using _flushall() seems to repair a code and I dont understand why

Question

I wrote a function that remove a given char from a string.

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

char* input_long(void);
void removeChar(char str[], char ch);

void main()
{
    char *str, ch;

    str = input_long();
    printf("\nplease enter a char to be removed from the string: ");
    scanf("%c", &ch);

    removeChar(str, ch);
    printf("the string after the removal is %s \n", str);

    free(str);
}

void removeChar(char str[], char ch) //this function removing a given char from a given string//
{
    int i, j = 0;
    for (i = 0; str[i] != '\0'; i++) //looping the string until its end//
    {
        if (str[i] != ch) //by that we ensure the char will be erased from the string //
        {
            str[j] = str[i];
            j++;
        }
    }
    str[j] = '\0';   //the end of the new string after the whole process//
}

char* input_long(void) //this function gets a string dynamically allocated//
{
    char tempstr[80], *str;

    printf("enter a string\n");
    gets(tempstr);
    str = (char*) malloc((strlen(tempstr) + 1) * sizeof(char));
    strcpy(str, tempstr);

    return str;
}

My code didnt run well; when I run it, it seemed to lead the statement which asks the user to enter a string. I repaired the code by adding _flushall() , and now the code is running well:

_flushall();
 str=input_long();
 printf("\nplease enter a char to be removed from the string: ");
 scanf("%c", &ch);

I dont really understand why adding that statement indeed repaired it.