptStrings[4]);

strncpy(au1CLIPromptStrings[3],newPrompt,strlen(au1CLIPromptStrings[3])) ;
strncpy(au1CLIPromptStrings[4],newPrompt,strlen(au1CLIPromptStrings[4])) ;
}

edited Mar 19, 2013 at 7:07

answered Mar 19, 2013 at 5:59

Barath Ravikumar

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score 1 · Accepted Answer · 2013-03-19 10:18:27Z

static unsigned char* au1CLIPromptStrings [] =
{
    "",
    "Login: ",
    "Password: ",
    "0123456789012345678901234",
    "0123456789012345678901234",
};

This defines an array, in which each member is a pointer pointing to char type. And after initializing, each member is pointing to a static area that you can't modify. Yes you can change that pointer to point to another memory block, as you did in

au1CLIPromptStrings[3] = "# \0";
au1CLIPromptStrings[4]  = "# \0";

But actually you were changing the value of a pointer (or just say point to another memory block), not changing the original memory block. The original one is READ-ONLY.

My code for your second question:

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

int main()
{
    unsigned char **newPrompt1 =NULL;
    static unsigned char* au1CLIPromptStrings [] =
    {
        "",
        "Login: ",
        "Password: ",
        "0123456789012345678901234",
        "0123456789012345678901234",
    };

    newPrompt1 = au1CLIPromptStrings;
    printf("a1 = %s and a2 = %s\n", au1CLIPromptStrings[3],au1CLIPromptStrings[4]);
    printf("a1 = %s and a2 = %s\n", newPrompt1[3],newPrompt1[4]); 

    return 0;
}

Under cygwin with gcc it prints:

a1 = 0123456789012345678901234 and a2 = 0123456789012345678901234
a1 = 0123456789012345678901234 and a2 = 0123456789012345678901234

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