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    <p>The compiler simply interprets this as calling <code>printf</code> with two strings as arguments (but see Zack's comment).</p> <h3>This happens at compile time (i.e. the compiler does this):</h3> <p>The strings (<code>"%%%s"</code> and <code>"hello"</code>) are copied directly into the executable, the compiler leaves them as-is.</p> <h3>This happens at runtime (i.e. the C standard library does this when the app is running):</h3> <p><code>printf</code> stand for 'print formatted'. When this function is called, it needs at least one argument. The first argument is the format. The next arguments are "arguments" to this format. They are formatted as specified in the first argument.</p> <p><img src="https://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Printf.svg/500px-Printf.svg.png" alt="img"></p> <hr> <h1>About optimization</h1> <p>I have written an example and ran Clang/LLVM with <code>-S</code>:</p> <pre><code>$ emacs printftest.c $ clang printftest.c -S -o printftest_unopt.s # not optimized $ clang printftest.c -S -O -o printftest_opt.s # optimized: -O flag </code></pre> <h2>C code</h2> <pre><code>#include &lt;stdio.h&gt; int main() { printf("%%%s", "hello"); return 0; } </code></pre> <h2>printftest_unopt.s (not optimized)</h2> <pre><code>; ... _main: pushq %rbp movq %rsp, %rbp subq $16, %rsp movl $0, %eax movl $0, -4(%rbp) movl %eax, -8(%rbp) xorb %al, %al leaq L_.str(%rip), %rdi leaq L_.str1(%rip), %rsi callq _printf ; printf called here &lt;---------------- movl %eax, -12(%rbp) movl -8(%rbp), %eax addq $16, %rsp popq %rbp ret .section __TEXT,__cstring,cstring_literals L_.str: .asciz "%%%s" L_.str1: .asciz "hello" ; ... </code></pre> <h2>printftest_opt.s (optimized)</h2> <pre><code>; ... _main: pushq %rbp movq %rsp, %rbp leaq L_.str(%rip), %rdi leaq L_.str1(%rip), %rsi xorb %al, %al callq _printf ; printf called here &lt;---------------- xorl %eax, %eax popq %rbp ret .section __TEXT,__cstring,cstring_literals L_.str: .asciz "%%%s" L_.str1: .asciz "hello" ; ... </code></pre> <h2>Conclusion</h2> <p>As you can see (in the <code>__TEXT,__cstring,cstring_literals</code> section <strong>and</strong> the <code>callq</code> to <code>printf</code>), LLVM (a very, very good compiler) does <strong>not</strong> optimize <code>printf("%%%s", "hello");</code>. :)</p>
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    1. POprintf("%%%s","hello")
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    1. CO....... Winner!
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    2. COThe compiler *might* (but is not required to) interpret it for optimization purposes. The compiler is allowed to know what the standard says the `stdio.h` functions do, and to make optimizations based on that knowledge -- for instance, `printf("%%%s", "Hello")` could be transformed to `printf("%%Hello")` or even `fputs("%Hello", stdout)` in the executable.
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    3. COYou are right, @Zack, I forgot. I have edited the answer. :)
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