When Do File Permissions Set by os.FileMode Differ from Expectations?

Discrepancies in File Permissions Set by os.FileMode

Intro

When using os.FileMode to set file permissions, users may encounter discrepancies between the expected and actual file modes. This article delves into the intricacies of converting permissions from various representations into file flags.

Issues with Decimal and Octal Representations

The use of decimal numbers without leading zeros, such as 700, is treated differently from octal representations like 0700. When 700 is interpreted without the leading zero, it is not seen as an octal number. Instead, it is parsed as an integer literal with the value 448 (7 64 0 8 0 * 1). When converted to binary using this integer representation, the resulting value is 1-010-111-100. The extra bit in the leading position, which is not part of the expected 9-bit Linux file mode, causes discrepancies.

Interpretation of Octal Representations

Go's os.FileMode expects octal representations of file modes. When an octal representation is used, such as 0700, it is interpreted as a base-8 number, resulting in a value of 448. The 9 least significant bits of this value map to file permissions, while the remaining bits are set and indicate special file features that are not relevant for this discussion.

Impact on File Permissions

The confusion arises because the 1-010-111-100 binary representation of 700 decimal does not match the expected permissions 0700. This is due to the extra leading bit mentioned earlier. In this case, it results in permissions of 0254 (-rw-r-xr-) instead of 0274 (-rwx------) as one might expect.

Resolution

To resolve this issue, it is essential to use leading zeros when specifying octal numbers for FileMode. This ensures that Go interprets the input as an octal value and not as a decimal number. By following this guideline, you can avoid the discrepancies in file permissions.

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