Why is the `os.OpenFile` function throwing a 'bad file descriptor' error when appending to a log file in Go?

Bad File Descriptor in Golang While Appending to a Logging File

When attempting to append to a logging file within a Go routine, users may encounter the following error:

write ./log.log: bad file descriptor

Despite ensuring that the target file exists and has the appropriate permissions, the issue persists. Initial attempts to resolve the issue using a mutex failed.

Solution

The resolution lies in adding the O_WRONLY flag to the os.OpenFile call:

if f, err := os.OpenFile("./log.log", os.O_APPEND|os.O_WRONLY, os.ModeAppend); err != nil { /*[...]*/ }

Explanation

According to the Linux documentation on open, one of the following access modes must be included in the flags argument: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file in read-only, write-only, or read/write mode, respectively.

By default, Go's os.OpenFile call opens the file in read-only mode. As such, it is necessary to explicitly specify O_WRONLY to allow writing to the logging file.

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