How to Correctly Convert Permission Values in os.FileMode for Go

Understanding os.FileMode Function in Go: Converting Permissions from Integers

The os.FileMode function in Go allows developers to set file permission flags. However, questions have arisen about how it handles permission conversions from integers, octal numbers, and other formats.

Originally reported as a permission bug involving decimal and octal values, the underlying issue stems from the base format of the input number. Go treats all input to os.FileMode as integers, regardless of any prefix or formatting.

To clarify, calling os.FileMode(700) does not result in octal interpretation but retains the integer value 700. Despite the expectation of '--w-rwxr--' permissions (274 in octal), the actual result is '--w-r-xr--' (254 in octal).

To address this, always specify integers as octal values by prefixing them with 0. In the example below, os.FileMode(0700) correctly sets the permissions as expected: '-rwx------' (700 in octal).

Alternatively, converting the decimal value 700 to octal manually using the following code:

<code class="go">mode := uint(0) // 0 is the octal separator
mode |= 1 << 6 // read permission for owner
mode |= 1 << 5 // write permission for owner
mode |= 1 << 3 // execute permission for owner

fmt.Println(os.FileMode(mode).String()) // Output: -rwx------</code>

The go docs highlight that the FileMode type is a uint32, with the nine LSB representing file permissions and the top 12 bits indicating special file features. Using decimal values in os.FileMode without the leading octal separator may trigger unexpected behaviors due to base interpretation.

Therefore, to avoid confusion and ensure correct permission settings, always follow these guidelines:

• Treat input to os.FileMode as integers and avoid using decimal numbers without the octal separator prefix.
• Use os.FileMode(0700) or the equivalent integer-to-octal conversion method to specify octal permission values.

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