Getting no output why is that? - c

Question

I', learning C and I'm getting no output for some reason, probably I don't return as I should but how I should? (described the problem in the comments below :D)
Any help is appreciated!
#include <stdio.h>
#include <ctype.h>
#include <string.h>
char *makeUpperCase (char *string);
int main()
{
printf(makeUpperCase("hello")); //Here there is no output, and when I'm trying with the format %s it returns null
return 0;
}
char *makeUpperCase(char *string)
{
char str_out[strlen(string) + 1];
for (int i = 0; i < strlen(string); ++i)
str_out[i] = toupper(string[i]);
printf(str_out); //Here I get the output.
return str_out;
}

Answer 1

You declared within the function a local variable length array that will not be alive after exiting the function
char str_out[strlen(string) + 1];
So your program has undefined behavior.
If the function parameter declared without the qualifier const then it means that the function changes the passed string in place. Such a function can be defined the following way
char * makeUpperCase( char *string )
{
for ( char *p = string; *p != '\0'; ++p )
{
*p = toupper( ( unsigned char )*p );
}
return string;
}
Otherwise you need to allocate dynamically a new string. For example
char * makeUpperCase( const char *string )
{
char *str_out = malloc( strlen( string ) + 1 );
if ( str_out != NULL )
{
char *p = str_out;
for ( ; *string != '\0'; ++string )
{
*p++ = toupper( ( unsigned char )*string );
}
*p = '\0';
}
return str_out;
}
Here is a demonstration program.
#include <stdop.h>
#include <stdlib.h>
#include <string.h>
char *makeUpperCase( const char *string )
{
char *str_out = malloc( strlen( string ) + 1 );
if (str_out != NULL)
{
char *p = str_out;
for (; *string != '\0'; ++string)
{
*p++ = toupper( ( unsigned char )*string );
}
*p = '\0';
}
return str_out;
}
int main( void )
{
char *p = makeUpperCase( "hello" );
puts( p );
free( p );
}
The program output is
HELLO

Answer 2

The problem is that printf() is buffering output based on a bit complex mechanism. When you are outputting to a terminal, printf() just buffers everything until the buffer fills (which is not going to happen with just the string "hello", or until it receives a '\n' character (which you have not used in your statement)
So, to force a buffer flush, just add the following statement
fflush(stdout);
after your printf() call.