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Old 10-12-2017, 11:51 PM   #1
aimal
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Post Needs of a C Library-- Building GCC


Hi,

I have a very basic question may be dummiest... from this https://github.com/kowalski100/gcc-t...ster/script.sh

Following are the steps to build GCC..
1. Build binutils
2. Build gcc
3. Build C-library
4. Again Build GCC

My qestions:
1. Why we build gcc twice, before and after c-library?
2. Why exactly we need C-Library especially if we are building toolchain for non-OS like arm-none-eabi??

Kind regards,
 
Old 10-13-2017, 02:53 AM   #2
hazel
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This looks somewhat similar to the way LFS builds its toolchain. You start by building a statically linked gcc and binutils. The --with-newlib option tells the compiler not to use your host's libc. You use the new tools to build libc, then rebuild gcc to link to it. In LFS, you do that with binutils too.

You might like to compare with the Cross-LFS book at http://trac.clfs.org/wiki/read#CLFS3.0.0. I couldn't find a version of this book for ARM, but any of the versions will give you a general idea of how CLFS does it and why.

You need a C library on any system, whatever kind of processor you are using. It's the one library that every program needs to use.
 
Old 10-13-2017, 04:43 AM   #3
aimal
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@Hazel, thank you so much for your kind reply. Let me explain what i understood from your answer.
We build GCC without any library, than using this newly build compiler to build c library. Then we build gcc again to link recently build c library to gcc.
BUT again:
Can you please explain a little bit how every C program uses C library?? What it offers to every program??
As far as i know only runtime library is required for every program which provide starup code for program... while the library we link is standard library. So how every program needs it?

Last edited by aimal; 10-13-2017 at 04:58 AM.
 
Old 10-13-2017, 06:40 AM   #4
hazel
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It would be very hard to write a program that didn't use the standard C library! libc provides:
1) All the functions for input and output (screen, keyboard or files)
2) All the arithmetic functions
3) Functions for opening and closing files and using directories
4) Functions for handling time differences
5) Functions for handling character sets in different languages
6) Functions for communicating with the kernel (system calls) and with other processes

...and lots more!

You can get a better idea by reading the GNU libc manual at https://www.gnu.org/software/libc/ma...mono/libc.html.
 
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Old 10-13-2017, 08:18 AM   #5
aimal
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Quote:
Originally Posted by hazel View Post
It would be very hard to write a program that didn't use the standard C library! libc provides:
1) All the functions for input and output (screen, keyboard or files)
2) All the arithmetic functions
3) Functions for opening and closing files and using directories
4) Functions for handling time differences
5) Functions for handling character sets in different languages
6) Functions for communicating with the kernel (system calls) and with other processes

...and lots more!

You can get a better idea by reading the GNU libc manual at https://www.gnu.org/software/libc/ma...mono/libc.html.
Thanks Hazel. It means the data types are also defined by C library?? And arent there special instructions for arithematic operation like ADD, MUL, SUB. Which are independent of C library.
 
Old 10-13-2017, 12:11 PM   #6
hazel
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Quote:
Originally Posted by aimal View Post
Thanks Hazel. It means the data types are also defined by C library?? And arent there special instructions for arithematic operation like ADD, MUL, SUB. Which are independent of C library.
Basic data types like int, float and char are defined by the C language. The language also provides standard arithmetical operators like +, -, * and /. But all mathematical functions (trigonometry, logs, square roots and so on) come from the math library which is part of libc.
 
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