Very simple I/O - c

Question

So I am trying to do some very simple read/writes on a file. Since it's for an assignment I can't use more sophisticated functions using File*.
I can easily create a file and write to it but if I try to read back my content (it's the same content but my problem boils down to this) I don't get what I expect and I can't yet see why.
Here the code snippet that causes me problems:
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char** argv){
int fdisk = open("testfile.txt", O_RDWR | O_CREAT | O_APPEND);
char buff [20] = "Just a short text!!!";
write(fdisk, buff, sizeof(buff));
char buff2[20];
read((fdisk), buff2, sizeof(buff2));
printf("Context of deleted file: %c\n",buff[1]);
printf("Context of deleted file: %c\n",buff2[1]);
return 0;
}

Answer 1

When you create a file you need to specify the file access mode:
int fdisk = open("testfile.txt", O_RDWR | O_CREAT | O_APPEND, 0666);
Otherwise the access mode is some indeterminate value.
And before reading it back you need to rewind it:
lseek(fdisk, 0, SEEK_SET); // rewind

Answer 2

The problem is that the write call leaves the file descriptor pointing to just after the data written (so more writes will go after that rather than overwriting the same data), so the following read call tries to read data after that which was written, and probably gets nothing.

Answer 3

I think it is a combination of issues:
You are not rewinding or re-opening the file, so when you read you are always reading from the end of the file.
You are using append mode, so it will add data to the end of the file. This means that after the first run you will be writing data at the end of the file but always reading from the beginning (assuming you address the first problem).
You are not setting the permissions, so you get random file permissions and the file may not be readable after creating it.
Your print statement is only printing the second character from each buffer, rather than a full string.
Here is a minimal working example. This compiles and runs with the expected results on my machine.
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char** argv){
int fdisk = open("testfile.txt", O_RDWR | O_CREAT, 0666);
char buff[] = "Just a short text!!!";
write(fdisk, buff, sizeof(buff));
lseek(fdisk, 0, SEEK_SET);
char buff2[sizeof(buff)];
read((fdisk), buff2, sizeof(buff2));
printf("Context of deleted file: %s\n",buff);
printf("Context of deleted file: %s\n",buff2);
return 0;
}
As a final note, the functions you are using (read/write) all return values indicating whether the operation was successful. You should check them. They would have indicated that the read operation in your problem was not actually reading any data (because it was at the end of the file).