How Does os.FileMode Convert Permissions Before Setting Flags?

How os.FileMode Converts Permissions Before Setting Flags

Original Concern

When using the os.FileMode function with octal or decimal numbers, the resulting file permissions do not always seem to match the expected behavior. Specifically, passing a decimal number (without leading zero) results in different file attributes than passing the octal equivalent.

Conversion Logic

os.FileMode accepts an integer as input and internally represents it as a 32-bit unsigned integer. The nine least significant bits correspond to Unix file permissions, while the remaining 12 bits are unused. When converting an octal number to an integer, the language specification interprets the number as base 8 if it begins with a leading zero ("0"). Otherwise, it interprets the number as base 10.

Example

Consider the octal number "0700" which represents the permissions "rwx------".

• Passing "0700" to os.FileMode will result in the correct permissions being set.
• Passing "700" (without leading zero) to os.FileMode will also result in the correct permissions being set, as the language specification automatically interprets it as octal because it starts with a digit.
• Passing "1274" (the decimal representation of "0700") to os.FileMode will result in incorrect permissions being set. This is because the language specification interprets "1274" as a base 10 number.

Additional Note

The unused 12 bits in the os.FileMode representation indicate special file features. However, these are not relevant for basic file permissions.

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