Purpose of "$Id: ..." line in the header of source files - coding-style

/* $Id: file.c,v 1.0 2010/09/15 01:12:10 username Exp $ */
I find this line in many source code files in the comment at the top (header) of the file. Why? Is it aimed for the version control software? -Thanks.

These sort of comments are automatically modified by various source code control systems, things like author, date, history and so forth.
See here for some common ones for RCS which is the first source code control system I ever saw to implement this sort of thing (that doesn't mean it was the first, just that RCS was the first I ever used and it had that capability).
One particular trick we used to use was to put the line:
static char *fileId = "$Id: $";
into the source file (and header files as well, although the names had to be unique) so that, when it was built, it would automatically have the ID of the files in the executable itself.
Then we could use something like strings to find out which source files were used to build the executable. Ideal for debugging problems in the field.

It tells CVS (and other VCSs) to expand the value of the Id at check-out time, so anybody reading the source file in question will know what version exactly was checked out for it. Not very popular any more (you can always ask your VCS for such info if you keep the source file in a client / repository / working directory -- or however else your VCS calls such things;-).

I believe you are correct. It appears to be a keyword substitution string for CVS.
Take a look at this question $id: name of file, date/time creation Exp $

Related

How do I not commit the development team lines in project.pbxproj without deselecting those lines manually?

