Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer p is accomplished 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 your program's
native language, and allowing gdb to output values in a manner
consistent with the syntax of your program's native language. The
language you use to build expressions is called the working
language.
14.1. Switching between source languages
There are two ways to control the working language--either have gdb
set it automatically, or select it manually yourself. You can use the
set language command for either purpose. On startup, gdb
defaults to setting the language automatically. The working language is
used to determine how expressions you type 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.
Refer to Section 14.2 Displaying the language.
This is most commonly a problem when you use a program, such
as cfront or f2c, that generates 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.
14.1.1. 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
- .m
Objective-C source file
- .f, .F
Fortran source file
- .mod
Modula-2 source file
- .s, .S
Assembler source file. This actually behaves almost like C, but
gdb does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename
extension.
Refer to Section 14.2 Displaying the language.
14.1.2. Setting the working language
If you allow gdb to set the language automatically,
expressions are interpreted the same way in your debugging session and
your program.
If you wish, you may set the language manually. To do this, issue the
command set language lang, where lang is the name of
a language, such as
c or modula-2.
For a list of the supported languages, type set language.
Setting the language manually prevents gdb from updating the working
language automatically. This can lead to confusion if you try
to debug a program when the working language is not the same as the
source language, when an expression is acceptable to both
languages--but means different things. For instance, if the current
source file were written in C, and gdb was parsing Modula-2, a
command such as:
might not have the effect you intended. In C, this means to add
b and c and place the result in a. The result
printed would be the value of a. In Modula-2, this means to compare
a to the result of b+c, yielding a BOOLEAN value.
14.1.3. Having gdb infer the source language
To have gdb set the working language automatically, use
set language local or set language auto. gdb
then infers the working language. That is, when your program stops in a
frame (usually by encountering a breakpoint), gdb sets the
working language to the language recorded for the function in that
frame. If the language for a frame is unknown (that is, if the function
or block corresponding to the frame was defined in a source file that
does not have a recognized extension), the current working language is
not changed, and gdb issues a warning.
This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using set language auto in this
case frees you from having to set the working language manually.
