I am asking this just out of curiosity.
I am using Microchip C30 compiler to develop EEPROM driver code for PIC24F.
During this I used C30 builtin functions such as
__builtin_tblwtl(), __builtin_tbloffset etc.
How can I find the location of builtin functions? From Wiki I found this:
"Some compilers (for example, GCC[7]) provide built-in versions of many
of the functions in the C standard library; that is, the
implementations of the functions are written into the compiled object
file, and the program calls the built-in versions instead of the
functions in the C library shared object file"
Does this mean that these functions are written inside the object files of compiler?So does that mean we cannot see it as a code?
I have searched the entire C30 directory and didn't find these functions.
Thank you
user defined functions only stored in objects file of the compiler and above mention functions __builtin_tblwtl(), __builtin_tbloffset etc are previously written in library of Microchip C30 compiler.So you only get link to that library. for example in linux gcc is compiler while glibc are the runtime libraries.
Related
I'm studying C language nowadays. In this book, it is said that the "compiler provides these library functions: 'printf','scanf'…".
I can't understand. Those functions are defined in the header file <stdio.h> aren't they?
Why does this book explain those functions are provided by the compiler?
printf, scanf, and the other standard library functions are provided as part of the implementation.
A C implementation is made up of several components. The compiler is just one of them. The library is another; it consists of headers (commonly provided as source files like stdio.h) and some form of object code files containing the code that actually implements the library functions.
The header stdio.h only declares these functions; it doesn't define them. The declaration of printf is something like:
int printf(const char *format, ...);
The definition of printf is the code that actually does the job of parsing the format string, accessing the arguments, and sending the formatted output to stdout. That's typically (but not necessarily) written in C and provided as some kind of linkable object code.
For some C implementations, the compiler and the library are provided by the same organization. For others, they might be provided separately (for example MinGW combines the gcc compiler with Microsoft's library).
The functions are provided by the standard library, which is a collection of precompiled code that is typically written by the compiler authors (but it is indeed not a part of the compiler itself).
Note, though, that the functions are only declared in the header files. The definition resides in source files that have already been compiled.
By saying, "Compiler provides these library functions , 'printf','scanf'..", the author of the book is being sloppy.
A standard conforming C implementation provides declarations of those functions in header files and implementations of those functions in some short of library. A compiler is just one aspect of a C programming environment.
The compiler does not provide those functions. The goal of the compiler is to translate your high-level language code into another form, in particular an executable binary.
The standard C library contains the functions in stdio.h and stdlib.h.
The compiler links the standard library with your code so that your code can call those functions.
For almost all libraries, you have to tell the compiler what libraries you want to link. It so happens that for some compilers, the library (libc) for stdio.h and stlib.h is automatically linked without you needing to specify them.
Those functions provided by standard library and GCC includes built-in versions of many of the functions in the standard C library.
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
As per Wikipedia there are many variants of standard C library based on operating system and compilers.
Ref: http://en.wikipedia.org/wiki/C_standard_library
But I want to understand that how plenty of functions which are declared in different headers(eg: stdio.h, string.h, stdlib.h etc. ) are defined in single library. Is the source code file is same for all these header files or there are different libraries for stdio.h, string.h etc? As I am beginner to programming I don't know if multiple source code files can generate a single library like executable. If it is possible then I can understand that libc contains definition of all the standard header files. Where can I see the source code of standard C library?
Is it a static library or dynamic library? If both versions are present in my environment(OS/IDE) which one get linked when I include any standard header file in my source code. Is it IDE dependent? But in case of gcc, programmer does not include libc explicitly.
Is libc a standard name for standard C library?
In windows operating system/environment is it already present or not? If it is present what is the name of it(is it libc only)?
Is there any other standard C library like libm?
Generally speaking, a header (.h) file contains the declarations of functions and variables. Implementation files (.c) contain the actual implementation of the declared functions. Since several implementation files can be translated and linked into a single library binary, you can have one library with multiple headers. Many C library implementations are Open Source, and you can look at their source code at their relative project pages. GNU libc and RedHat newlib are the most prominent. I am sure people will add more in comments.
Implementation defined. You can translate the very same sources into either a static or a dynamic library. It is not uncommon to have both versions installed on your system. Since virtually every executable requires libc, it is usually added to the linker input by default so you don't have to add -lc to every command line.
No. The standard name for the standard C library is "The Standard C Library". Note that virtually all implementations of the standard library extend the library with non-standard functions. These remain non-standard, even if they come as part of the standard library. (alloca() springs to mind.)
MSVCRT.dll or somesuch, if I remember correctly.
libm stands for the math section of the standard library, which is not added to the linker input by default as it is seldom required. There is only one standard C library, the one described by the ISO/IEC 9899 language standard (hence the name). There are many other libraries that can readily be assumed to be present on a given system, but only what's described in the ISO/IEC documents is "the standard".