I am collaborating with my friend on an iOS app. We use different Apple IDs in our Xcodes, so in "Signing and Capabilities" tab of project settings, we select different teams in the "Team" field:
From my observation, changing this affects the MyProject.xcodeproj/project.pbxproj file, which stores the file references that the Xcode project has, in addition to the "Team". Here's a snippet of what is changed:
buildSettings = {
ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
CODE_SIGN_STYLE = Automatic;
DEVELOPMENT_TEAM = <my team ID>; /* this is changed */
INFOPLIST_FILE = MyProject/Info.plist;
LD_RUNPATH_SEARCH_PATHS = (
"$(inherited)",
"#executable_path/Frameworks",
);
PRODUCT_BUNDLE_IDENTIFIER = io.github.sweeper777.MyApp;
PRODUCT_NAME = "$(TARGET_NAME)";
SWIFT_VERSION = 5.0;
TARGETED_DEVICE_FAMILY = 1;
};
The problem arises, when one of us commits this file and the other person pulls. The "puller" will now have the "Team" set to something invalid. When this person then tries to run the app on a real device, there will be code signing errors for obvious reasons. To solve this, this person must tediously go through all the targets that we have, and set each "Team" to their own team.
How can we make it so that on each person's computer, the "Team" stays the same after pulling, but any other changes to MyProject.xcodeproj/project.pbxproj is applied?
Remarks:
Putting the entire MyProject.xcodeproj/project.pbxproj in .gitignore doesn't work, because that would ignore every other change to it. Adding a new file to the project, for example, also changes MyProject.xcodeproj/project.pbxproj, and we want to be able to pull that change.
Manually deselecting the lines that say "DEVELOPMENT_TEAM = ..." when committing is as tedious as reselecting the correct team every time, so that's not a solution.
I found this. Apparently, I can configure git to run sed before git checkout and git add. However, that answer seems ignore the line by deleting it completely. This means that my friend, when he pulls, would still have to reselect the correct team. What I want is the kind of "ignore" that simply stops tracking that line. That is, if there is a local version of that line, use that.
I am also aware that this all wouldn't be a problem if we are on the same team. But if I understand this correctly, I can't have multiple people on my team unless I have a Company account, and not only can I not afford that, I don't own a Company.
I don't use Xcode itself and do not know how to smuggle Git hooks and scripts past the Xcode interface, so you'll need more than just this answer. But you mention sed in comments, and given your proposed file format, that may well be the way to go:
buildSettings = {
ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME = AccentColor;
CODE_SIGN_STYLE = Automatic;
DEVELOPMENT_TEAM = <my team ID>; /* this is changed */
INFOPLIST_FILE = MyProject/Info.plist;
LD_RUNPATH_SEARCH_PATHS = (
"$(inherited)",
"#executable_path/Frameworks",
);
PRODUCT_BUNDLE_IDENTIFIER = io.github.sweeper777.MyApp;
PRODUCT_NAME = "$(TARGET_NAME)";
SWIFT_VERSION = 5.0;
TARGETED_DEVICE_FAMILY = 1;
};
Git has the ability to run what it calls clean and smudge filters. These can be used to run any arbitrary program you like, including sed, the "stream editor", which is particularly good at making single-line changes based on regular expression matches.
There is another method that may also work, and may "play better" with Xcode, or may play worse. I'll go over that too, after covering clean and smudge filters.
Before we dive into writing clean and smudge filters, and using them from Git—you'll need to know all of these details as you will have to write your own custom filters—we should start with a simple fact about Git commits: No part of any commit can ever be changed. Once you make a commit, the stuff that's inside the commit—the stored data in all of its files—is the way it is, forever. So these filters have to work within that system. Remember that, as it will help with understanding what we're doing.
How Git makes and stores objects
The files inside a commit are not files, exactly: they're not the same thing as files in your file system, at least. Instead, they are what Git calls objects, specifically blob objects. A blob object holds the file's data; other objects hold the file's name; and commit objects collect everything together to be used all at once. There's one more internal object type for annotated tags but we'll stop here as we're really only interested in the blob-object part.
When Git extracts a commit, it reads the internal blob objects and runs them through internal code to decompress and format them into regular files. This can include doing end-of-line hacking (turning LFs into CRLFs) if desired. Normally, all this happens entirely inside Git, and the end result is that Git writes out an ordinary everyday file for you to use. This ordinary file is what you will work on / with, in Xcode or any other editor and compiler system and so on. These ordinary files are in your working tree.
After you've extracted some commit, you'll do some work on it, by changing some or all of the files in your working tree, to achieve whatever result you wanted. This can include changing the buildSettings, editing Swift code, editing Objectionable-C Objective-C code, and so on. You might add all-new files to the working tree, some of which you never commit at all (you can help make sure this never happens by listing such files in .gitignore).
Eventually, though, you'd like to commit the updated code. To do so, you must run git add, or maybe have your IDE run git add for you (perhaps Xcode has clicky buttons to do this). This invokes code in Git that converts the working tree file(s) back to internal blob objects if and as needed.
Again, normally this is all handled entirely inside of Git. Git will read the working tree file, maybe do CRLF-to-LF-only changes, compress the text, search for duplicate objects, and do all the other complicated things necessary to prepare the file, so that it is ready to be committed. The resulting data need not match what's in your working tree at all: it just has to be something that, when Git later goes to extract the file, produces what you will need in your working tree.
Clean and smudge filters
This is where these clean and smudge filters come in. I said, above, that normally, Git does the extraction and insertion all on its own. For binary files, the only thing Git does here is apply lossless compression.1 For text files, Git can do CRLF/LF substitutions as well. But what if you'd like to do your own operations?
You can: Git will let you do whatever you want during the extract process with a smudge filter, and will let you do whatever you want during the compress process with a clean filter. The clean filter replaces the in-file data, using a stream-edit type process,2 and then Git does its CRLF hacking if any and compressing on the "cleaned" data. The smudge filter replaces the decompressed, post-CRLF-hacking data coming out of Git with the data that should go into the working tree.
Hence you can write, as your clean filter, a sed script of the form:
s/DEVELOPMENT_TEAM = .*;/DEVELOPMENT_TEAM = DEVTEAMTEMPLATE;/
With that as the entire sed script, what sed will do is edit the incoming data stream and replace any actual development team text with the word DEVTEAMTEMPLATE.
Your smudge filter has to work slightly harder: it must find the template line and adjust it so that it contains the correct team ID. Where will you get the correct team ID? That's up to you: perhaps you can store it in a file in your home directory, or in a file that you create in the working tree but never commit in Git. You'll have to write this one or two or however-many-liner sed and/or shell script yourself.
1There are multiple phases of compression; git add does just one, and git checkout undoes all—including reading from "pack" files—as needed. The deeper level of compression, using delta encoding techniques, is entirely invisible at the "object" level, so nobody ever really has to think about it.
2With the advent of Git-LFS, Git gained the ability to run long-lived filters. Before that, Git always used simple stream filtering. The stream filtering is easier to understand, but is less efficient for doing en-masse operations on many files. Here, we're only interested in one file per repository anyway, so there's no need to go into the fancier long-lived filter details.
Defining clean and smudge filters
The tricky part here, with Git, is that you must define the filters in one place—in $HOME/.gitconfig or .git/config, for instance—and then tell Git to invoke them from another place, using the .gitattributes file. This is described in the gitattributes documentation. This documentation is pretty thorough, so read it. You can ignore all the long-running filter discussion, as noted above. I will quote one bit from the documentation here for emphasis, though, and expound on it:
Note that "%f" is the name of the path that is being worked on. Depending on the version that is being filtered, the corresponding file on disk may not exist, or may have different contents. So, smudge and clean commands should not try to access the file on disk, but only act as filters on the content provided to them on standard input.
When Git is running the smudge filter, it:
has opened some internal object (which may or may not be packed);
has decompressed it, or is in the process of decompressing it, and pumped / is-pumping out the data; and
this data is being fed to your filter, but is not written out to any file anywhere.
Your filter can use %f to know the name of the target output file, but the data are not in that file yet. The data bytes are only in some OS-level pipes or sockets or whatever your OS uses for connecting the output of one program (Git's internal decompressors) to another (your filter). Your smudge filter must read its standard input to get the data, and write the smudged data to standard output so that Git can read it (if necessary) and/or redirect that output to the correct file. Do not attempt to open the file by name!
(The same holds for the clean filter, except that in many cases, the input to your filter is just the raw data already in the file, so that opening the file and reading it mostly works. So this can mislead you, if you do your tests using a clean filter.)
Note that you can implement this scheme without a clean filter at all: your smudge filter can replace whatever is in the committed file even if it's a real team ID, rather than just a template. If you choose to do this, however, you'll "see" the team ID changing every time a different team-ID commits the file. The nice thing about using the clean filter is that once the committed copies of the file use the template line, every future cleaned file also uses the template line, so that it never changes.
Alternative: a template file
In general, it's unwise to commit actual configurations. Clean and smudge tricks can work, but they can only go so far: this particular file format works well because the change you want made is on a single line, and Git itself shows you file changes on a line-by-line basis, and sed works well with line-oriented input, and so on.
A lot of configuration files, though, wind up storing at least slightly-sensitive data, or perhaps very-sensitive data such as cleartext passwords. Such files should not be stored in Git at all if at all possible. Instead, you would store a template file in Git.
In this case, for instance, instead of storing MyProject.xcodeproj/project.pbxproj, you might have Git store MyProject.xcodeproj/project.pbxproj.template. This file would have template-ized contents. When you clone and check out the repository, you'd subsequently copy the template file into place and do any required adjustments.
Should the MyProject.xcodeproj/project.pbxproj file itself need to change, e.g., to acquire a new SWIFT_VERSION setting, you'd instead edit the template file, add that to Git, and commit. You would then use the usual "convert template to mine" process, or manually update the MyProject.xcodeproj/project.pbxproj file. Since this file is never committed—and is listed in .gitignore—it never goes into any commit and you never have to worry about collisions within it. Only the template file goes into Git.

