Write a function that removes all whitespace from a string
Error analysis:
This is because when the character constant is a space character, you cannot enter only two single quotes, you must also enter a space between the two single quotes; Otherwise, an error will be reported when compiling, prompting null characters, as shown in the figure:

After consulting materials, it is not allowed to put nothing between two single quotation marks. After modification, it is shown in the following figure:

Code:

#include <stdio.h>
#include <ctype.h>
#include <conio.h>
#include <stdlib.h>
void fun (char *str)
{
    int i=0;
    char *p=str;
    while(*p){
        if(*p!=' '){     //Del Space key
            str[i++]=*p;
        }
        p++;
    }
    str[i]='\0';        //add Terminator
}
main()
{
  char str[81];
  char Msg[]="Input a string:";
  int n;
  FILE *out;
  printf(Msg);
  gets(str);
  puts(str);
  fun(str); 
  printf("*** str: %s\n",str); 
  /******************************/
  out=fopen("out.dat","w");
  fun(Msg);
  fprintf(out,"%s",Msg);
  fclose(out);
  /******************************/
}