Node:Large executable, Next:, Previous:Large image, Up:Compiling

8.14 Why are DJGPP .exe files so large?

Q: I compiled a trivial "Hello world" program and got a 280KB executable file. That's ridiculously bloated!

Q: I switched to GCC 2.95.1, and my C++ executables are considerably larger than when compiled with GCC!

A: Did you link with -s switch to gcc, or run strip on the output of the linker? If not, the executable includes the debugging symbols, which makes it quite a lot larger. (It is not recommended to strip the symbols except when distributing production programs, because this makes debugging very hard indeed; that is why -s is not passed to gcc by default.)

A stripped "Hello world" program written in C should be about 42KB on disk; an analogous program written in C++ should be about 140KB on disk (the additional overhead is due to the C++ classes that are linked in to support cout).

C++ programs could be further bloated because the release of Binutils 2.8.1 was configured in a way that caused the assembler to put into the symbol table local labels generated when compiling code that uses exceptions. Later uploads of GNU Binutils should solve this problem, so consider upgrading to the latest

Some other compilers with which people keep comparing the size of DJGPP programs use shared libraries or DLLs, so the size of the executable doesn't include the libraries. If you have an immediate question "Why won't DJGPP use DLLs as well", read the following section.

In general, judging code sizes by looking at the size of "Hello" programs is meaningless, because such programs consist mostly of the startup code. The DJGPP startup code does many things in preparation for running a protected-mode program in a Posix-compliant environment. This includes switching the processor to protected mode (which requires a lot of code), wildcard expansion, long command-line support, and loading the environment from a disk file; these usually aren't available with other DOS compilers. Exception and signal handling (not available at all in v1.x), FPU detection and emulator loading (which were part of go32 in v1.x), are now also part of the startup code.

Most of the power of all these features goes wasted in "Hello" programs. There is no point in running all that code just to print a 15-byte string and exit. However, the overhead induced by the code needed to set up the DJGPP run-time environment is additive; the larger the program, the smaller the overhead relative to the program size. For non-trivial programs, the code produced by DJGPP is usually smaller than what other compilers produce. For example, the DJGPP version of the Povray program is smaller by about 200KB than the same program compiled with the Watcom compiler.

If your program doesn't need parts of the startup code, it can be made smaller by defining certain functions with empty bodies. These functions are __crt0_glob_function, __crt0_load_environment_file, and __crt0_setup_arguments. If you define empty substitutes for all three of these, and compile with -O2 -s, you can make the "Hello" program be 31KB on disk. These functions are documented in the DJGPP libc reference, which see. Here's an example of definitions for these functions that will make the startup code as small as it gets15:

  #include <crt0.h>

  char **__crt0_glob_function (char *arg) { return 0; }
  void   __crt0_load_environment_file (char *progname) { }
  void   __crt0_setup_arguments (void) { }

(To do this in a C++ program, prepend the extern "C" qualifier to each one of the three lines that define the substitute functions.)

Note that if you define an empty substitute for __crt0_setup_arguments, your program will not be able to access its command-line arguments via the argv[] array. So this is only recommended for programs which don't accept any arguments at all.

You can make your program image still smaller by compressing it with a compressor called DJP. DJP is a DJGPP-specific executable file compressor. It is fast and has no memory overhead. It also knows about DJGPP Dynamically Loaded Modules (DLM) technology. (Note that DJP before version 1.06 was incompatible with Binutils 2.8.1 and later16, so you should always use the latest DJP version available on SimTel.NET mirrors.)

DJP is not actively developed anymore; its successor is the UPX compressor, currently in beta testing. UPX is written by the same people who wrote DJP, compresses better, and supports a broader class of executable formats, including DOS .exe, .com and .sys, DJGPP's COFF, Watcom's LE, Win32 PE, and Linux's ELF. UPX is available via the Web.