Rules for file extensions?

Are there any rules for file extensions? For example, I wrote some code which reads and writes a byte pattern that is only understood by that specific programm. I'm assuming my anti virus programm won't be too happy if I give it the name "pleasetrustme.exe"... Is it gerally allowed to use those extensions? And what about the lesser known ones, like ".arw"?
You can use any file extension you want (or none at all). Using standard extensions that reflect the actual type of the file just makes things more convenient. On Windows, file extensions control stuff like how the files are displayed in Windows Explorer and what happens when you double click on it.
I wrote some code which reads and writes a byte pattern that is only
understood by that specific programm.
A file extension is only an indication of what type of data will be inside, never a guarantee that certain data formatted in a specific way will be inside the file.
For your own specific data structure it is of course always best to choose an extension that is not already in use for other file formats (or use a general extension like .dat or .bin maybe). This also has the advantage of being able to use an own icon without it being overwritten by other software using the same extension - or the other way around.
But maybe even more important when creating a custom (binary?) file format, is to provide a magic number as the first bytes of that file, maybe followed by a file header structure containing a version number etc. That way your own software can first check the header data to make sure it's the right type and version (for example: anyone could rename any file type to your extension, so your program needs to have a way to do some checks inside the file before reading the remaining data).

Expressions in a build rule "Output Files"?

Can you include expressions in the "Output Files" section of a build rule in Xcode? Eg:
$(DERIVED_FILE_DIR)$(echo "/dynamic/dir")/$(INPUT_FILE_BASE).m
Specifically, when translating Java files with j2objc, the resulting files are saved in subfolders, based on the java packages (eg. $(DERIVED_FILE_DIR)/com/google/Class.[hm]). This is without using --no-package-directories, which I can't use because of duplicate file names in different packages.
The issue is in Output Files, because Xcode doesn't know how to search for the output file at the correct location. The default location is $(DERIVED_FILE_DIR)/$(INPUT_FILE_BASE).m, but I need to perform a string substitution to insert the correct path. However any expression added as $(expression) gets ignored, as it was never there.
I also tried to export a variable from the custom script and use it in Output Files, but that doesn't work either because the Output Files are transformed into SCRIPT_OUTPUT_FILE_X before the custom script is ran.
Unfortunately, Xcode's build support is pretty primitive (compared to say, make, which is third-odd years older :-). One option to try is splitting the Java source, so that the two classes with the same names are in different sub-projects. If you then use different prefixes for each sub-project, the names will be disambiguated.
A more fragile, but maybe simpler approach is to define a separate rule for the one of the two classes, so that it can have a unique prefix assigned. Then add an early build phase to translate it before any other Java classes, so the rules don't overlap.
For me, the second alternative does work (Xcode 7.3.x) - to a point.
My rule is not for Java, but rather for Google Protobuf, and I tried to maintain the same hierarchy (like your Java package hierarchy) in the generated code as in the source .proto files. Indeed files (.pb.cc and .pb.h) were created as expected, with their hierarchies, inside the Build/Intermediates/myProject.build/Debug/DerivedSources directory.
However, Xcode usually knows to continue and compile the generated output into the current target - but that breaks as it only looks for files in the actual ${DERIVED_FILE} - not within sub-directories underneath.
Could you please explain better "Output Files are transformed into SCRIPT_OUTPUT_FILE_X" ? I do not understand.

