C-Code

#include <stdlib.h>
#include <string.h>
#include <wchar.h>
#include <wctype.h>
#include <stdio.h>
#include <locale.h>

#define BUFLEN 256

/* convert multi byte string from upper to lower an vice versa */
char* ul2lu(char *out, const char *in) {
	mbstate_t ps, pst;
	const char *mbin;
	char *mbout;
	wchar_t wc, wct;
	size_t wcl, wctl;

	memset(&ps, 0, sizeof(mbstate_t));
	memset(&pst, 0, sizeof(mbstate_t));
	mbin = in;
	mbout = out;
	while ((wcl = mbrtowc(&wc, mbin, MB_CUR_MAX, &ps)) > 0) {
		wct = (iswlower(wc))?(wchar_t)towupper(wc):(wchar_t)towlower(wc);
		wctl = wcrtomb(mbout, wct, &pst);
		mbout += wctl;
		mbin += wcl;
	}
	wcrtomb(mbout, L'\0', &pst);	/* terminate mb string */
	return out;
}

int main(int argc, char **argv) {
	char outbuf[BUFLEN];

	printf("locale = %s\n", setlocale(LC_ALL, ""));
	if (argc > 1) {
		printf("your string: %s\n", argv[1]);
		printf("  converted: %s\n", ul2lu(outbuf, argv[1]));
	}
	return 0;
}

C-Code

int main (void){
    printf("geben Sie einen String nur mit Kleinbuchstaben ein ein\n");
    char string[100];
    scanf("%s" , &string);
    int i=0;
    while(string[i] != '\0')
    {
        string[i]-=32;
        i++;
    }
    printf("%s", string\n);
    
    
    printf("geben Sie einen String nur mit Großbuchstaben ein ein\n");
    
    scanf("%s" , &string);
    
    i=0;
    while(string[i] != '\0')
    {
        string[i]+=32;
        i++;
    }
    printf("%s", string);
    
    return 0;
}

C-Code

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

int main() {
    char string[100];
    
    printf("Enter string: ");
    gets(string);
    printf("You entered: %s", string);
    
    unsigned long length = strlen(string);
    
    for(int i = 0; i < length; i++){
        
        //lower case letters to upper case letters
        if((int)string[i]>=97 && string[i]<=122){
           
            string[i] = string[i] - 32;
        }
        
        //upper to lower
        else if((int)string[i]>=65 && string[i]<=90){
            
            string[i] = string[i] + 32;
        }
    }
    
    printf("\nConverted string: %s\n",string);
    
    return 0;
}