#include<stdio.h>
int main(){
  int i=0,j=0;
  char *str1,*str2,*str3;
  puts("Enter first string");
  gets(str1);
  puts("Enter second string");
  gets(str2);
  printf("Before concatenation the strings are\n");
  puts(str1);
  puts(str2);
  while(*str1){
      str3[i++]=*str1++;
  }
  while(*str2){
      str3[i++]=*str2++;
  }
  str3[i]='\0';
  printf("After concatenation the strings are\n");
  puts(str3);
  return 0;
}

#include<stdio.h>
int main(){
   char str[20];
   int i=0;
   printf("Enter a string: ");
   gets(str);
   printf("%c",*str);
   while(str[i]!='\0'){
       if(str[i]==' '){
            i++;
            printf("%c",*(str+i));
       }
       i++;
   }
   return 0;
}
Sample output:
Enter a string: Robert De Niro
RDN

#include<string.h>
#include<stdio.h>
int main(){
  char *p;
  char s[20],s1[1];
  printf("\nEnter a string: ");
  scanf("%[^\n]",s);
  fflush(stdin);
  printf("\nEnter character: ");
  gets(s1);
  p=strpbrk(s,s1);
  printf("\nThe string from the given character is: %s",p);
  return 0;
}

#include<stdio.h>
#include<string.h>
int main(){
    char str[50];
    char *rev;
    printf("Enter any string : ");
    scanf("%s",str);
    rev = strrev(str);
   
    printf("Reverse string is : %s",rev);
   
    return 0;
}

#include<stdio.h>
int main(){
    char str[50];
    char rev[50];
    int i=-1,j=0;

    printf("Enter any string : ");
    scanf("%s",str);
   
    while(str[++i]!='\0');

    while(i>=0)
     rev[j++] = str[--i];

    rev[j]='\0';
  
    printf("Reverse of string is : %s",rev);
  
    return 0;
}
Sample output:
Enter any string : cquestionbank.blogspot.com
Reverse of string is : moc.topsgolb.knabnoitseuqc