If you consider using an existing utility, take a look at the GNU readline library which provides a pretty neat implementation of what you're looking for.
There some other helpful features like moving the cursor in your input, providing an input history and an input shell-like prompt.

This library's functionality works identical on different platforms.

As this example from Wikipedia shows, the key to indicate the completion can be set easily:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <readline/readline.h>
#include <readline/history.h>

int main()
{
    char* input, shell_prompt[100];

    // Configure readline to auto-complete paths when the tab key is hit.
    rl_bind_key('\t', rl_complete);

    for(;;) {
        // Create prompt string from user name and current working directory.
        snprintf(shell_prompt, sizeof(shell_prompt), "%s:%s $ ", getenv("USER"), getcwd(NULL, 1024));

        // Display prompt and read input (NB: input must be freed after use)...
        input = readline(shell_prompt);

        // Check for EOF.
        if (!input)
            break;

        // Add input to history.
        add_history(input);

        // Do stuff...

        // Free input.
        free(input);
    }
    return 0;
}

Because in C there is no portable solution for knowing what you have typed in a promt without pressing enter you need to read character by character.

• In windows you can use _getch() function to read one character without the need of pressing enter. The function is defined in conio.h
• In linux you can use getch() function.

Note: you may want to use getche() in order to echo the character back.

You will need to define a buffer where you will store character by character everything the user press, until the user hit enter or it press enter. When you find \t you will analyze the content from buffer and take a decision based on that.

The following code is a oversimplified example:

char c;
int i = 0;

 do {
     c = _getche();
         if (c == '\t') {
             // Autocomplete
             // Print the rest of the word here
         }
    else {
        buff[i++] = c;
    }

 } while (c != '\n' && c!='\r');
 buff[i] = '\0'; // Let's keep the standard

It's not clear whether your asking for completion performed by

1. The program itself while it is running. Presumably you would be implementing some custom command line.
2. The operating system command line (e.g., bash) before you've started your program and you just want to auto-complete the arguments passed to it

Your grep example suggests your asking about shell completion. Here is one example.