Unix O_CREAT flag without mode specified

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The POSIX standard (IEEE 1003.1:2008) prototypes open() as:

int open(const char *path, int oflag, ...);

The section describing the behaviour of O_CREAT doesn’t say what will happen if you omit the necessary third argument, which means the behaviour is undefined – anything is possible.

In practice, the use of part of the stack that was intended to be stack frame or return address or something similar is quite likely – unto a reasonable approximation, that can be considered a random integer.

The POSIX 2008 standard has some interesting new (and useful) flags for open(), including:

  • O_FDCLOEXEC to specify close-on-exec at open.
  • O_DIRECTORY to specify that the file must be a directory.
  • O_NOFOLLOW to specify not to chase symlinks.

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