Thread: i know the wind keeps changing but this is getting silly

i have the following silly program

Code:

int main()
{
    const int x = 146641;
    char buffer[6] = { '\0' };

    sprintf( buffer, "%d", x);
    printf("%s and x is %d\n", buffer, x);
    printf("Length of buffer is %lu\n", strlen(buffer));

    char RevNum[6] = { '\0' };

    for (int j = 6, i = 0; i < strlen( buffer ) ; i++ )
    {
        strcpy(&RevNum[j], &buffer[i]);
        j--;
    }

    if ( strcmp( RevNum, buffer ) == 0 )
    {
        printf("%d is palindromic!!\n", x);
    }
    return 0;
}

any guesses as to what number is printed with the last printf statement/? ill give you a hint its not 146641 despite x never being changed and being constant!!

First, #include both stdio.h and string.h.

Then correct the errors:

Code:

.../foo.c: In function ‘main’:
.../foo.c:15:30: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
   15 |     for (int j = 6, i = 0; i < strlen( buffer ) ; i++ )
      |                              ^
.../foo.c:9:25: warning: ‘sprintf’ writing a terminating nul past the end of the destination [-Wformat-overflow=]
    9 |     sprintf( buffer, "%d", x);
      |                         ^
.../foo.c:9:5: note: ‘sprintf’ output 7 bytes into a destination of size 6
    9 |     sprintf( buffer, "%d", x);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~

Increase the size of buffer to allow for the Nul byte.
Cast the return value from strlen() to int.

Then run the program.

Turn on the error checking, and turn up to the highest level.

how do you turn the warning level up on code blocks as i have wall and gcc -Wall -g -Wall -pedantic turned on

gcc -Wall -g -Wall -pedantic -c "/home//Documents/my code/problem_36 /main.c" -o obj/Debug/main.o
gcc -o "bin/Debug/problem_36 " obj/Debug/main.o -lm
Output file is bin/Debug/problem_36 with size 17.78 KB
Process terminated with status 0 (0 minute(s), 0 second(s))
0 error(s), 0 warning(s) (0 minute(s), 0 second(s))

ok why does this work for the above number but not 4664 with the buffer adjusted

ok i got it working now
the issue was the strcpy command the only way it didn't whine about pointers and double pointers was when i sent in the addresses of the strings but that was obviously wrong

here is the working code

Code:

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

int main()
{
    const int x = 343;
    char buffer[4] = {'\0'};

    sprintf( buffer, "%d", x);
    printf("%s and x is %d\n", buffer, x);
    printf("Length of buffer is %d\n", (int) strlen(buffer));

    char RevNum[4] = {'\0'};
    int y = (int)strlen(buffer);
    for (int j = y - 1, i = 0; i < y; i++ )
    {
        RevNum[j] = buffer[i];
        j--;
    }

    if ( strcmp( RevNum, buffer) == 0 )
    {
        printf("%d is palindromic!!\n", x);
    }
    return 0;
}

Originally Posted by cooper1200

ok why does this work for the above number but not 4664 with the buffer adjusted

When the Nul byte was written to the first byte beyond the end of buffer, it stepped on the first byte in the 4 byte integer x.

Make sure you correct all errors and warnings before examining the results of the program.

My gcc command line is:

"gcc -std=c18 -Wall -Wextra -Wpedantic -g -o program program.c"

I don't use Code::Blocks. Please refer to the user manual.

Also, check the links I found in a Google search.

thanks for that.

i think i saw on another thread the other day that you used to teach this. Please could you recommend a good book on pointers

Originally Posted by cooper1200

thanks for that.

i think i saw on another thread the other day that you used to teach this. Please could you recommend a good book on pointers

There is no one book on pointers. I recommend 3 different books on the C Programming language that would give you the information you need on pointers along with all the other details on the language.

You need to study one of the following books on the C Programming Language, cover to cover, and do all the exercises at the end of each chapter!

C Programming, A Modern Approach, 2nd Edition
Author: K. N. King

C Primer Plus, 6th Edition
Stephen Prata

C How to Program, 8/e
Deitel & Deitel

Studying one of these books, and writing code, you will have a much better understanding of the entire C Programming language, not just pointers.