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authorAndrew Burgess <aburgess@redhat.com>2023-03-23 12:12:38 +0000
committerAndrew Burgess <aburgess@redhat.com>2023-05-30 21:49:21 +0100
commit901682e4a4270fbd40ed4bfab0571c98d19c6557 (patch)
tree51b77d4d47c5367bba2df468e3940f39efb48a9b /gdb/NEWS
parentec517d1040bf02e15e9724b14df9dba73665883f (diff)
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gdb: add support for %V to printf command
This commit adds a new format for the printf and dprintf commands: '%V'. This new format takes any GDB expression and formats it as a string, just as GDB would for a 'print' command, e.g.: (gdb) print a1 $a = {2, 4, 6, 8, 10, 12, 14, 16, 18, 20} (gdb) printf "%V\n", a1 {2, 4, 6, 8, 10, 12, 14, 16, 18, 20} (gdb) It is also possible to pass the same options to %V as you might pass to the print command, e.g.: (gdb) print -elements 3 -- a1 $4 = {2, 4, 6...} (gdb) printf "%V[-elements 3]\n", a1 {2, 4, 6...} (gdb) This new feature would effectively replace an existing feature of GDB, the $_as_string builtin convenience function. However, the $_as_string function has a few problems which this new feature solves: 1. $_as_string doesn't currently work when the inferior is not running, e.g: (gdb) printf "%s", $_as_string(a1) You can't do that without a process to debug. (gdb) The reason for this is that $_as_string returns a value object with string type. When we try to print this we call value_as_address, which ends up trying to push the string into the inferior's address space. Clearly we could solve this problem, the string data exists in GDB, so there's no reason why we have to push it into the inferior, but this is an existing problem that would need solving. 2. $_as_string suffers from the fact that C degrades arrays to pointers, e.g.: (gdb) printf "%s\n", $_as_string(a1) 0x404260 <a1> (gdb) The implementation of $_as_string is passed a gdb.Value object that is a pointer, it doesn't understand that it's actually an array. Solving this would be harder than issue #1 I think. The whole array to pointer transformation is part of our expression evaluation. And in most cases this is exactly what we want. It's not clear to me how we'd (easily) tell GDB that we didn't want this reduction in _some_ cases. But I'm sure this is solvable if we really wanted to. 3. $_as_string is a gdb.Function sub-class, and as such is passed gdb.Value objects. There's no super convenient way to pass formatting options to $_as_string. By this I mean that the new %V feature supports print formatting options. Ideally, we might want to add this feature to $_as_string, we might imagine it working something like: (gdb) printf "%s\n", $_as_string(a1, elements = 3, array_indexes = True) where the first item is the value to print, while the remaining options are the print formatting options. However, this relies on Python calling syntax, which isn't something that convenience functions handle. We could possibly rely on strictly positional arguments, like: (gdb) printf "%s\n", $_as_string(a1, 3, 1) But that's clearly terrible as there's far more print formatting options, and if you needed to set the 9th option you'd need to fill in all the previous options. And right now, the only way to pass these options to a gdb.Function is to have GDB first convert them all into gdb.Value objects, which is really overkill for what we want. The new %V format solves all these problems: the string is computed and printed entirely on the GDB side, we are able to print arrays as actual arrays rather than pointers, and we can pass named format arguments. Finally, the $_as_string is sold in the manual as allowing users to print the string representation of flag enums, so given: enum flags { FLAG_A = (1 << 0), FLAG_B = (1 << 1), FLAG_C = (1 << 1) }; enum flags ff = FLAG_B; We can: (gdb) printf "%s\n", $_as_string(ff) FLAG_B This works just fine with %V too: (gdb) printf "%V\n", ff FLAG_B So all functionality of $_as_string is replaced by %V. I'm not proposing to remove $_as_string, there might be users currently depending on it, but I am proposing that we don't push $_as_string in the documentation. As %V is a feature of printf, GDB's dprintf breakpoints naturally gain access to this feature too. dprintf breakpoints can be operated in three different styles 'gdb' (use GDB's printf), 'call' (call a function in the inferior), or 'agent' (perform the dprintf on the remote). The use of '%V' will work just fine when dprintf-style is 'gdb'. When dprintf-style is 'call' the format string and arguments are passed to an inferior function (printf by default). In this case GDB doesn't prevent use of '%V', but the documentation makes it clear that support for '%V' will depend on the inferior function being called. I chose this approach because the current implementation doesn't place any restrictions on the format string when operating in 'call' style. That is, the user might already be calling a function that supports custom print format specifiers (maybe including '%V') so, I claim, it would be wrong to block use of '%V' in this case. The documentation does make it clear that users shouldn't expect this to "just work" though. When dprintf-style is 'agent' then GDB does no support the use of '%V' (right now). This is handled at the point when GDB tries to process the format string and send the dprintf command to the remote, here's an example: Reading symbols from /tmp/hello.x... (gdb) dprintf call_me, "%V", a1 Dprintf 1 at 0x401152: file /tmp/hello.c, line 8. (gdb) set sysroot / (gdb) target remote | gdbserver --once - /tmp/hello.x Remote debugging using | gdbserver --once - /tmp/hello.x stdin/stdout redirected Process /tmp/hello.x created; pid = 3088822 Remote debugging using stdio Reading symbols from /lib64/ld-linux-x86-64.so.2... (No debugging symbols found in /lib64/ld-linux-x86-64.so.2) 0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2 (gdb) set dprintf-style agent (gdb) c Continuing. Unrecognized format specifier 'V' in printf Command aborted. (gdb) This is exactly how GDB would handle any other invalid format specifier, for example: Reading symbols from /tmp/hello.x... (gdb) dprintf call_me, "%Q", a1 Dprintf 1 at 0x401152: file /tmp/hello.c, line 8. (gdb) set sysroot / (gdb) target remote | gdbserver --once - /tmp/hello.x Remote debugging using | gdbserver --once - /tmp/hello.x stdin/stdout redirected Process /tmp/hello.x created; pid = 3089193 Remote debugging using stdio Reading symbols from /lib64/ld-linux-x86-64.so.2... (No debugging symbols found in /lib64/ld-linux-x86-64.so.2) 0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2 (gdb) set dprintf-style agent (gdb) c Continuing. Unrecognized format specifier 'Q' in printf Command aborted. (gdb) The error message isn't the greatest, but improving that can be put off for another day I hope. Reviewed-By: Eli Zaretskii <eliz@gnu.org> Acked-By: Simon Marchi <simon.marchi@efficios.com>
Diffstat (limited to 'gdb/NEWS')
-rw-r--r--gdb/NEWS11
1 files changed, 11 insertions, 0 deletions
diff --git a/gdb/NEWS b/gdb/NEWS
index d97e3c1..649a3a9 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -67,6 +67,17 @@
break foo thread 1 task 1
watch var thread 2 task 3
+* The printf command now accepts a '%V' output format which will
+ format an expression just as the 'print' command would. Print
+ options can be placed withing '[...]' after the '%V' to modify how
+ the value is printed. E.g:
+ printf "%V", some_array
+ printf "%V[-array-indexes on]", some_array
+ will print the array without, or with array indexes included, just
+ as the array would be printed by the 'print' command. This
+ functionality is also available for dprintf when dprintf-style is
+ 'gdb'.
+
* New commands
maintenance print record-instruction [ N ]