# fd
*** ## Solution Given a C file. Let's do some basic checking on it. 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char buf[32];
int main(int argc, char* argv[], char* envp[]){
        if(argc<2){
                printf("pass argv[1] a number\n");
                return 0;
        }
        int fd = atoi( argv[1] ) - 0x1234;
        int len = 0;
        len = read(fd, buf, 32);
        if(!strcmp("LETMEWIN\n", buf)){
                printf("good job :)\n");
                system("/bin/cat flag");
                exit(0);
        }
        printf("learn about Linux file IO\n");
        return 0;

}
Based on the source code given, we can see there are 2 lines that are very interesting.
Line 10 * `atoi()` function in C converts a string to an integer * it takes `argv[1]` as its argument and substract it with **0x1234** which is **4660** in decimal * the result is stored in variable `fd`
Line 13 * it compares a strings in variable `buf` with "LETMEWIN\n" * that's it
Between the lines, we can see there is a `read()` function which takes file descriptor, some buffer and number of bytes. We need to make the **file descriptor equals 0** so that we can write a string to the program and pass the **if statement** in Line 13. So we input **4660** which then it will substract with 0x1234 and returns 0. Then write **LETMEWIN** to the program.
Yeah. Nice challenge btw :thumbsup: *** ## Flag mommy! I think I know what a file descriptor is!! --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://n3r.gitbook.io/6e3372/ctf-writeups/pwnable.kr/toddlers-bottle/fd.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.