[PATCH] Fix formatted print of functions

[PATCH] Fix formatted print of functions

[Tom Tromey]

A user noticed that formatted printing of functions is strange.  That
is, printing a function normally does what you'd expect, but "print/d"
or "print/x" will, bizarrely, instead print the value of the first
byte of the function's body:

    (gdb) print main
    $1 = {<text variable, no debug info>} 0x393c20 <main>
    (gdb) print/x main
    $2 = 0x41

This doesn't seem useful or intuitive to me.

This patch changes gdb to instead simply print the function pointer
using the specified format.

Regression tested on x86-64 Fedora 43.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34685
---
 gdb/testsuite/gdb.base/printcmds.exp |  7 +++++++
 gdb/testsuite/gdb.cp/printmethod.exp | 11 ++++++++++-
 gdb/valprint.c                       |  8 ++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
index 9de1bcdd1a3..940622abb76 100644
--- a/gdb/testsuite/gdb.base/printcmds.exp
+++ b/gdb/testsuite/gdb.base/printcmds.exp
@@ -1176,6 +1176,13 @@ proc test_printf_convenience_var {prefix} {
     }
 }
 
+# Test formatted print with a function.  See PR gdb/34685.
+proc test_function_print {} {
+    set value [get_var_address main]
+    gdb_test "print/x main" " = $value"
+    gdb_test "print/d main" " = [expr {$value}]"
+}
+
 clean_restart
 
 gdb_test "print \$pc" "No registers\\."
diff --git a/gdb/testsuite/gdb.cp/printmethod.exp b/gdb/testsuite/gdb.cp/printmethod.exp
index fdc1c91bf23..e19646b0366 100644
--- a/gdb/testsuite/gdb.cp/printmethod.exp
+++ b/gdb/testsuite/gdb.cp/printmethod.exp
@@ -49,4 +49,13 @@ gdb_test "print theA->nonvirt" \
     "${::valnum_re} = {void \\(A \\* const\\)} $hex <A::nonvirt\\((void|)\\)>" \
     "print nonvirtual method."
 
-gdb_exit
+proc test_formatted {exp} {
+    set out [get_valueof "" $exp ""]
+    regexp -- [subst_vars {^.* ($::hex) .*$}] $out ignore value
+    gdb_test "print/x $exp = $value"
+    gdb_test "print/d $exp" " = [expr {$value}]"
+}
+
+# Test for PR gdb/34685.
+test_formatted A::nonvirt
+test_formatted theA->virt
diff --git a/gdb/valprint.c b/gdb/valprint.c
index 83a442dcef5..211bdca04f1 100644
--- a/gdb/valprint.c
+++ b/gdb/valprint.c
@@ -1300,6 +1300,14 @@ value_print_scalar_formatted (struct value *val,
 
   gdb_assert (val != NULL);
 
+  /* Treat a function or a method as if it were a pointer.  */
+  if (type->code () == TYPE_CODE_FUNC
+      || type->code () == TYPE_CODE_METHOD)
+    {
+      val = value_addr (val);
+      type = check_typedef (val->type ());
+    }
+
   /* If we get here with a string format, try again without it.  Go
      all the way back to the language printers, which may call us
      again.  */

base-commit: 9decc44bc67d9fb4248c2afe4fde77174e8abf80
-- 
2.55.0

Re: [PATCH] Fix formatted print of functions

[Simon Marchi]

On 9/29/26 1:37 PM, Tom Tromey wrote:
> A user noticed that formatted printing of functions is strange.  That
> is, printing a function normally does what you'd expect, but "print/d"
> or "print/x" will, bizarrely, instead print the value of the first
> byte of the function's body:
> 
>     (gdb) print main
>     $1 = {<text variable, no debug info>} 0x393c20 <main>
>     (gdb) print/x main
>     $2 = 0x41
> 
> This doesn't seem useful or intuitive to me.
> 
> This patch changes gdb to instead simply print the function pointer
> using the specified format.
> 
> Regression tested on x86-64 Fedora 43.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34685
> ---
>  gdb/testsuite/gdb.base/printcmds.exp |  7 +++++++
>  gdb/testsuite/gdb.cp/printmethod.exp | 11 ++++++++++-
>  gdb/valprint.c                       |  8 ++++++++
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
> index 9de1bcdd1a3..940622abb76 100644
> --- a/gdb/testsuite/gdb.base/printcmds.exp
> +++ b/gdb/testsuite/gdb.base/printcmds.exp
> @@ -1176,6 +1176,13 @@ proc test_printf_convenience_var {prefix} {
>      }
>  }
>  
> +# Test formatted print with a function.  See PR gdb/34685.
> +proc test_function_print {} {
> +    set value [get_var_address main]
> +    gdb_test "print/x main" " = $value"
> +    gdb_test "print/d main" " = [expr {$value}]"
> +}

This proc is never called.

> +
>  clean_restart
>  
>  gdb_test "print \$pc" "No registers\\."
> diff --git a/gdb/testsuite/gdb.cp/printmethod.exp b/gdb/testsuite/gdb.cp/printmethod.exp
> index fdc1c91bf23..e19646b0366 100644
> --- a/gdb/testsuite/gdb.cp/printmethod.exp
> +++ b/gdb/testsuite/gdb.cp/printmethod.exp
> @@ -49,4 +49,13 @@ gdb_test "print theA->nonvirt" \
>      "${::valnum_re} = {void \\(A \\* const\\)} $hex <A::nonvirt\\((void|)\\)>" \
>      "print nonvirtual method."
>  
> -gdb_exit
> +proc test_formatted {exp} {
> +    set out [get_valueof "" $exp ""]
> +    regexp -- [subst_vars {^.* ($::hex) .*$}] $out ignore value
> +    gdb_test "print/x $exp = $value"
> +    gdb_test "print/d $exp" " = [expr {$value}]"

The quotes are wrong on the first gdb_test.

Simon