C: Difference between revisions

← Older edit

C (view source)

Revision as of 20:25, 25 September 2018

1,034 bytes removed , 5 years ago

m

Reverted edits by Carver (talk) to last revision by Sinetek

[unchecked revision]

VisualWikitext

Glauxosdever

241

edits

Revision as of 18:12, 22 July 2008 (view source) osdev>Adek336 mNo edit summary ← Older edit		Latest revision as of 20:25, 25 September 2018 (view source) Glauxosdever (talk \| contribs) m (Reverted edits by Carver (talk) to last revision by Sinetek)
(12 intermediate revisions by 8 users not shown)
Line 6: == Libraries == {{Main\|C Library}} The C library implements the standard C functions (i.e., the things declared in <stdlib.h>, <math.h>, <stdio.h> etc.) and provides them in binary form suitable for linking with user-space applications. In addition to standard C functions (as defined in the ISO standard), a C library might (and usually does) implement further functionality, which might or might not be defined by some standard. The standard C library says nothing about networking, for example. For Unix-alike systems, the POSIX standard defines what is expected from a C library; other systems might differ fundamentally. It should be noted that, in order to implement its functionality, the C library must call kernel functions through system calls. So, for your own OS, you can of course take a ready-made C library and just recompile it for your OS - but that requires that you tell the library how to call your kernel functions, and your kernel to actually provide those functions~~. The good news is that [http://sourceware.org/newlib/libc.html#SEC195 relatively few of the library's functions do use some system call]~~. It should also be noted that ~~in most cases~~ you can not use a usermode C library directly in your kernel as kernel code needs to be compiled specially. Things such as malloc, free, memcpy need to implemented by you before being used~~. In GCC the "--nobuiltin" flag tells GCC to not use pre-existing builtin functions such as memcpy~~.▼ Available libraries include the [http://www.gnu.org/software/libc/ GNU C library] (with info about porting the glibc), [http://sources.redhat.com/newlib/ newlib] (with info on the required OS functions detailed in the manual), and [http://www.uclibc.org/ uClibC] (although that is highly optimized to be used with an embedded Linux). ▲It should also be noted that in most cases you can not use a usermode C library directly in your kernel. Things such as malloc, free, memcpy need to implemented by you before being used. In GCC the "--nobuiltin" flag tells GCC to not use pre-existing builtin functions such as memcpy. == Things C can't do== Line 32: To fix possible issues due to optimizations performed by processors on runtime (e.g. reordering instructions and memory accesses, caching memory) make sure you use [[atomic operation]]s or [[memory barriers]] where neccessary. This however isn't an optimization specific to the C/C++ language but rather platform and hardware specific. However, although optimizations increase the severity and apparent number of small bugs in C code, they will never break correct C, assuming the compiler is stable and multi-threading issues are accounted for. Because of this, it is sometimes best to debug with optimizations enabled at all different levels, to make sure that code is absolutely correct. If optimizations are turned off, the bugs are still there, just less noticeable. A release-quality project should work while enabling any combination of optimizations, although it is much harder to make progress while doing so. == Learning C ==▼ ~~If you don't know C and plan to use it for osdeving, it might be profitable to begin with some C language exercises.~~ ~~A good start (you still need to find tutorials, these are just exercises):~~ * program that writes a string, asks for your name, writes two line breaks and writes another string and the provided name; * implement strcopy(), strncopy(), strncmp() and other string manipulation functions. What is a buffer overflow? * implement memmove() or memcpy(). Can you do it with inline assembly so it be faster? * use inline assembly to perform CPUID and interpret the results; * use typedefs to name uint32 for a 32-bit unsigned integer type; what type is signed 16-bit? * use structs and pointers to implement a singly linked lists. * divide the program with singly linked lists to main.c, list.c and list.h. What is a header guard? * learn about inline, static, volatile; ternary operator; casts; * use C preprocessor to write MAX, MIN macros; to print out the current function name, line number; * use C preprocessor to write a log function which takes a level for the first argument, and uses printf() on the rest of the arguments if the level is higher or equal to current logging threshold. == See also == ▲==* [[Books#C\|Learning C ==]] * [[C preprocessor]] * [[C++]] * [[C MinusMinus\|C--]] * [[Bare Bones]] [[Category:C]] [[Category:Languages]] [[de:C]]