Question

I’ve been studying C, and I decided to practice using my knowledge by creating some functions to manipulate strings. I wrote a string reverser function, and a main function that asks for user input, sends it through stringreverse(), and prints the results.

Basically I just want to understand how my function works. When I call it with ‘tempstr’ as the first param, is that to be understood as the address of the first element in the array? Basically like saying &tempstr[0], right?

I guess answering this question would tell me: Would there be any difference if I assigned a char* pointer to my tempstr array and then sent that to stringreverse() as the first param, versus how I’m doing it now? I want to know whether I’m sending a duplicate of the array tempstr, or a memory address.

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

int main()
{
    char* stringreverse(char* tempstr, char* returnptr);

    printf("\nEnter a string:\n\t");

    char tempstr[1024];

    gets(tempstr);
    char *revstr = stringreverse(tempstr, revstr); //Assigns revstr the address of the first character of the reversed string.

    printf("\nReversed string:\n"
           "\t%s\n", revstr);

    main();
    return 0;
}

char* stringreverse(char* tempstr, char* returnptr)
{
    char revstr[1024] = {0};

    int i, j = 0;

    for (i = strlen(tempstr) - 1; i >= 0; i--, j++)
    {
        revstr[j] = tempstr[i]; //string reverse algorithm
    }

    returnptr = &revstr[0];
    return returnptr;
}

Thanks for your time. Any other critiques would be helpful . . only a few weeks into programming 😛

EDIT: Thanks to all the answers, I figured it out. Here’s my solution for anyone wondering:

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

void stringreverse(char* s);

int main(void)
{
    printf("\nEnter a string:\n\t");

    char userinput[1024] = {0}; //Need to learn how to use malloc() xD

    gets(userinput);
    stringreverse(userinput);

    printf("\nReversed string:\n"
           "\t%s\n", userinput);

    main();
    return 0;
}

void stringreverse(char* s)
{
    int i, j = 0;
    char scopy[1024]; //Update to dynamic buffer
    strcpy(scopy, s);

    for (i = strlen(s) - 1; i >= 0; i--, j++)
    {
        *(s + j) = scopy[i];
    }
}

Answer 1

First, a detail:

int main()
{
    char* stringreverse(char* tempstr, char* returnptr);

That prototype should go outside main(), like this:

char* stringreverse(char* tempstr, char* returnptr);

int main()
{

As to your main question: the variable tempstr is a char*, i.e. the address of a character. If you use C’s index notation, like tempstr[i], that’s essentially the same as *(tempstr + i). The same is true of revstr, except that in that case you’re returning the address of a block of memory that’s about to be clobbered when the array it points to goes out of scope. You’ve got the right idea in passing in the address of some memory into which to write the reversed string, but you’re not actually copying the data into the memory pointed to by that block. Also, the line:

returnptr = &revstr[0];

Doesn’t do what you think. You can’t assign a new pointer to returnptr; if you really want to modify returnptr, you’ll need to pass in its address, so the parameter would be specified char** returnptr. But don’t do that: instead, create a block in your main() that will receive the reversed string, and pass its address in the returnptr parameter. Then, use that block rather than the temporary one you’re using now in stringreverse().