| Author | beginning argument ( Replies received: 4 ) |
| pandoraems |
 Posted 03-09-2008 at 23:32 |
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09-03-2008
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following is a snip from STm's example of using DMA with ADC:
u16 ADC_ConvertedValue[512];
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DMA_InitStructure.DMA_MemoryBaseAddr = (u32)&ADC_ConvertedValue;
I am wondering about why it needs the ampersand in front of ADC_ConvertedValue ? I would think that DMA_MemoryBaseAddr should contain the address of the first element of the array , which what ADC_ConvertedValue is. And not the address to a pointer to the first member of ADC_ConvertedValue[] (which seems like what the example code does and is quite unintuitive)
am i confused? or is the example incorrect?
Thanks
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| lanchon |
 Posted 04-09-2008 at 05:40 |
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11-02-2008
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well, *ADC_ConvertedValue is the first u16, and I guess ADC_ConvertedValue is a const pointer to u16 and since it's const it needn't be allocated in memory at all. I think the result of taking the address of such a const should at lest be undefined and might be illegal (maybe only in C++). so I'd say it's a bug. the horrible structure of the lib which require casts all over may be hiding the error until runtime.
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| andreas1 |
Posted 04-09-2008 at 16:22 |
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In ANSI C, &ADC_ConvertedValue is valid and is a pointer of type pointer-to-512-element-array-of-u16, as opposed to using ADC_ConvertedValue without the ampersand, which decays into a pointer of type pointer-to-u16. Both point to the same location, the first element of the array. In this example, the type of the pointer is irrelevant since it is immediately cast to u32 anyway.
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| pandoraems |
Posted 04-09-2008 at 16:23 |
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Thanks for reply
I've tried both and surprisingly both seemed to work! (i.e with and without &),
which boggles my mind.
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| lanchon |
Posted 04-09-2008 at 23:20 |
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> In ANSI C, &ADC_ConvertedValue is valid and is a pointer of type pointer-to-512-element-array-of-u16
thanks. so I guess a pointer-to-512-element-array-of-u16 autoconverts to pointer-to-u16, but size of *pointer-to-512-element-array-of-u16 covers all the array.
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what should DMA_MemoryBaseAddr be pointing to?