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15.1.1 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.

Ada source file.
C source file
C++ source file
D source file
Objective-C source file
Fortran source file
Modula-2 source file
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. See Displaying the Language.