⇠ back to blog posts

Misusing go's `fmt.Sscanf`

Recently I was building a new website in go at $work and needed to do some URL parsing to grab some expected parameters in the URL. The URL was expected to look something like the following: /path/:id1/:id2, where I was trying to grab id1 and id2 out of the URL.

In the past, I’ve done something along the lines of:

splitPath := strings.Split(r.URL.Path, "/")

// error handling, etc.

// splitPath is ["", "path", ":id1", ":id2"],
// because of the way `strings.Split` works
id1 := splitPath[2]
id2 := splitPath[3]

However, I had come across the following code, from the source code of builds.sr.ht:

var (
  jobId int
  op    string
_, err := fmt.Sscanf(r.URL.Path, "/job/%d/%s", &jobId, &op)

fmt.Sscanf? Haven’t seen that in use often! But it seemed like a great fit for the exact problem I was solving.

So, I tried it:

var id1, id2 string
matchCount, err := fmt.Sscanf(r.URL.Path, "/path/%s/%s", &id1, &id2)
if err != nil {
    fmt.Printf("err: %s, match count: %d\n", err, matchCount)
    fmt.Printf("id1: %s, id2: %s", id1, id2)
} else {
    fmt.Printf("no err, id1: %s, id2: %s", id1, id2)

Here’s a slightly modified version on play.golang.org.

Spoiler: here's the output. err: unexpected EOF, match count: 1
id1: abc/def, id2:

So, the first match hit by the scan, because it’s %s, continues through the / character and picks up id2’s value as well, leaving nothing for id2.

So why did it work in the line of code shown above?

Because the %d format specifier only captures numeric characters, so the / character breaks up the scan.

I messed around with this approach for awhile, but ultimately gave up and went back to my approach using strings.Split. But, it’s a nice reminder that format strings in go can be used both for formatting output and for scanning input, even if there are some footguns attached.