> For the complete documentation index, see [llms.txt](https://n3r.gitbook.io/6e3372/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://n3r.gitbook.io/6e3372/ctf-writeups/pwnable.kr/toddlers-bottle/fd.md). # 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!!