Concat two strings through pointers not working

Question

I made a function that concat string t to the end of string s, for my exercise I have to use pointers for this, so I did this way but its not working:

#include <stdio.h>

void strcat(char *s, char *t)
{
    while (*s++);

    for (*s = *t; *s = *t; s++, t++);
}

int main()
{
    char c[100] = "hello";
    char *w = " world\n";
    
    strcat(c, w);

    printf(c);
    
    return 0;
}

The output of c always return "hello" instead of "hello world\n"

Answer 1

This while loop within the function

while (*s++);

is incorrect. After the while loop the pointer s points to after the terminating zero character '\0' due to the postfix increment operator.

The function can be declared and defined the following way

char * strcat(char *s, const char *t)
{
    char *p = s;

    while( *p ) ++p;

    while ( ( *p++ = *t++ ) != '\0' );

    return s;
}

Also you should rename the function because there is already standard string function strcat in C.

Here is a demonstration program.

#include <stdio.h>

char *string_cat( char *s, const char *t )
{
    char *p = s;

    while (*p) ++p;

    while (( *p++ = *t++ ) != '\0');

    return s;
}

int main( void )
{
    char c[100] = "hello";
    const char *w = " world";

    puts( string_cat( c, w ) );
}

The program output is

hello world