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fmt: Scanf supports %X for integers but not strings. [Patch+Tests included] #12940

benthor opened this issue Oct 14, 2015 · 4 comments

fmt: Scanf supports %X for integers but not strings. [Patch+Tests included] #12940

benthor opened this issue Oct 14, 2015 · 4 comments


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@benthor benthor commented Oct 14, 2015

The fmt Documentation states two things:

  • Firstly, for Printing:
String and slice of bytes (treated equivalently with these verbs):

%s  the uninterpreted bytes of the string or slice
%q  a double-quoted string safely escaped with Go syntax
%x  base 16, lower-case, two characters per byte
%X  base 16, upper-case, two characters per byte
  • Secondly, for Scanning:
The formats behave analogously to those of Printf with the following exceptions:

%p is not implemented
%T is not implemented
%e %E %f %F %g %G are all equivalent and scan any floating point or complex value
%s and %v on strings scan a space-delimited token
Flags # and + are not implemented.

(Note how %X is not listed among the exceptions)

So given the following:

var (
    hex = "48C3A43F"
    b   []byte
    i   int
    s   string

fmt.Sscanf(hex, "%X", &b)
fmt.Sscanf(hex, "%X", &i)
fmt.Sscanf(hex, "%X", &s)
fmt.Println("Bytes:  ", b)
fmt.Println("Integer:", i)
fmt.Println("String: ", s)

one would assume that b, i and s now respectively contain byte and integer representations of the hex-value 48C3A43F. However, this is not the case! Instead, the following output is generated:

Bytes:   []
Integer: 1220781119

Only the integer variable gets correctly assigned.

But: Substituting "%X" with lower-case "%x" in the example above gives the desired output:

Bytes:   [72 195 164 63]
Integer: 1220781119
String:  Hä?

I conclude that this behavior is a bug. Either the verb "%X" should be explicitly listed as an exception in the Scanning-documentation or it should be fully supported for all relevant types.

I therefore submit the following patch (including additional relevant tests) which adds support for upper-case "%X" when scanning into []byte or strings:

diff --git a/src/fmt/scan.go b/src/fmt/scan.go
index e3e0fd0..68cb8bb 100644
--- a/src/fmt/scan.go
+++ b/src/fmt/scan.go
@@ -813,7 +813,7 @@ func (s *ss) scanComplex(verb rune, n int) complex128 {
 // convertString returns the string represented by the next input characters.
 // The format of the input is determined by the verb.
 func (s *ss) convertString(verb rune) (str string) {
-   if !s.okVerb(verb, "svqx", "string") {
+   if !s.okVerb(verb, "svqxX", "string") {
        return ""
@@ -821,7 +821,7 @@ func (s *ss) convertString(verb rune) (str string) {
    switch verb {
    case 'q':
        str = s.quotedString()
-   case 'x':
+   case 'x', 'X': // s.hexString() supports both upper and lower-case hex strings, so we can support both verbs here.
        str = s.hexString()
        str = string(s.token(true, notSpace)) // %s and %v just return the next word
diff --git a/src/fmt/scan_test.go b/src/fmt/scan_test.go
index 334c4a6..1924c02 100644
--- a/src/fmt/scan_test.go
+++ b/src/fmt/scan_test.go
@@ -255,12 +255,14 @@ var scanfTests = []ScanfTest{
    // Strings
    {"%s", "using-%s\n", &stringVal, "using-%s"},
    {"%x", "7573696e672d2578\n", &stringVal, "using-%x"},
+   {"%X", "7573696E672D2558\n", &stringVal, "using-%X"},
    {"%q", `"quoted\twith\\do\u0075bl\x65s"` + "\n", &stringVal, "quoted\twith\\doubles"},
    {"%q", "`quoted with backs`\n", &stringVal, "quoted with backs"},

    // Byte slices
    {"%s", "bytes-%s\n", &bytesVal, []byte("bytes-%s")},
    {"%x", "62797465732d2578\n", &bytesVal, []byte("bytes-%x")},
+   {"%X", "62797465732D2558\n", &bytesVal, []byte("bytes-%X")},
    {"%q", `"bytes\rwith\vdo\u0075bl\x65s"` + "\n", &bytesVal, []byte("bytes\rwith\vdoubles")},
    {"%q", "`bytes with backs`\n", &bytesVal, []byte("bytes with backs")},
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@nodirt nodirt commented Oct 14, 2015

Normally patches are sent on Gerrit.

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@benthor benthor commented Oct 14, 2015

Thanks will do

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@gopherbot gopherbot commented Oct 14, 2015

CL mentions this issue.

@adg adg closed this in 26fe24c Oct 20, 2015
@golang golang locked and limited conversation to collaborators Oct 24, 2016
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