Why does write continuously leave 4K bytes in the buffer?
You can find the answer in man 2 write:
It is not an error if this number is smaller than the number of bytes requested; this may happen for example because the disk device was filled.
And from the write() man page description:
ssize_t write(int fd, const void *buf, size_t count);According to POSIX.1, if
countis greater thanSSIZE_MAX, the result is implementation-defined; see NOTES for the upper limit on Linux.NOTES
On Linux,
write()(and similar system calls) will transfer at most0x7ffff000(2,147,479,552) bytes, returning the number of bytes actually transferred. (This is true on both 32-bit and 64-bit systems.)