[v2] Fix formatted print of functions
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Commit Message
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.
v2: Fixed all the mistakes in the tests.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34685
---
gdb/testsuite/gdb.base/printcmds.exp | 13 +++++++++++++
gdb/testsuite/gdb.cp/printmethod.exp | 11 ++++++++++-
gdb/valprint.c | 8 ++++++++
3 files changed, 31 insertions(+), 1 deletion(-)
base-commit: 9decc44bc67d9fb4248c2afe4fde77174e8abf80
Comments
On 9/30/26 2:03 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
> $2 = 0x41
>
> This doesn't seem useful or intuitive to me.
>
Agreed.
While reviewing this patch, I stumbled on some inconsistency on handling
ptype *main vs p *main, which I filed here (
https://sourceware.org/bugzilla/show_bug.cgi?id=34700 ).
> This patch changes gdb to instead simply print the function pointer
> using the specified format.
>
Reviewed-By: Tom de Vries <tdevries@suse.de>
Thanks,
- Tom
> Regression tested on x86-64 Fedora 43.
>
> v2: Fixed all the mistakes in the tests.
>
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34685
> ---
> gdb/testsuite/gdb.base/printcmds.exp | 13 +++++++++++++
> gdb/testsuite/gdb.cp/printmethod.exp | 11 ++++++++++-
> gdb/valprint.c | 8 ++++++++
> 3 files changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/gdb/testsuite/gdb.base/printcmds.exp b/gdb/testsuite/gdb.base/printcmds.exp
> index 9de1bcdd1a3..2ae19e9e440 100644
> --- a/gdb/testsuite/gdb.base/printcmds.exp
> +++ b/gdb/testsuite/gdb.base/printcmds.exp
> @@ -1176,6 +1176,18 @@ proc test_printf_convenience_var {prefix} {
> }
> }
>
> +# Test formatted print with a function. See PR gdb/34685.
> +proc test_function_print {} {
> + with_test_prefix function {
> + gdb_test_no_output "set print address on"
> + set out [get_valueof "" main ""]
> + gdb_test_no_output "set print address off"
> + }
> + regexp -- [subst_vars {^.* ($::hex) .*$}] $out ignore value
> + gdb_test "print/x main" " = $value"
> + gdb_test "print/d main" " = [expr {$value}]"
> +}
> +
> clean_restart
>
> gdb_test "print \$pc" "No registers\\."
> @@ -1267,3 +1279,4 @@ test_print_symbol
> test_repeat_bytes
> test_radices
> test_repeat_bytes_limit
> +test_function_print
> diff --git a/gdb/testsuite/gdb.cp/printmethod.exp b/gdb/testsuite/gdb.cp/printmethod.exp
> index fdc1c91bf23..0da08b1f7ee 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
@@ -1176,6 +1176,18 @@ proc test_printf_convenience_var {prefix} {
}
}
+# Test formatted print with a function. See PR gdb/34685.
+proc test_function_print {} {
+ with_test_prefix function {
+ gdb_test_no_output "set print address on"
+ set out [get_valueof "" main ""]
+ gdb_test_no_output "set print address off"
+ }
+ regexp -- [subst_vars {^.* ($::hex) .*$}] $out ignore value
+ gdb_test "print/x main" " = $value"
+ gdb_test "print/d main" " = [expr {$value}]"
+}
+
clean_restart
gdb_test "print \$pc" "No registers\\."
@@ -1267,3 +1279,4 @@ test_print_symbol
test_repeat_bytes
test_radices
test_repeat_bytes_limit
+test_function_print
@@ -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
@@ -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. */