int setvbuf(FILE* stream, char* buffer, int mode, size_t size);

#include 
#include 

#define SIZE 1024

using namespace std;

int main()
{
    char buffer[SIZE] = "...";
    char str[] = "This is first line\nThis is second line";
    FILE *fp = fopen("test.txt","wb+");

    /* no buffering, buffer remains unchanged */
    setvbuf(fp,buffer,_IONBF,SIZE);
    fwrite(str, sizeof(str), 1, fp);
    cout << buffer << endl;

    /* line buffering, only a single line is buffered */
    setvbuf(fp,buffer,_IOLBF,SIZE);
    fwrite(str, sizeof(str), 1, fp);
    cout << buffer << endl;

    /* full buffering, all the contents are buffered */
    setvbuf(fp,buffer,_IOFBF,SIZE);
    fwrite(str, sizeof(str), 1, fp);
    cout << buffer << endl;

    fclose(fp);
    return 0;
}

...
This is second line
This is first line
This is second line