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copied!<p><code>l1[]</code> and <code>l2[]</code> are removed during compiler optimisation because both are <code>local and unused variables</code> (no chance that both variable will be use any where else). </p> <p>you can compile your code with <code>-S</code> option to generate assembly code: and there is no definition for <code>l1[]</code> and <code>l2[]</code> in main not even any where else: </p> <p><sub>input file was <code>x.c</code>, compiled with command <code>gcc -S x.c</code> to generate assembly file <code>x.s</code></sub> </p> <pre><code>main: pushl %ebp movl %esp, %ebp subl $16, %esp movb $28, -4(%ebp) movb $44, -3(%ebp) movb $60, -2(%ebp) movb $76, -1(%ebp) leave ret .size main, .-main .data .type l2.1250, @object .size l2.1250, 4 </code></pre> <p>But you can find definition for <code>g1[] and g2[]</code>. </p> <pre><code> .file "x.c" .globl g1 .data .type g1, @object .size g1, 4 g1: .byte 26 .byte 42 .byte 58 .byte 74 .type g2, @object .size g2, 4 g2: .byte 27 .byte 43 .byte 59 .byte 75 .text .globl main .type main, @function </code></pre> <p><strong>Additionally</strong> , It would be interesting to know if you compile if you compile you code with flag <code>-O3</code> <em>optimisation flag level 3</em> then only definition of <code>g1[]</code> is present. and global static variables(private to the file) are also removed. </p> <p><sub>input file was <code>x.c</code>, compiled with command <code>gcc -S -O3 x.c</code> to generate assembly file <code>x.s</code></sub> </p> <p>below: </p> <pre><code> .file "x.c" .text .p2align 4,,15 .globl main .type main, @function main: pushl %ebp movl %esp, %ebp popl %ebp ret .size main, .-main .globl g1 .data .type g1, @object .size g1, 4 g1: .byte 26 .byte 42 .byte 58 .byte 74 .ident "GCC: (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5" .section .note.GNU-stack,"",@progbits </code></pre> <p><code>g1[]</code> is global data only present, and <code>g2[]</code> is removed in <code>-O3</code>. </p> <p><code>g2[]</code> use defined <code>static unsigned char g2[]</code> so access only within this file and not use so again unused. But <code>g1[]</code> is global it may be useful by other program if other file include it this. And compiler is not allowed to optimisation away global objects. </p> <p>Reference: <a href="https://stackoverflow.com/questions/1229430/how-do-i-prevent-my-unused-global-variables-being-compiled-out">How do I prevent my 'unused' global variables being compiled out?</a> </p> <p>So, this all due to compiler optimisation! </p>

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