c - Confused with read() system call

Question

I am very confused with read() calls to get inotify events.

Here is the code:

#define EVENT_SIZE sizeof(inotify_event)
int fd = inotify_init();
int wd = inotify_add_watch(fd, dir, IN_MODIFY);
void* p = malloc(sizeof(EVENT_SIZE));
read(wd, p, (EVENT_SIZE + 10));

My test file is a.txt. The output after debug in gdb is:

{wd = 0, mask = 0, cookie = 0, len = 0, name = 0x558f05d002d0 ""}

Now, when I change the last line to read(fd, p, (EVENT_SIZE + 16));, the output that I get in gdb is:

{wd = 1, mask = 2, cookie = 0, len = 16, name = 0x5625cdd422d0 ".a.txt.swp"}

Q1. Why don't I get an overflow error because in both cases, I am writing more than the allocated buffer p?

Q2. If there is no error, then my first program should also work because my filename is less than 10, but it doesn't work and it only works with 16. What am I missing here?

compiler - g++ 9.3.0

os - ubuntu 20.04

Thank you.

question from:https://stackoverflow.com/questions/65860234/confused-with-read-system-call

Answer 1

1. The behaviour of writing outside the boundaries of an array is undefined. Additionally, read reads up to the given number of bytes.
2. ???