### Analysis

Reading the source code with `cat re.c` presents the following:

```c
#include <stdlib.h>

int win(){
int fd = open("./flag.txt",0);
sendfile(1,fd,0,100);
exit(0);
}

int main() {
srand(time(0));
int rand_num;
int user_num;

printf("Give me a number: ");
scanf("%d", &user_num);

rand_num = rand();

if (rand_num == user_num) {
printf("Correct \n ");
win();
}

printf("Wrong!\n");
printf("I randomly generated:\n%d", rand_num);
}
```

From line 22 (`if (rand_num == user_num)`), we can see that in order to fulfill the win condition, we have to input a string to the program which matches the randomly generated number. The `srand()` function used on line 13 seeds the pseudorandom number generator with the current time, meaning that the "random" number will be the same whenever executed in the same millisecond.

### Testing

This can be tested easily in Bash:
```bash
for _ in {1..9} do # Repeat 9 times
echo "test" | ./re # Run the program with a dummy input string
echo -e "\n" # Print a blank line to delimit each loop, for readability
done
```

This provides an output such as the following example:

```bash
Give me a number: Wrong!
I randomly generated:
1606725990

Give me a number: Wrong!
I randomly generated:
1606725990

Give me a number: Wrong!
I randomly generated:
1606725990

# <Repeats 6 more times>
```

### Exploitation

We can see that the randomly generated number is displayed on the third line each time, so we can extract just the line containing the number using `sed -n 3p`. Then, we can pipe that number back into the program, since the random number will be unchanged, assuming execution happens within the same millisecond.

Our final code is this:

```bash
echo "test" | ./re | sed -n 3p | ./re
```

Executing it will provide the following output:
```bash
Give me a number: Correct
SAH{Can_I_Have_Your_Number?}
```

### Flag

#### ? `SAH{Can_I_Have_Your_Number?}`