Understanding the Interpretation of Non-Octal Integers in os.FileMode

How Does os.FileMode Handle Permission Conversions?

A common pitfall in Go programming is the subtle difference in permissions when using os.FileMode to set file permissions. Calling os.FileMode with integer values, such as 700, can lead to unexpected results.

Understanding Decimal vs. Octal Representations

os.FileMode accepts integer values for setting file permissions. However, there is a crucial distinction between decimal and octal representations. In Go, integer literals without a leading "0" are treated as decimal numbers. To specify an octal number, you must prefix it with "0," as in "0700."

Binary Representation and Leading Bits

The os.FileMode function maps the lowest 9 bits of an integer to the standard Unix file permission flags. Therefore, when using an integer representation, the leading bits are ignored. For example, the integer 700 (1-010-111-100 in binary) has the following permissions:

0700 -> -rwx------

Interpreting Non-Octal Integers

If an integer without the "0" prefix is provided to os.FileMode, it is interpreted as a decimal number. In this case, the leading bits are not ignored. The decimal number 700 translates to binary 1274, which is not a valid Unix permission code.

Results and Confusion

When calling os.FileMode(700), the result is "-w-r-xr--" (octal 0254) instead of the expected "-rwx------" (octal 0700). This is because the integer 700 is interpreted as a decimal and not octal.

To set file permissions correctly, you should always use octal representations with leading "0" when calling os.FileMode. For example, os.FileMode(0700) will produce the correct "-rwx------" permissions.

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