While I think the grandparent is right about POSIX not supporting this, I think this should be possible in Linux:
1. Open the file with open(2), passing the O_TMPFILE flag. This creates a temporary, unnamed file. (As if you had opened it, then immediately unlinked it, but atomically.)
2. Write to the file.
3. Link the file into the filesystem with linkat(2), passing the AT_EMPTY_PATH flag (which tells linkat(2) to target a file descriptor that we pass it, not a path.)
(I'm not sure if step 3 helps in the case that you want to atomically replace the contents of an existing named file; I suspect that linkat(2) will error out because the file exists. Perhaps someday linkat(2) will grow a "atomic replace" flag.)
1. Open the file with open(2), passing the O_TMPFILE flag. This creates a temporary, unnamed file. (As if you had opened it, then immediately unlinked it, but atomically.)
2. Write to the file.
3. Link the file into the filesystem with linkat(2), passing the AT_EMPTY_PATH flag (which tells linkat(2) to target a file descriptor that we pass it, not a path.)
(I'm not sure if step 3 helps in the case that you want to atomically replace the contents of an existing named file; I suspect that linkat(2) will error out because the file exists. Perhaps someday linkat(2) will grow a "atomic replace" flag.)