How to reverse a string with pointers only?

Question

I'm trying to reverse a string, but it just stays the same. I don't use any modules except <string.h> and <stdio.h> .

void rev(s){
    char i, temp;
    char *sf = s;
    char ri = strlen((s) - 1);
    char *sl = &s[ri];
    for (i = 0; i < ri; i++){
        if (*sf != *sl){
            temp = *sf++;
            s[i] = *sl--; //
            s[ri--] = temp; //those two seems to be getting new characters, but it won't
        }
        else {
            ri--;
            sf++;
            sl--;
        }
    }
    printf("%s", s);
}

Answer 1

The function will not compile at least because the parameter does not have a type specifier.

void rev(s){

The type char has a little range of acceptable values. So you shall not use it for calculating the length of a string.

The call of strlen in this declaration

char ri = strlen((s) - 1);

invokes undefined behavior. It seems you mean

char ri = strlen(s) - 1; 

that also can invoke undefined behavior for an empty string.

This loop

for (i = 0; i < ri; i++){

does not use pointers.

The function can be defined the following way as it is shown in the demonsytrative program below.

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

char * reverse( char *s )
{
    if ( *s )
    {
        for ( char *first = s, *last = s + strlen( s ); first < --last; ++first )
        {
            char c = *first;
            *first = *last;
            *last = c;
        }
    }
    
    return s;
}

int main( void ) 
{
    char s1[] = "1";
    char s2[] = "12";
    char s3[] = "123";
    
    puts( reverse( s1 ) );
    puts( reverse( s2 ) );
    puts( reverse( s3 ) );
}   

The program output is

1
21
321

Answer 2

Your code has plenty of issues (bugs 🐛):

char ri = strlen((s) - 1); has to be size_t ri = strlen((s)) - 1;

Other code is very hard to analyze as you use not self explanatory variable names.

Here you have a bit more simple code and much easier to analyse.

char *reverseString(char *str)
{
    char *wrk = str, *end;
    if(str && *str)
    {
        end = str + strlen(str) - 1;
        while(end > wrk)
        {
            char temp = *wrk;
            *wrk++ = *end;
            *end-- = temp;
        }
    }
    return str;
}


int main(void)
{
    char str[] = "1234567890";
    printf("reversed: %s\n", reverseString(str));
}

Answer 3

A simple solution:

char *sl = sf;
while (*sl != 0)
    ++ sl;
-- sl; 
while (sf < sl)
{
    char c = *sf;
    *sf = *sl;
    *sl = c;

    ++sf, --sl;
}

Find the end of the string by skipping all characters until you find the NUL (zero) character.
Then step back one character (decrement sl ) so that you have pointers to the first and the last character of the string.

Then walk both pointers towards one another and swap characters until the pointers meet or cross.