Background: I'm working in C (and very C-ish C ) on Linux. The parent process has an open file descriptor that it writes data to in a "sectioned" format. The child process uses it for this same purpose. As long as the child process is running, it is guaranteed that the parent will not attempt to write more data to its copy of the file descriptor. The child exits, the parent waits for it, and then it writes more data to the file.
It appears to be working correctly, but I'm still suspicious of it. Do I need to re-seek to the end in the parent? Are there any synchronization issues I need to handle?
CodePudding user response：
File descriptors come from Unix in general, and Linux in particular, and a lot of the behaviour is standardized by POSIX. You say that the parent and child processes share the same file descriptor. That's not possible; file descriptors are specific to one process.
Suppose the parent opens a file (assume the file descriptor is 3) and therefore there is also a new open file description; then the parent process forks. After the fork, each process has a separate file descriptor (but they're both using file descriptor 3), but they share the same open file description. Yes: 'open file descriptors' and 'open file descriptions' are different! Every open file descriptor has an open file description, but a single open file description can be associated with many open file descriptors, and those descriptors need not all be associated with the same process.
One of the critical bits of data in the open file description is the current position (for reading or writing — writing is what matters here). Consequently, when the child writes, the current position moves for both the parent and the child. Therefore, whenever the parent does write, it writes after the location where the child finished writing.
What you are seeing is guaranteed. At least under the circumstances where the parent opens the file and forks.
Note that in the scenario discussed, the O_APPEND flag was not needed or relevant. However, if you are worried about it, you could open the file with the O_APPEND flag, and then each normal write (not written via pwrite()) will write the data at the current end of the file. This will work even if the two processes do not share the same open file description.
POSIX specification:
CodePudding user response：
If you are talking about POSIX file descriptors, then each write call to a file descriptor is atomic and affects the underlying kernel resource object indepently of what other processes might do with file descriptors that refer to the same object. If two processes do write at approximately the same time, the operations will get ordered by the kernel with one happening completely (though it might write less data than requested) and then the other happening.
In your case, it sounds like you are synchronizing such that you know all parent writes happen either before the child has started (before fork) or after it has completed (after wait), which guarentees the ordering of the write calls.