C segfault when modifiy array value [duplicate]

Question

Im getting a segfault when changing dna[i] to U, Ive debbuged but I still cant understand why.

Also I was comparing the value at a position against T with strcmp, but from what I understand thats for string literals and I can simply compare it with dna[i] == 'T'. Is that right? thanks.

#include <string.h>

char *dna_to_rna(char *dna) {
    int size = (int)( sizeof(dna) / sizeof(dna[0]));
    char *retour[size];
    strcpy(retour, dna);
    for (int i = 0; i < size; i++) {
        if (dna[i] == 'T') {
            dna[i] = 'U';
        }
    }
    return (char *) retour;
}

int main() {
    char *dna[] = {
            "TTTT",
            "GCAT",
            "GACCGCCGCC"
    };

    char *actual, *expected;
    size_t n;
    for (n = 0; n < 3; ++n) {
        actual = dna_to_rna(*(dna + n));
    }
    return 0;
}

Answer 1

You are passing to the function dna_to_rna a pointer to a string literal

actual = dna_to_rna(*(dna + n));

and then within the function you are trying to change the string literal

    if (dna[i] == 'T') {
        dna[i] = 'U';
    }

Any attempt to change a string literal results in undefined behavior.

Also the expression with the sizeof operator in this declaration

int size = (int)( sizeof(dna) / sizeof(dna[0]));

does not make a sense. It evaluates the size of a pointer of the type char *.

Instead you should use the standard string function strlen.

And this declaration is incorrect

char *retour[size];

At least you need a character array instead of an array of pointers.

 char retour[size];

And the function returns a pointer to an array with automatic storage duration that will not be alive after exiting the function

char *dna_to_rna(char *dna) {
    //...
    char retour[size];
    //...
    return (char *) retour;
}

that is the function returns an invalid pointer.

You should dynamically allocate a character array within the function with the length strlen( dna ) + 1 and change and return this array.

It seems what you mean is something like the following

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

char * dna_to_rna( const char *dna ) 
{
    size_t n = strlen( dna );
    char *retour = malloc( n + 1 );

    if ( retour != NULL )
    {
        strcpy( retour, dna );

        for ( size_t i = 0; i < n; i++ ) 
        {
           if ( retour[i] == 'T' ) retour[i] = 'U';
        }
    }

    return retour;
}