Debugging with GDB (September 2007)

Chapter 9: Using GDB with Different Languages 79
9 Using GDB with Different Languages
Although programming languages generally have common aspects, they are rarely ex-
pressed in the same manner. For instance, in ANSI C, dereferencing a pointer p is accom-
plished by *p, but in Modula-2, it is accomplished by p^. Values can also be represented
(and displayed) differently. Hex numbers in C appear as ‘0x1ae’, while in Modula-2 they
appear as ‘1AEH’.
Language-specific information is built into GDB for some languages, allowing you to
express operations like the above in the native language of your program, and allowing
GDB to output values in a manner consistent with the syntax of the native language. The
language you use to build expressions is called the working language.
9.1 Switching between source languages
There are two ways to control the working language. You can have GDB set it automat-
ically, or you can select it manually. You can use the set language command for either
purpose. On startup, GDB sets the default language automatically. The working language
is used to determine how expressions are interpreted, how values are printed, etc.
In addition to the working language, every source file that GDB knows about has its
own working language. For some object file formats, the compiler might indicate which
language a particular source file is in. However, most of the time GDB infers the language
from the name of the file. The language of a source file controls whether C++ names are
demangled—this way backtrace can show each frame appropriately for its own language.
There is no way to set the language of a source file from within GDB, but you can set the
language associated with a filename extension. See Section 9.2 [Displaying the language],
page 80.
This is a common problem when you use a program, such as cfront or f2c, that gen-
erates C but is written in another language. In that case, make the program use #line
directives in its C output; that way GDB will know the correct language of the source code
of the original program, and will display that source code, not the generated C code.
9.1.1 List of filename extensions and languages
If a source file name ends in one of the following extensions, then GDB infers that its
language is the one indicated.
‘.c’ C source file
‘.C’
‘.cc’
‘.cp’
‘.cpp’
‘.cxx’
‘.c++’ C++ source file