c - writing a glibc api for a system call

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Need help with glibc source

I understand how to implement our own system calls in linux kernel. I know we can call this with syscall() or with _asm() in a c program.

1. But I want to understand how to write glibc api for this new system call?.

2. How the open() and read() glibc function calls mapping into system call in kernel?.

   char      message[ ] = "Hello!\n";
   
   int main( void )
   {
           write( 1, message, 7 );
           exit( 0 );
   }
   

When I convert the above program into assembly it is giving

main:
    leal    4(%esp), %ecx
    andl    $-16, %esp
    pushl   -4(%ecx)
    pushl   %ebp
    movl    %esp, %ebp
    pushl   %ecx
    subl    $20, %esp
    movl    $7, 8(%esp)
    movl    $message, 4(%esp)
    movl    $1, (%esp)
    call    write
    movl    $0, (%esp)
    call    exit
    .size   main, .-main
    .ident  "GCC: (Debian 4.3.2-1.1) 4.3.2"
    .section        .note.GNU-stack,"",@progbits

~

3, In "call write" I think write is glibc call here ?. what happens after that? how it maps the glibc call to system call?

Answer 1

参见例如这个答案和类似问题的那个答案。阅读更多关于系统调用、linux内核、 linux系统调用概述和汇编方法的信息。

glibc 中的write函数不是真正的系统调用。sysenter它是一个包装器（通过例如机器指令执行 sycall ，可能使用VDSO和设置errno）。您可以使用它strace来了解某些程序所做的系统调用。

例如，MUSL libc有这个write.c实现，用于write. 对于 GNU libc，请查看它的write.c。