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fmt: Sscanf %c skips a blank in Go1.5; not in Go1.3.3, nor in C #12275

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strickyak opened this issue Aug 23, 2015 · 4 comments
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fmt: Sscanf %c skips a blank in Go1.5; not in Go1.3.3, nor in C #12275

strickyak opened this issue Aug 23, 2015 · 4 comments
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@strickyak
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@strickyak strickyak commented Aug 23, 2015

What version of Go are you using (go version)?

I found this when upgrading from Go 1.3.3 to Go 1.5

What operating system and processor architecture are you using?

GOARCH="amd64"
GOOS="linux"

What did you do?

$ cat scango.go

package main

import "fmt"

func main() {
    var x int

    n, err := fmt.Sscanf("A", "%c", &x)
    println("n=", n, "x=", x, "err=", err)

    n, err = fmt.Sscanf(" A", "%c", &x)
    println("n=", n, "x=", x, "err=", err)

    n, err = fmt.Sscanf(" ", "%c", &x)
    println("n=", n, "x=", x, "err=", err)
}

What did you expect to see?

I expected what Go 1.3.3 did: it always gets the first character in the scanned string, either "A" or " ":
$ go run scango.go
n= 1 x= 65 err= (0x0,0x0)
n= 1 x= 32 err= (0x0,0x0)
n= 1 x= 32 err= (0x0,0x0)

That's also what my C compiler does
/* gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04) */
$ cat scanc.c

#include <stdio.h>

main() {
  char x[33];
  int n = sscanf("A", "%c", x);
  printf("n=%d <%d>\n", n, *x);
  n = sscanf(" A", "%c", x);
  printf("n=%d <%d>\n", n, *x);
  n = sscanf(" ", "%c", x);
  printf("n=%d <%d>\n", n, *x);
}

$ gcc scanc.c
$ ./a.out
n=1 <65>
n=1 <32>
n=1 <32>

What did you see instead?

Under Go 1.5, it skips the blank, and advances to the "A" (in the second clause) or EOF (in the third clause):
$ go run scango.go
n= 1 x= 65 err= (0x0,0x0)
n= 1 x= 65 err= (0x0,0x0)
n= 0 x= 65 err= (0x7f555757b028,0xc82000a1d0)

@mikioh mikioh changed the title Sscanf %c skips a blank in Go1.5; not in Go1.3.3, nor in C fmt: Sscanf %c skips a blank in Go1.5; not in Go1.3.3, nor in C Aug 23, 2015
@mikioh
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@mikioh mikioh commented Aug 23, 2015

/CC @robpike

@robpike
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@robpike robpike commented Aug 23, 2015

1.4 also gets a space. Arguably, it's doing what the docs say, but this new behavior, a consequence of a major internal restructuring, is clearly a change in behavior. Should perhaps be fixed. I wish instead we could just tell everyone not to use fmt.Scan*. I wish they had never been written.

@robpike robpike self-assigned this Aug 23, 2015
@robpike robpike added this to the Go1.6Early milestone Aug 23, 2015
@gopherbot
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@gopherbot gopherbot commented Aug 23, 2015

CL https://golang.org/cl/13821 mentions this issue.

@robpike robpike closed this in b1eec18 Aug 24, 2015
strickyak added a commit to strickyak/aphid that referenced this issue Sep 2, 2015
@adg adg modified the milestones: Go1.5.1, Go1.6Early Sep 8, 2015
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@gopherbot gopherbot commented Sep 8, 2015

CL https://golang.org/cl/14395 mentions this issue.

robpike added a commit that referenced this issue Sep 8, 2015
…kip spaces at %c

In short, %c should just give you the next rune, period.
Apparently this is the design. I use the term loosely.

Fixes #12275

Change-Id: I6f30bed442c0e88eac2244d465c7d151b29cf393
Reviewed-on: https://go-review.googlesource.com/13821
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/14395
@golang golang locked and limited conversation to collaborators Sep 8, 2016
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