I'm a total noob to learning Zig so my question may be naïve.
I'm trying to create a variety of strings in my code using the string formatting functions in the standard library such as std.fmt.allocPrint()
It was working when I was just doing one string at a time but when I decided to abstract it into a function the compiler is telling me that allocPrint's format string is comptime.
I can't find any way to achieve the same thing using a runtime format string.
Is there a totally different way that involves not using std.fmt or am I missing something basic due to being a beginner?
Related
This question already exists:
Is it possible to have gcc analyze my code and give me info about it [closed]
Closed 2 years ago.
I want to write a script that analyzes a C header file.
The header contains a few declarations of functions
The scrip needs to understand how many arguments each function has, and what are their types.
can gcc (or other tool) do this?
It comes down to how accurate you want to be when parsing a C header file. You could implement the C grammar for a function declaration and be guaranteed to parse it 100% correctly, but it will be a lot of work.
Conversely, a simple regex matcher is the easiest approach, but could have false-positive matches. You can implement a regex parser using C++, python or even other scripting languages such as bash or powershell. I would suggest python as its beginner friendly.
If you don't know what regex is, it's a simple language used to parse strings and extract data from strings. So you could create a regex expression to find the c function declaration then use that expression to pull out all of the arguments. Should be straight forward.
Create a regex expression on https://regex101.com/ and test it against teat strings, then implement that expression in python to do the work.
Is there a way to use a string as a delimiter?
We can use characters as delimiters using sscanf();
Example
I have
char url[]="username=jack&pwd=jack123&email=jack#example.com"
i can use.
char username[100],pwd[100],email[100];
sscanf(url, "username=%[^&]&pwd=%[^&]&email=%[^\n]", username,pwd,email);
it works fine for this string. but for
url="username=jack&jill&pwd=jack&123&email=jack#example.com"
it cant be used...its to remove SQL injection...but i want learn a trick to use
&pwd,&email as delimiters..not necessarily with sscanf.
Update: Solution doesnt necessarily need to be in C language. I only want to know of a way to use string as a delimiter
Just code your own parsing. In many cases, representing in memory the AST you have parsed is useful. But do specify and document your input language (perhaps using EBNF notation).
Your input language (which you have not defined in your question) seems to be similar to the MIME type application/x-www-form-urlencoded used in HTTP POST requests. So you might look, at least for inspiration, into the source code of free software libraries related to HTTP server processing (like libonion) and HTTP client processing (like libcurl).
You could read an entire line with getline (or perhaps fgets) then parse it appropriately. sscanf with %n, or strtok might be useful, but you can also parse the line "manually" (consider using e.g. your recursive descent parser). You might use strchr or strstr also.
BTW, in many cases, using common textual representations like JSON, YAML, XML can be helpful, and you can easily find many libraries to handle them.
Notice also that strings can be processed as FILE* by using fmemopen and/or open_memstream.
You could use parser generators such as bison (with flex).
In some cases, regular expressions could be useful. See regcomp and friends.
So what you want to achieve is quite easy to do and standard practice. But you need more that just sscanf and you may want to combine several things.
Many external libraries (e.g. glib from GTK) provide some parsing. And you should care about UTF-8 (today, you have UTF-8 everywhere).
On Linux, if permitted to do so, you might use GNU readline instead of getline when you want interactive input (with editing abilities and autocompletion). Then take inspiration from the source code of GNU bash (or of RefPerSys, if interested by C++).
If you are unfamiliar with usual parsing techniques, read a good book such as the Dragon Book. Most large programs deal somewhere with parsing, so you need to know how that can be done.
What's the best way to concatenate a string using Win32? If Understand correctly, the normal C approach would be to use strcat, but since Win32 now deals with Unicode strings (aka LPWSTR), I can't think of a way for strcat to work with this.
Is there a function for this, or should I just write my own?
lstrcat comes in ANSI and Unicode variants. Actually lstrcat is simply a macro defined as either lstrcatA or lstrcatW.
These functions are available by importing kernel32.dll. Useful if you're trying to completely avoid the C runtime library. In most cases you can just use wcscat or _tcscat as roy commented.
Also consider the strsafe.h functions, such as StringCchCat These come in ANSI and Unicode variants as well, but they help protect against buffer overflow.
I need a function to encode base64 and a function to decode base64 string in c. I found http://base64.sourceforge.net/b64.c but the functions work on files, not strings, and add line breaks. I need one that simply encodes/decodes strings. Where can I find such a sourcecode?
Get the functions from libb64.
If you have openssl available to you (which most *nix distros seem to have out-of-the-box these days), it provides robust, well-tested base64 encoding/decoding out of the box. This site has a decent code sample: Howto base64 decode with C/C++ and OpenSSL
When I needed to use Base64 encoding to build an encrypted email server, I decided to build my own implementation.
Currently, it's placed within a C++ class; but I wrote the encoding functions without using any c++ specific code, so you can copy and paste as you please.
This implementation is not approved by any organizations; but it should help you with learning how the algorithm works, while also giving you access to just base64 encoding. IE: no extra libraries get included.
https://github.com/AlexBestoso/Base64
Use as you please. The functions take chars, maps them to an integer via bit-wise operations, and then produces your result.
Currently, there's no newline or carriage return, which specified in the MIME implementation.
If you decide to use the code and find any bugs, let me know through github.
I'm building my own language using Flex, but I want to know some things:
Why should I use lexical analyzers?
Are they going to help me in something?
Are they obligatory?
Lexical analysis helps simplify parsing because the lexemes can be treated as abstract entities rather than concrete character sequences.
You'll need more than flex to build your language, though: Lexical analysis is just the first step.
Any time you are converting an input string into space-separated strings and/or numeric values, you are performing lexical analysis. Writing a cascading series of else if (strcmp (..)==0) ... statements counts as lexical analysis. Even such nasty tools as sscanf and strtok are lexical analysis tools.
You'd want to use a tool like flex instead of one of the above for one of several reasons:
The error handling can be made much better.
You can be much more flexible in what different things you recognize with flex. For instance, it is tough to parse a C-format hexidecimal value properly with scanf routines. scanf pretty much has to know the hex value is comming. Lex can figure it out for you.
Lex scanners are faster. If you are parsing a lot of files, and/or large ones, this could become important.
You would consider using a lexical analyzer because you could use BNF (or EBNF) to describe your language (the grammar) declaratively, and then just use a parser to parse a program written in your language and get it in a structure in memory and then manipulate it freely.
It's not obligatory and you can of course write your own, but that depends on how complex the language is and how much time you have to reinvent the wheel.
Also, the fact that you can use a language (BNF) to describe your language without changing the lexical analyzer itself, enables you to make many experiments and change the grammar of your language until you have exactly what it works for you.