Question about .exe files

[Peter Swinkels]

I like to examine executable files with a viewer or the Microsoft Windows Editor (Edit.com) out of curiosity. I often see readable text which contain symbols such as %ld, %s, %d and %u. Those symbols almost always seem to appear where some variable text would appear when the text is displayed during the execution of the program. These symbols often seem to refer to file names and appear often in error messages for example.

I have tried hex editing (a copy of course) of several of these executables, changing %s to %u for example. Usually this does result in other text being displayed (that is if the executable still works) but nothing I can make sense of. Does any one know what these symbols are and what they mean? These symbols appear both in Windows and MS-DOS executables.