Write() Syscall | 32 Bit OS Dev (in C)
28Expanding the write() syscall to work for open()-ed files and File Descriptors (FDs).
Brain was a bit slow on this one, hope that improves.
Errata:
- memcpy32() is wrong, it does not fill out the correct len % 4 number of bytes, or the right bytes at all in dest, from src. It was later changed to be:
for i = 0; i less than len / 4; i++
((uint32_t *)dst)[i] = ((uint32_t *)src)[i];
i *= 4; // NEW
for j = 0; j less than len % 4; i++, j++ // NEW
((uint8_t *)dst)[i] = ((uint8_t *)src)[i];
return dst;
Notes:
- One thing I should've added was a function to add a disk block to a file's data: allocate and add a bit in the data bitmap, and add that bit's disk block to the file's extents, extending the last extent currently used in the file. There is already code to do that in fs_impl.h/create_file() I think, but it would also be used in the write() syscall and possibly other places, so it should be extracted out into a function to make easier changes later.
Next video:
- adding a (buggy) read() syscall, to check that write() actually writes data to files
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Outline:
0:00 basic write() logic
2:07 start implementing write()
23:39 add helper function to update inode on disk
44:32 add no. of pages allocated to file table entries
57:01 add test for write()
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