Parsing Permissions for FileMode Function in Go

The os.FileMode function converts permissions from various formats before setting the flags. These formats include integers, octal representations, and potentially other forms.

When converting from integers to os.FileMode, the function does not explicitly discern whether the integer is represented in octal or in decimal. The decimal representation is interpreted as a regular integer.

The octal representation of a number is commonly used to represent file permissions in Unix-like systems. To explicitly specify an octal number in Go, a leading zero is prefixed to the literal. For example, 0700 represents the octal number 700.

One important aspect to note is that os.FileMode represents permissions as a 32-bit unsigned integer. The nine least significant bits of this integer correspond to the standard Unix file permission structure. The remaining 12 most significant bits are reserved for indicating special file features.

In your example, calling os.FileMode(700) should result in the binary value 1-010-111-100, which translates to the octal representation 274. However, you observed that the permissions on the created file were instead 254, which corresponds to the binary representation --w-r-xr--.

This discrepancy can be attributed to the fact that one leading bit in the tenth position is set in your binary representation. This bit is located in the unused territory of the os.FileMode representation.

To clarify further, let's break down the binary representation of the permissions:

• 1-010-111-100 (274 in octal): This represents the correct permissions you were expecting (--w-rwxr--).
• 1-010-111-100-000000000000 (extended binary representation): The leading bit in the tenth position is unset, resulting in the expected permissions.
• 1-010-111-100-000000000001 (with a leading bit set): This extended binary representation corresponds to the permissions you observed (--w-r-xr--).

Therefore, when converting from integers to os.FileMode, it is crucial to ensure that the integer representation is either a decimal number or an explicitly specified octal number. This will avoid any unintended conversions that could result in incorrect permissions being set on the file.