getting compile-time date and time without macros

Question

using c++ I compile my code on an automated schedule and need to use the time at which the code was compiled in the code itself. Currently I'm just using the `__DATE__`, `__TIME__` macros to get the compile- time date and time. However, this causes the binaries to change even if no changes have been made to the source (macros will inflate at compile time) which is not good (i don't want the setup to think that the binary changed if there have been no changes to the source). Is it possible to get the compile-time without using any means that would cause the source to change? Thanks

Accepted Answer

The standard `__DATE__` and `__TIME__` macros do what you observe, return a time dependent string.

It depends upon the system (and perhaps the compiler) and notably the build system (like GNU make for example).

A possible idea could be to link in a seperate timestamp file, something like (in `make` syntax)

```
timestamp.c:
        date +'const char timestamp[]="%c";' > $@

program: $(OBJECTS) timestamp.c
        $(LINKER.cc) $^ -o $@ $(LIBES)
        rm -f timestamp.c
```

The `timestamp.o`would then be regenerated and your `program`would be relinked at every `make` (so the generated program will indeed change, but most of the code -thru `$(OBJECTS)` make variable- will stay unchanged).

Alternatively, you could e.g. log inside some database or textual log file the time of linking, e.g.

```
program: $(OBJECTS)
      $(LINKER.cc) $^ -o $@ $(LIBES)
      date +'$@ built at %c' >> /var/log/build.log
```

(you might use `logger` instead of `date` to get that logged in the syslog)

Then the generated `program` won't change, but you'll have logged somewhere a build timestamp. BTW you could log also some checksum (e.g. `$(shell md5sum program)` in `make` syntax) of your binary program.

getting compile-time date and time without macros

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