Why am I getting such accurate results from `filesize`?

Question

When I run this code:

<?php
$handle = fopen('/tmp/lolwut', 'w') or die("Cannot open File");    
fwrite($handle, "1234567890");
fclose($handle);

print_r(filesize('/tmp/lolwut'));
?>

I get the result 10, which is the correct number of characters in the file.

However, because filesystem blocks are much larger than this, I expected the file size to be "rounded up" to more like 512 bytes or even 1KB. Why is it not?

Answer 1

Do not confuse "file size" for "file size on disk"; PHP's filesize function gives you the former, not the latter.

Although not explicitly documented as such, filesize is basically implemented in terms of stat, and on Linux stat makes a distinction between filesize and "file size on disk":

All of these system calls return a stat structure, which contains the following fields:

struct stat {
    // [...]
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for file system I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    // [...]
};

The value you're expecting is st_blocks * st_blksize, but the "true" filesize st_size is available regardless.

(This appears to be the case on Windows, too.)