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POIs function declaration essential to C programming?
 

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  1. POIs function declaration essential to C programming?
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    <p>I used to believe that we should declare a function which is defined in another file before use it, but recently I changed my way of thinking due to an experience of programming. For three files, <code>C</code> and <code>ASM</code>:</p> <p><em><strong>main.c</em></strong></p> <pre><code>extern test_str; /*extern myprint*/ --&gt; If I add the line, gcc will report an error: called object ‘myprint’ is not a function void Print_String() { myprint("a simple test", test_str); } </code></pre> <p><em><strong>kernel.asm</em></strong></p> <pre><code>extern Print_String [section .text] global _start global test_str test_str dd 14 _start: call Print_String jmp $ </code></pre> <p><em><strong>another.asm</em></strong></p> <pre><code>[section .text] global myprint myprint: mov edx, [esp + 8] mov ecx, [esp + 4] mov ebx, 1 mov eax, 4 int 0x80 ret </code></pre> <p><em>compile</em></p> <pre><code>nasm -f elf another.asm -o another.o gcc -c -g main.c -o main.o nasm -f elf kernel.asm -o kernel.o ld -o final main.o kernel.o another.o </code></pre> <p><em>result</em></p> <pre><code>./final a simple test </code></pre> <p>In my view, if I want to use the function <code>myprint</code> in <strong><em>main.c</em></strong>, I should declare it using <code>extern</code> beforehand, because <code>myprint</code> is defined in another file, but the result is exactly opposite. Just as <strong><em>main.c</em></strong> shows above. If I add the line <code>extern myprint</code>, I will get an error. However, without that declaration, I will get the right result. What's more, I didn't define function <code>myprint</code> in <strong><em>main.c</em></strong>, why can I use that function? Shouldn't I declare it beforehand?</p>
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    1. COWhat about if you compile with `gcc -c -g -W -Wall -pedantic -o main.o main.c`? What does GCC thing about this, then?
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      1. USMartin Green
    2. COTry using extern myprint() to tell the compiler it's a function.
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