Question

I have the code below:

#include <stdio.h>
#include <unistd.h>

int main () {

    int fd = open("filename.dat", O_CREAT|O_WRONLY|O_TRUNC, 0600);
    int result = write(fd, "abcdefghijklmnopqrstuvxz", 100);
    printf("\n\nfd = %d, result = %d, errno = %d", fd, result, errno);
    close(fd);
    return 0;
}

I am trying to understand what happens when I try to write more bytes to a file than I have available. So I am calling write and asking the program to write 100 bytes while I have much less than that. The result: a bunch of stuff from stdout ends up on filename.dat. If instead of 100 I use strlen("abcdefghijklmnopqrstuvxz"), I get the desired result. My question then is: why is the program trying to write beyond the '\0' character on my string? Is there some undefined behavior going on here?

Answer 1

My question then is: why is the program trying to write beyond the
‘\0’ character on my string?

The function write(2) doesn’t care about 0-terminators. It actually doesn’t care about buffer contents at all: it will try to write as many bytes as you tell it.

Is there some undefined behavior going on here

Of course, trying to write more than you have might incur the wrath of the OS who could decide to terminate your process if it touches inaccessible memory.