I was searching for the source code of the C standard libraries. What I mean with it is, for example, how are cos, abs, printf, scanf, fopen, and all the other standard C functions written, I mean to see their source code.
So while searching for this, I came across with GLIBC, but I don't know what it actually is. It is GNU C Library, and it contains some source codes, but what are they actually, are they the source code of the standard functions or are they something else? And what is it used for?
Its the implementation of Standard C library described in C standards plus some extra useful stuffs which are not strictly standard but used frequently.
Its main contents are :
1) C library described in ANSI,c99,c11 standards. It includes macros, symbols, function implementations etc.(printf(),malloc() etc)
2) POSIX standard library. The "userland" glue of system calls. (open(),read() etc. Actually glibc does not "implement" system calls. kernel does it. But glibc provides the user land interface to the services provided by kernel so that user application can use a system call just like a ordinary function.
3) Also some nonstandard but useful stuff.
"use the force, read the source "
$git clone git://sourceware.org/git/glibc.git
(I was recently pretty enlightened when i looked through malloc.c in glibc)
There are several implementations of the standard. Glibc is the implementation that most Linuxes use, but there are others. Glibc also contains (as Aftnix states) the glue functions which set up the scene for jumps into the kernel (also known as system calls). So many of glibc's 'functions' don't do the actual work but only delegate to the kernel.
To read the source of Glibc, just google for it. There are myriad sites which carry it, and also several variations.
Windows uses Microsoft's own implementation, which I believe is called MSVCR.DLL. I doubt that you will find the source code to that library anywhere. Also note that some functions which a Linux hacker might think of as 'standard', simply don't exist on Windows (notably fork). The reverse is also true.
Other systems will have their own libc.
The glibc package contains standard libraries which are used by multiple programs on the system. In order to save disk space and memory, as well as to make upgrading easier, common system code iskept in one place and shared between programs. This particular package contains the most important sets of shared libraries: the standard C library and the standard math library. Without these two libraries, a Linux system will not function. The glibc package also contains national language (locale) support.
Yes, It's the implementation of standard library functions.
More specifically, it is the implementation for all GNU systems and in almost all *NIX systems that use the Linux kernel.
Here are a few "hands-on" points of view:
it implements the POSIX C API on top of the Linux kernel: What is the meaning of "POSIX"?
it contains several assembly hand-optimized versions of ANSI C functions for several different architectures, e.g. strlen:
sysdeps/x86_64/strlen.S
sysdeps/aarch64/strlen.S
how to modify its source, recompile and use it understand it better: How to compile my own glibc C standard library from source and use it?
how to GDB step debug it with QEMU and Buildroot: https://github.com/cirosantilli/linux-kernel-module-cheat/tree/9693c23fe6b2ae1409010a1a29ff0c1b7bd4b39e#gdbserver-libc
I know that when you compile and link a C program, you link it with
C library
C runtime startup code
I wonder if I write a program (in a new language, or just C without linking to this code) and link it directly to a C code shared library (say zlib or gsl or fftw or something) and omitting the C library and C startup code (assuming my program will load the external lib itself using its magic), will this "just work"?
I know there is some initialization code in the CRT startup, so I wonder how I can call the required functions without having my application itself depend on a C library: so loading the external C library will at that point call the necessary initialization code (if any, this is the question), and otherwise just load the OS libraries/interfaces.
The reason I ask is that I want to write a language with a Standard library that hooks into the OS API directly, unlike most C++ implementations, that are built on top of the C library.
Take a look here https://blogs.oracle.com/ksplice/entry/hello_from_a_libc_free
So you can startup your program without depend to any library included libc, then libraries can be load and use as needed later.
I have used C shared libraries from a number of other languages. Whether you must explicitly initialize the shared library depends on the library. Normally, it will be implicitly initialized on load, but some libraries require extra initialization. Read the documentation.
And the code of my program (C or other language) must of course be initialized too, but that is what the compiler/linker usually take care of, by linking to the startup code by default.
I use dev c++ for my c projects,because it's simple for me.I installed it with the mingw extension.Well,I included stdlib.h and made a call to mrand which according to manpages belongs to that header but I got a linker error.I looked in mingw's headers and found no declaration for mrand although the glibc has one in stdlib.Am I missing something?I thought mingw and gcc were the same.If they are different I suppose that there isn't a way to get gcc's full power.Right?Thank you.
mrand is not part of the standard C library, nor is it present in standard Linux manpages. Whatever compiler you previously used may have had it as a proprietary extension, but since you haven't mentioned which (it's not GCC or MSVC, at least), I can't tell what mrand is supposed to do, and so it's hard to suggest an alternative function to use.
Note that glibc does offer a mrand48(). Since this is a POSIX function, not a standard C function, it may or may not be present in other C libraries - but note that this is a function of the C library (glibc), not the compiler (gcc/mingw).