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copied!<p><code>errno</code> is not always a global variable. It is only required to be an lvalue, and may be a macro hiding a function call. From <a href="http://pubs.opengroup.org/onlinepubs/009695399/functions/errno.html" rel="nofollow">the specification</a>:</p> <blockquote> <p>It is unspecified whether errno is a macro or an identifier declared with external linkage.</p> </blockquote> <p>In fact, it is usually <em>not</em> a global variable since it is usually implemented to be thread-local (and so requires a function call to retrieve a pointer to the TLS block).</p> <p>You are better off having a C wrapper that calls the assembly function and sets <code>errno</code>.</p> <hr> <p>EDIT: Since you can't use a C function (which, IMO, is senseless, since <code>errno</code> is firmly a C/POSIX concept), you will have to implement the <code>errno</code> gathering yourself. Find the definition of <code>errno</code> in <code>errno.h</code>, and implement whatever is there in assembly. For example, my <code>errno.h</code> defines <code>errno</code> as</p> <pre><code>extern int * __error(void); #define errno (*__error()) </code></pre> <p>Therefore, I would make a call to the <code>__error</code> function (which returns an <code>int *</code>), then store to the returned address. For example, here's the assembly my system produces for setting <code>errno</code>:</p> <pre><code>$ gcc -xc - -o- -S <<EOF #include <errno.h> main() { errno = 3; return 0; } EOF .section __TEXT,__text,regular,pure_instructions .globl _main .align 4, 0x90 _main: Leh_func_begin1: pushq %rbp Ltmp0: movq %rsp, %rbp Ltmp1: subq $16, %rsp Ltmp2: callq ___error movl $3, (%rax) ... </code></pre> <p>Your system will probably have a different implementation of <code>errno</code>.</p>

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