Contents|Index|Previous|Next Merging Files with If-then-else You can use diff to merge two files of C source code. The output of diff in this format contains all the lines of both files. Lines common to both files are output just once; the differing parts are separated by the C preprocessor directives, #ifdef name or #ifndef name, #else, and #endif. When compiling the output, you select which version to use by either defining or leaving undefined the macro name. To merge two files, use diff with the '-D name' or '--ifdef=name' option. The argument, name, is the C preprocessor identifier to use in the #ifdef and #ifndef directives. For example, if you change an instance of wait (&s) to waitpid (-1, &s, 0) and then merge the old and new files with the '--ifdef=HAVE_WAITPID' option, then the affected part of your code might look like the following.    do { #ifndef HAVE_WAITPID       if ((w = wait (&s)) < 0 && errno != EINTR) #else /* HAVE_WAITPID */ if ((w = waitpid (-1, &s, 0)) < 0 && errno != EINTR) #endif /* HAVE_WAITPID */       return w; }    while (w != child); You can specify formats for languages other than C by using line group formats and line formats, as described in Line Group Formats, Line Formats, Detailed Description of If-then-else Format and An Example of If-then-else Format.