GDB/MI: Add new "--language LANG" command option.

Frontend sometimes need to evaluate expressions that are
language-specific. For instance, Eclipse uses the following
expression to determine the size of an address on the target:

    -data-evaluate-expression "sizeof (void*)"

Unfortunately, if the main of the program being debugged is not C,
this may not work. For instance, if the main is in Ada, you get...

    -data-evaluate-expression "sizeof (void*)"
    ^error,msg="No definition of \"sizeof\" in current context."

... and apparently decides to stop the debugging session as a result.
The  recommendation sent was to specifically set the language to C
before trying to evaluate the expression.  Something such as:

    1. save current language
    2. set language c
    3. -data-evaluate-expression "sizeof (void*)"
    4. Restore language

This has the same disadvantages as the ones outlined in the "Context
Management" section of the GDB/MI documentation regarding setting
the current thread or the current frame, thus recommending the use of
general command-line switches such as --frame, or --thread instead.

This patch follows the same steps for the language, adding a similar
new command option: --language LANG. Example of use:

    -data-evaluate-expression --language c "sizeof (void*)"
    ^done,value="4"

gdb/ChangeLog:

        * mi/mi-parse.h (struct mi_parse) <language>: New field.
        * mi/mi-main.c (mi_cmd_execute): Temporarily set language to
        PARSE->LANGUAGE during command execution, if set.
        * mi/mi-parse.c: Add "language.h" #include.
        (mi_parse): Add parsing of "--language" command option.

        * NEWS: Add entry mentioning the new "--language" command option.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-language.exp: New file.

gdb/doc/ChangeLog:

        * gdb.texinfo (Show): Add xref anchor for "show language" command.
        (Context management): Place current subsection text into its own
        subsubsection.  Add new subsubsection describing the "--language"
        command option.

2 files changed, 29 insertions, 0 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 968201e..6ffdefa 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,10 @@
+2013-11-14  Joel Brobecker  <brobecker@adacore.com>
+
+	* gdb.texinfo (Show): Add xref anchor for "show language" command.
+	(Context management): Place current subsection text into its own
+	subsubsection.  Add new subsubsection describing the "--language"
+	command option.
+
 2013-11-13  Joel Brobecker  <brobecker@adacore.com>
 
 	* gdb.texinfo (GDB/MI Miscellaneous Commands): Delete
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 84acd5c..a164785 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -13340,6 +13340,7 @@ working language, and also what language source files were written in.
 
 @table @code
 @item show language
+@anchor{show language}
 @kindex show language
 Display the current working language.  This is the
 language you can use with commands such as @code{print} to
@@ -28816,6 +28817,8 @@ the user interface.
 @node Context management
 @subsection Context management
 
+@subsubsection Threads and Frames
+
 In most cases when @value{GDBN} accesses the target, this access is
 done in context of a specific thread and frame (@pxref{Frames}).
 Often, even when accessing global data, the target requires that a thread
@@ -28866,6 +28869,25 @@ all subsequent commands.  No frontend is known to do this exactly
 right, so it is suggested to just always pass the @samp{--thread} and
 @samp{--frame} options.
 
+@subsubsection Language
+
+The execution of several commands depends on which language is selected.
+By default, the current language (@pxref{show language}) is used.
+But for commands known to be language-sensitive, it is recommended
+to use the @samp{--language} option.  This option takes one argument,
+which is the name of the language to use while executing the command.
+For instance:
+
+@smallexample
+-data-evaluate-expression --language c "sizeof (void*)"
+^done,value="4"
+(gdb) 
+@end smallexample
+
+The valid language names are the same names accepted by the
+@samp{set language} command (@pxref{Manually}), excluding @samp{auto},
+@samp{local} or @samp{unknown}.
+
 @node Asynchronous and non-stop modes
 @subsection Asynchronous command execution and non-stop mode