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POAccessing linker defined symbol in C
 

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    <p>I have a linker symbol (let's call it __BASE_ADDR), defined in a linker command script, which contains the address of the longword <em>after</em> a longword that I need to use (e.g. I need to check longword at address 0x0000000C and __BASE_ADDR is equal to 0x00000010). So in the code I try to use __BASE_ADDR and subtract 4 from it to access 0x0000000C.</p> <p>My linker map output correctly shows the value of __BASE_ADDR to be 0x00000010. However when I try to access the location using the following C code it doesn't work:</p> <pre><code>extern unsigned char *__BASE_ADDR; void someFunc( void ) { unsigned long Val = *((unsigned long *) (__BASE_ADDR - 4)); /* Use Val for some comparison...*/ /* ... */ } </code></pre> <p>Val gets set to the wrong value. What appears to be happening is that whatever longword is at __BASE_ADDR is taken as an address, 4 is subtracted, and the contents of the resulting address is put into Val.</p> <p>But when I do the following, it does work fine and Val is set correctly to whatever resides at address 0x0000000C:</p> <pre><code>extern unsigned char __BASE_ADDR[]; void someFunc( void ) { unsigned long Val = *((unsigned long *) (__BASE_ADDR - 4)); /* Use Val for some comparison...*/ /* ... */ } </code></pre> <p>Can anyone shed light on this? I understand why you can't access a linker symbol's contents (it technically has no contents), but why would there be a difference between accessing the linker symbol as a pointer and accessing it as an array?</p> <p>Thanks.</p>
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