C Program to print string from commandline in reverse

Question

/* Pgm to print string from commandline and reverse it */
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#include<string.h>

int main(int argc, char *argv[]) {
  if(argc<1){
    perror("Not enough arguments");
  }
  // Printing the string
  for(int i=1; i<argc ; i++){
   printf("%s\n",argv[i]);
  }
  //Part to print the string in reverse
  char *arr = (char*) malloc(sizeof(argv[1])+1);    // +1 for the NULL terminator
  strcpy(arr,argv[1]);
  char *str = (char*) malloc(sizeof(argv[1])+1);       //buffer array

//Reverse part begins
int j=0;
  for(int i= sizeof(argv[1]); i>=0 ; i--){
    str[j] = arr[i];
    j++;
  }

  for(int j=0;j<sizeof(argv[1]);j++){        // Printing the reverse string
  printf("R=%s\n",&str[j]);
 }

  free(arr);
  free(str);
  return 0;
}

This program is expected to print the text from argv[1] on commandline in reverse order. But the output I get is weird. output

user@DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ gcc linex.c -o linex -Wall -pedantic -std=c99
user@DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ ./linex hello
hello
R=
R=
R=
R=
R=olleh
R=lleh
R=leh
R=eh

Also when the input is above a certain number of characters, it automatically truncates it:

user@DESKTOP-KI53T6C:/mnt/c/Users/user/Documents/C programs$ ./linex strcmpppssdsdssdsd
strcmpppssdsdssdsd
R=spppmcrts
R=pppmcrts
R=ppmcrts
R=pmcrts
R=mcrts
R=crts
R=rts
R=ts

All I want is the output to be : 'olleh' when I type 'hello'

`sizeof` doesn't give you the length of a string. Use `strlen()` for that. — r3mainer, Feb 16 '21 at 09:11
There are a LOT of misunderstandings here. I would recommend finding a good C fundamentals book and understanding everything chapter by chapter. — Qix - MONICA WAS MISTREATED, Feb 16 '21 at 09:20
Also, it's not like there aren't thousands of code examples of how to reverse a string — klutt, Feb 16 '21 at 09:25

Vlad from Moscow · Accepted Answer · 2021-02-16T09:29:00.277

The expression argv[1] has the type char *. That is it is a pointer. sizeof( char * ) that is equivalent to the expression sizeof( argv[1] ) is equal to 4 or 8 depending on the used system.

So for example in this line

char *arr = (char*) malloc(sizeof(argv[1])+1);

there are allocated not enough memory to store a string pointed to by the pointer argv[1]. Instead of the expression sizeof( argv[1] ) you need to use expression strlen( argv[1] ).

Also to output a string in the reverse order there is no any need to allocate dynamically a character array. This is just a bad idea.

Here is a demonstrative program that shows how to output a string in the reverse order.

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

int main(void) 
{
    const char *s = "Hello";
    
    for ( size_t i = strlen( s ); i != 0; )
    {
        putchar( s[--i] );
    }
    
    putchar( '\n' );
    
    return 0;
}

The program output is

olleH

If you want not just to output a string in the reverse order but to create a new string in the reverse order then what you need is to write a function that performs copying of a string stored in one character array into another character array.

Here is a demonstrative program.

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

char * copy_reverse( char *s1, const char *s2 )
{
    size_t n = strlen( s2 );
    
    s1[n] = '\0';
    
    while ( *s2 ) s1[--n] = *s2++;
    
    return s1;
}

int main(void) 
{
    const char *s1 = "Hello";
    
    char *s2 = malloc( strlen( s1 ) + 1 );
    
    if ( s2 ) puts( copy_reverse( s2, s1 ) );
    
    free( s2 );

    return 0;
}

The program output is the same as shown above

olleH

Thanks! Changing it from sizeof to strlen fixed the issue. Now it is printing the text from commandline in reverse order. — technextgen, Feb 16 '21 at 09:42

C Program to print string from commandline in reverse

1 Answers1