Variable directory path for AC_CONFIG_FILES in configure.ac

I am writing a small tool in c++. It is actually more of a framework that is open to customization. It has the following directory structure (simplified example).
src/
main/myexec # linked to libapple.so
apple/
coder/libapple.so
john/libapple.so
.
.
james/libapple.so
Here, the directory "coder" is a generic dummy, with some example code to generate libapple.so. Different users can checkout this tool, create directories of their own, copy the template code from "coder" and customize as they wish. Depending on the configure option (indicating the user), the respective libapple.so needs to be generated.
As I mentioned, this is a simplified example. It is not a matter of generic programming, inheritance etc. In fact, similar to the "apple" folder there are others like "scripts", "docs", "configs" etc each having similar user specific folders. Also, the tool will be maintained at a single repository location to allow me to support & maintain all the code that is not specific to user. As a policy, users are expected to modify and check-in only the contents of their folders.
The problem I am facing is with "configure.ac". I do not want to use "AC_ARG_WITH" option as it would require each new user to edit configure.ac. Also for each user the AC_CONFIG_FILE entries would be exactly the same except for his folder name. I tried using "--enable-user=User" and then AC_SUBST(USERDIR), which also helps in setting "SUBDIRS = #USERDIR#" in Makefile.am. Everything looks good except for the fact that "Makefile.in" is not getting created under the user folder when I specify "AC_CONFIG_FILE = ([apple/${USERDIR}/Makefile])".
Please advice how to overcome this issue. In the worst case I may end up in creating softlinks :(
After one full day of scratching my head, following is the solution that I have come up with.
Create a file "project_makefiles.m4.in" like this
AC_CONFIG_FILES([ apple/USERDIR/Makefile ]
Add the below to configure.ac
m4_include([project_makefiles.m4])
Create a wrapper script like "build.sh" which will create "project_makefiles.m4" from "project_makefiles.m4.in" by replacing "USERDIR". This is done before the automake.

Can I automatically update msgids in gettext's .po files for trivial text changes?

With gettext, the original (usually English) text of messages serves as
the message key ("msgid") for the translations. This means that every time the
original text changes, the msgid must be updated in all the .po files.
For real changes of the text, this is obviously unavoidable, as the
translator must update the translation.
However, if the change of the original does not change its meaning,
re-translation is superflous (e.g. change in punctation, whitespace
changes, or correction of a spelling mistake).
Is there a way to update the .po files automatically in that case?
I tried to use xgettext & msgmerge (with fuzzy matching turned on), but
fuzzy matching sometimes fails, plus this produces lots of ugly
"#,fuzzy" flags.
Note: There is a similar question:
How to efficiently work with gettext PO files when making small edits to large text values
However, it's about large strings, thus about a more specific problem.
One way to avoid the problem is to leave the msgids alone, have a .po file for the original language and make the fix inside that.
It always strikes me as being more of a work around than a proper fix though. For the next iteration (where there will definitely be more msgid changes) the msgid is changed and either the translators translate it in their usual update or each language is updated by hand when the msgid is changed.
I've had exactly this issue when doing minor changes to a django project. What I do is the following:
Change message in code.
Run find and replace on all translation files ("django.po"), replacing the old message (msgid) with the new one.
Run django-admin makemessages.
If I have done things right, the last step is superflous (i.e, you have done the change for gettext). django uses the gettext utilities, so it shouldn't matter how you make your message files.
I find and replace like so:
find . -name "*.po" -print | xargs sed -i 's/oldmessageid/newmessageid/g' Courtesy of http://rushi.vishavadia.com/blog/find-replace-across-multiple-files-in-linux

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