Do strings in C language change in the middle of program execution?

Question

I was writing the code for Bit stuffing, but I am not getting appropriate answer, when I checked the values of the input string, there was another garbage value added at the end of the string.

#include<stdio.h>
#include<string.h>
void input(char[],int);
void stuff(char ch[],int n)
{
    char str1[6]="11111";
    char str2[6]="00000";
    printf("str1=%s\n",str1);
    printf("ch=%s\n",ch);
        if(!strcmp(ch,str1))
        {
            printf("\n111101");
        }
        else if(!strcmp(ch,str2))
        {
            printf("\n000010");
        }
        else
        {
            puts(ch);
        }
}
void main()
{
    int flag=0;
    char ch[5];
    input(ch,5);
    printf("ch0=%s\n",ch); //printing the input string
    for(int i=0;i<5;i++)
     {
       if((ch[i]!='0')&&(ch[i]!='1'))
       {
           flag=1;
       }
    }

   if(flag==0)
    {
        puts("Entered data:");
        for(int i=0;i<5;i++)
        {
            printf("%c",ch[i]);
        }
        puts("\nAfter stuffing");
        printf("ch1=%s\n",ch);    //getting garbage value here
        stuff(ch,5);
    }
    else
    {
         printf("Enter a valid data\n");
    printf("%d",flag);
    }

}
void input(char ch[],int n)
{
    printf("Enter 5 digits\n");
    for(int i=0;i<=n;i++)
    {
        scanf("%c",&ch[i]);
    }

}

The output of the code is as follows.

Enter 5 digits
11111
ch0=11111

Entered data:
11111
After stuffing
ch1=11111♣
str1=11111
ch=11111♣
11111♣

Process returned 0 (0x0)   execution time : 4.046 s
Press any key to continue.

I am using Code blocks IDE with MINGW. The code above should compare the string entered with the given sequence and stuff the bits if all five bits are homogenous.

You try to read 6 characters into an array that can only hold 5 characters. That is a string of length 4 plus terminating 0 byte. You also do not nul-terminate your input — Gerhardh, Jan 05 '23 at 10:38
```i<=5``` invokes undefined behaviour because it accesses memory out of bounds. — Harith, Jan 05 '23 at 10:38
Your `input` function is incorrect: it does not terminate `ch` with a null terminator, and it also has array out-of-bounds error. Compile and run with address sanitizer (not available in MinGW, though, only Visual Studio or Linux/macOS) to see more details. If CS50 has a forum, please bring up the issue there as well: it's not the first question I see with a problem from CS50 related to C-style strings lacking null terminator. — yeputons, Jan 05 '23 at 10:39

Vlad from Moscow · Answer 1 · 2023-01-06T10:43:26.393

Your code has undefined behavior. For example you declared a character array with 5 elements in main

char ch[5];

Then you are calling the function input

input(ch,5);

Within the function you are entering 6 characters in the for loop

void input(char ch[],int n)
{
    printf("Enter 5 digits\n");
    for(int i=0;i<=n;i++)
    {
        scanf("%c",&ch[i]);
    }

}

That is you are overwriting the memory outside the array.

Taking into account the program output

Enter 5 digits
11111
ch0=11111

the call of scanf

scanf("%c",&ch[i]);

also stored the new line character '\n' in the memory after the last element of the array. You should at least write

scanf(" %c",&ch[i]);
      ^^^^^

instead of

scanf("%c",&ch[i]);

Pay attention to the leading space in the format string. It allows to skip white space characters.

The array does not contain a string but you are trying to output it as if it contains a string

printf("ch0=%s\n",ch); //printing the input string

or to use it in calls of strcmp as for example

if(!strcmp(ch,str1))

that again invokes undefined behavior.

To output the array you could use the following format string

printf("ch0=%.*s\n", 5, ch); //printing the input string

and to compare character arrays you could use either strncmp or memcmp.

Pay attention to that according to the C Standard the function main without parameters shall be declared like

int main( void )

Harith · Accepted Answer · 2023-01-05T10:53:02.003

Lack of null-byte:

A string in C is an array of null-terminated bytes. You do not terminate ch in the input function.

strcmp does compare two strings, but it requires them to be null-terminated.

printf too requires a null-terminated string with the %s format specifier, otherwise it doesn't know when to stop printing. And your code would most probably result in a segmentation fault.

Accessing memory out of bounds:

This line:

for(int i=0;i<=n;i++)

invokes undefined behaviour because it's writing to out of bounds memory. ch has been declared to only contain 5 bytes, which must include the '\0' character.

Do strings in C language change in the middle of program execution?

2 Answers2

Lack of null-byte:

Accessing memory out of bounds: