What is wrong with my program reversing strings in C?

Question

I want to create a function to reverse a string in C. I found a couple pre-made on the internet but I wish to create mine. Here is the code:

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


char* inverseCarac(char* chaine){
    if(chaine==0||*chaine==0) return NULL;  

    int j, i;
    int taille=strlen(chaine); 
    char* inverse=malloc(taille*sizeof(char));

    for(j=taille-1, i=0; j>0; j--, i++){
        *(inverse+i)=*(chaine-j);
    }

    return inverse;

}

int main(void){
    char* test="bonjour";
    char* inv=inverseCarac(test);
    printf("%s", inv);
    return 0;
}

I can't figure out why I get a segmentation fault.

First thing - you do not malloc enough space for the null terminator. — Eugene Sh.
– Eugene Sh., Commented Sep 14, 2016 at 17:33
I don't see why using two indices. I think it should be i only. — Eugene Sh.
– Eugene Sh., Commented Sep 14, 2016 at 17:35
malloc(taille*sizeof(char)); --> malloc(taille+1); and add a nul terminator. — Weather Vane
– Weather Vane, Commented Sep 14, 2016 at 17:36
Thanks, I tried but the problem is it doesn't print anything. — S Nus
– S Nus, Commented Sep 14, 2016 at 17:44
@SNus you may be copying the null terminator into inverse[0] which wouldnt print anything. — Charles
– Charles, Commented Sep 14, 2016 at 21:33

Weather Vane · Accepted Answer · 2016-09-14 17:46:12Z

1

There were several errors in your code, the most significant being the offset from chaine in the wrong direction. Also, lack of space for a string terminator, and j ending prematurely.

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

char* inverseCarac(char* chaine){
    if(chaine==0||*chaine==0) return NULL;  

    int j, i;
    int taille=strlen(chaine); 
    char* inverse=malloc(taille+1);             // add 1 for terminator

    for(j=taille-1, i=0; j>=0; j--, i++){       // change j>0 to j >= 0
        *(inverse+i)=*(chaine+j);               // change -j to +j
    }
    inverse[taille] = '\0';                     // write terminator
    return inverse;
}

int main(void){
    char* test="bonjour";
    char* inv=inverseCarac(test);
    printf("%s\n", inv);
    return 0;
}

Program output

ruojnob

answered Sep 14, 2016 at 17:46

Weather Vane

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1 Comment

S Nus Over a year ago

THANK YOU VERY MUCH, that was it. You're great

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