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If you have ever used a p2p downloading software, they can download a file with multi-threading, and they created only one file, So I wonder how the threads write data into that file. Sequentially or in parallel?
Imagine that you want to dump a big database table to a file, and how to make this job faster?
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Multiple threads can read and write the same file in several situations: Multiple threads read the same file at the same time. In this case, there is no conflict. If multiple threads write the same file at the same time, write data will be lost.
While fread() and fwrite() are thread safe, the stream buffer represented by the FILE* is not. So you can have multiple threads accessing the same file, but not via the same FILE* - each thread must have its own, and the file to which they refer must be shareable - which is OS dependent.
In the same multithreaded process in a shared-memory multiprocessor environment, each thread in the process can run concurrently on a separate processor, resulting in parallel execution, which is true simultaneous execution.
Writing to the same file from multiple threads concurrently is not thread safe and may result in a race condition. Thread-safe means that writing or appending to the same file from more than one thread may result in a race condition.
You can use multiple threads writing a to a file e.g. a log file. but you have to co-ordinate your threads as @Thilo points out. Either you need to synchronize file access and only write whole record/lines, or you need to have a strategy for allocating regions of the file to different threads e.g. re-building a file with known offsets and sizes.
This is rarely done for performance reasons as most disk subsystems perform best when being written to sequentially and disk IO is the bottleneck. If CPU to create the record or line of text (or network IO) is the bottleneck it can help.
Image that you want to dump a big database table to a file, and how to make this job faster?
Writing it sequentially is likely to be the fastest.
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