I've compiled a project as a library for stm32 using iar, after adding the .a file the linker gives a warning that the used functions are declared implicity.
The function accepts a pointer to input buffer and return a pointer to output buffer, when assiging a pointer to save the return address the complier gives an error that a value of intcan't be assigned to pointer.
What could be the error, or. Is there something missing in the way of adding lib
You need the .h file as well with the data structures & function prototypes.
Related
I am learning how to write an operating system by using Rust. I am trying to config my target system, (Does the target system means the operating system I want to create?),and I added
"linker-flavor": "ld.lld",
"linker": "rust-lld",
to my x86_64-os.json file.
The tutorial said "Instead of using the platform’s default linker (which might not support Linux targets), we use the cross platform LLD linker that is shipped with Rust for linking our kernel."
I still don't understand what linker meaning. What is the function of the linker and who and whom does it connect.
"llvm-target": "x86_64-unknown-none",
"data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
"arch": "x86_64",
"target-endian": "little",
"target-pointer-width": "64",
"target-c-int-width": "32",
"os": "none",
"executables": true
"linker-flavor": "ld.lld",
"linker": "rust-lld",
What is the function of the linker and who and whom does it connect.
The linker is used to link several object files together. When you compile code, the compiler creates a platform specific object file (.exe for Windows, .elf for Linux, etc). The compiler will often transparently call the linker with the resulting object files. You often don't actually need to call it manually.
For example, if you call an undefined function in a C source file, the compiler knows that it might be defined later when the linker finds the actual definition. The compiler only requires an header declaration for that function. The compiler doesn't throw any error but, instead, leaves an unresolved call instruction and keeps the unresolved symbol in the object file. When the linker is called, it will look at the unresolved symbols and will attempt to find them in other object files. The linker will combine all the object files into one final executable and make sure to patch the call instruction depending on where the actual definition of the symbol is found in the final executable.
From the perspective of the function being called, the object file where the function is really defined will also keep a symbol for that function. The linker can thus find that both symbols correspond and make sure that the caller jumps at the right address to get there.
I am trying to call HAL_UART_Transmit() from my custom SA145.c file, which causes the error: identifier huart1 us undefined. huart1 is declared as extern in main.c.
How to call HAL_UART_Transmit() from the file other than main.c
error: identifier huart1 us undefined.
huart1 is declared as extern in main.c.
Something needs to be declared as extern in a file where it is, to use an everyday word, "borrowed".
But it needs to actually exist (ie, not be extern) is some file. Otherwise the linker will find only attempts to use it, but will never actually find an "it" to use.
At the same time, you may not really want to be calling the basic serial function from a lot of places, but may rather want to wrap it in something appropriate to why you are calling it, eg, to send data to a peripheral, or to create a debug printout, etc. Then you can keep the argument in just one file and not need to share it.
I need to use a function in Matlab that is defined by a dll file. I got an example that guy converted a dll to mexw32 file but i have known how I do this. I tried use loadlibrary but it didn't create any file.
How I can do this?
loadlibrary is MATLAB's implementation of FFI services, a mechanism of calling functions in external shared libraries. It involves converting between C-types and their equivalent MATLAB data types to pass data around.
MEX-files are also a kind of dynamically linked libraries (with .mex* extension), that can be run directly in MATLAB as a regular function without any special syntax.
The difference is that it has a specific gateway routine called mexFunction, which receives both input and output as mxArray type. mxArray is an opaque type defined in mex.h header file, which is the fundamental type underlying all MATLAB data. You usually manipulate this data using functions in the MEX library API.
I'm using a SDK in one of my projects and when adding in the libraries etc, I get an error from this line of code, which is from inside a .h file. I cannot change the .m since its not available to me. Its inside .a (lib)
-(enum scanDeviceID)ID;
And the error message is:
Semantic Issue
ISO C++ forbids forward references to 'enum' types
Are there any compilier/build setting that I can modify to make this work?
Failing that, I know very little C++, is there something on the line of code I can change to make it work?
The line of code says that this method returns an element that's named in the scanDeviceID enum. The problem is that the compiler doesn't see a declaration of that enum. Somewhere in a .h file you should have that declaration. Importing the file should fix it.
This is probably a totally noob question but I have missing links in my mind when thinking about linking libraries in iOS. I usually just add a new library that's been cross compiled and set the build and linker paths without really know what I'm doing. I'm hoping someone can help me fill in some gaps.
Let's take the OpenCV library for instance. I have this totally working btw because of a really well written tutorial( http://niw.at/articles/2009/03/14/using-opencv-on-iphone/en ), but I'm just wanting to know what is exactly going on.
What I'm thinking is happening is that when I build OpenCV for iOS is that your creating object code that gets placed in the .a files. This object code is just the implementation files( .m ) compiled. One reason you would want to do this is to make it hard to see the source code and so that you don't have to compile that source code every time.
The .h files won't be put in the library ( .a ). You include the .h in your source files and these header files communicate with the object code library ( .a ) in some way.
You also have to include the header files for your library in the Build Path and the Library itself in the Linker Path.
So, is the way I view linking libraries correct? If , not can someone correct me on this ?
Basically, you are correct.
Compiling the source code of a library produces one object file for each of the source files (in more than one, if compiled multiply times against different architectures). Then all the object files are archived (or packaged) into one .a file (or .lib on Windows). The code is not yet linked at this stage.
The .h files provide an interface for the functionality exposed by the library. They contain constants, function prototypes, possibly global declarations (e.g. extern int bad_global;), etc. -- basically, everything that is required to compile the code which is using the library.
.h files do not 'communicate' with object code in any way. They simply provide clues for the compiler. Consider this header file:
// library.h
extern int bad_global;
int public_func(int, const void*);
By including this file in your own code, you're simply telling the compiler to copy and paste these declarations into your source file. You could have written declarations for OpenCV library and not use the headers provided with it. In other words, you're asking the compiler to not issue errors about undefined symbols, saying "I have those symbols elsewhere, ok? Here are their declarations, now leave me alone!".
The header files need to be included in the search path in order for compiler to find them. You could simply include them via the full path, e.g. #include "path/to/file.h", or supply an -I option for your compiler, telling him where to look for additional headers, and use #include <file.h> instead.
When your code is compiled, the declarations in header files serve as an indication that symbols your code is using are defined somewhere. Note the difference between the words declaration and definition. Header files contain only declarations most of the time.
Now, when your code is compiled, it must be linked in order to produce the final executable. This is where the actual object code stored in the library comes into play. The linker will look at each symbol, function call, etc. in your object code and then try to find the corresponding definition for each such symbol. If it doesn't find one in the object code of your program, it will look the standard library and any other library you've provided it with.
Thus, it is important to understand that compilation and linkage are two separate stages. You could write any function prototypes at all and use them in your code, it will compile cleanly. However, when it comes to the linking stage, you have to provide implementation for symbols used in your code, or you won't get your executable.
Hope that makes sense!
The .a is the compiled version of the code.
The header files provided with a library are its public interface. They show what classes, methods, properties are available. They do not "communicate" with the binary code.
The compiler needs the headers to know that a symbol (a method name for example) is defined somewhere else. They are associated with the right "piece of code" in the library binary later during the "link" step.