C prog - need to check a part of string while user enters input

POSIX assumed - you can use the standard terminal control interface declared in <termios.h> (error checking omitted for clarity):

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

static void init_getc_unbuf()
{
    struct termios term = { 0 };
    tcgetattr(0, &term);

    term.c_lflag &= ~ICANON;
    term.c_cc[VMIN] = 1;
    term.c_cc[VTIME] = 0;
    tcsetattr(0, TCSANOW, &term);
}

int getc_unbuf()
{
    char c;
    static int initted = 0;
    if (!initted) {
        init_getc_unbuf();
        initted = 1;
    }

    read(STDIN_FILENO, &c, 1);
    return c;
}

int main(int argc, char *argv[])
{
    char line[128];
    char buf[128] = { 0 };
    char *p = buf;

    printf("Enter string to search for:\n");
    fgets(line, sizeof(line), stdin);
    char *nl = strchr(line, '\n');
    if (nl != NULL) *nl = 0;

    printf("Now start typing:\n");

    do {
        *p++ = getc_unbuf();
    } while(strstr(buf, line) == NULL);

    printf("\nThe string you are searching for has been entered.\n");
    return 0;
}

The only way you can possibly achieve "checking while the user types" is if you use "raw" input methods. Unfortunately, that's non-trivial, and depends on the actual system you are running on - it's COMPLETELY different for Windows and Linux, and slightly different from Linux if you are using, say, MacOS or Solaris.

The principle of "raw" input is that the line-editing and buffering is turned off, and the application will get every character as it is typed, instead of "cooked" mode where the data is held in a buffer until the user hits enter.

I'm sure a bit of googling will allow you to find how to do it on your OS - searching for "How to read input without enter on -insert-OS-here" should do it.

Beware however that you will most likely have to deal with all the line-editing - e.g. hitting backspace will just send "backspace" to your program, not erase the last typed character. Backspace is one of my "most hit keys"...

Then all you need is a state machine to track where you are in the sequence of t, o, g and o and set a flag when you hit the o after g. If something else comes in between those characters, obviously the state machine needs to restart.

For Windows, you could do:

#include <stdio.h>
#include <conio.h>

int main()
{
    char c;
    int flag=0,state=0;

    while(flag == 0)
    {
        c = getch();
        if(state==0 && c=='t') state = 1;
        else if(state==1 && c=='o') state = 2;
        else if(state==2 && c=='g') state = 3;
        else if(state==3 && c=='o') flag = 1;
        else state = 0;
    }
    printf("togo entered");

    return 0;
}