What permissions does a file written with fstream have?

Suppose I create a file for writing like this:

std::ofstream my_file("filename", std::ios_base::out | std::ios_base::trunc);

How are the permissions of this file determined? I've had a program run开发者_运维知识库ning overnight generating files about once a minute - some are 0644 but others are 0660, and there's nothing in my code that should make it change. (I've seen that some implementations allow a third argument to set the file permissions - I'm using gcc 4.1.2 which doesn't appear to support this).

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It depends on the umask.

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default attrs are set with umask command. it not explains your files attrs changes, but still may be useful for you