Calling C library function printf in arm assembly language

Today, when I was writing an ARM assembly code, I wanted to display string information in the program, but it didn't work. I thought it was because of the implementation of printf in the C library. I finally solved the problem in the afternoon. It turned out that the C library was not initialized in the assembly program.

When I encountered this problem, for the sake of simplicity, I directly used assembly to write a program that called the printf function to display the string hello, as follows:

    area |hello$code|,code,readonly
    code32
    entry
    import _printf 
    import ||Lib armlib||,weak
    
hello
    adr r0,strhello
    bl _printf

strhello
    dcb "hello!/n/0"
    end

As a result, during debugging, although the _printf function can be called, the string cannot be output in the console. But later I rewrote this section of assembly code into a procedure (removed entry, added registers and lr protection at the entry, and restored the protected registers before returning), and then added a C original program (main), called this procedure in main, and found that the string can be output in the console.
So I checked some information (Google) and finally found the problem. It turned out that the C library was not initialized in the assembly code. So I changed the assembly code to the following:
    area |.text|,code,readonly
    code32
    export main; Export main, main function,
    import _printf
    import __main; C Library Entry
    import ||Lib armlib||,weak
    
main
    stmfd sp!,{lr}
    adr r0,strhello
    bl _printf
    
    ldmfd sp!,{pc}

strhello
    dcb "hello!/n/0"
  
    end
The debugging is successful, and the string information can be successfully output to the console.

This program has a main function main, so the main symbol needs to be exported. In addition, the label __main (import __main) needs to be introduced to represent the beginning of C library initialization; because the _printf function is called in the program and the ARM C library is used, it is necessary to import |Lib$Request$armlib|. The weak at the end indicates that if the label of this line cannot be found, no connection error will be reported.
The meaning of |.text|: It indicates the code segment generated by the C compiler, or the code segment used to be associated with the C library in some way. Here it refers to the latter.

References:
1. ARM assembly optimization, http://blog.csdn.net/guanchanghui/archive/2007/06/27/1669513.aspx
2. RealView Compiler Tool 2.0 Assembler Guide, http://infocenter.arm.com/help/index.jsp