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POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
 

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  1. POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
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    <p>What is the proper way to allocate a specific region of memory using 'mmap' in C? I've read /proc/self/maps to determine that the area is available.</p> <p>I've tried the following, but it crashes when trying to write to the allocated memory:</p> <pre><code>// rdram is defined in header as: #define rdram ((unsigned int *)0x80000000) printf( "rdram=0x%x", (int)rdram ); printf( "munmapping" ); munmap ((void*)0x80000000, 0x800000); printf( "mmapping" ); if(mmap ((void*)0x80000000, 0x800000, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0) &lt;= 0) { printf( "mmap(0x80000000) failed" ); } else { for (i=0; i&lt;(0x800000/4); i++) { printf( "writing a zero at 0x%x", (0x80000000 + i) ); rdram[i]=0; // &lt;&lt;---------CRASH HERE--------&lt;&lt; } printf( "done writing zeros" ); } </code></pre> <p>It was suggested on my earlier question (which got closed for not being stated clearly), that I am not properly detecting when mmap failed, and that I should use posix_typed_mem_open instead of -1 for the fildes parameter. I thought I would try the following (please let me know if there is something wrong with this):</p> <pre><code> int m; int p; printf( "rdram=0x%x", (int)rdram ); printf( "munmapping" ); munmap ((void*)0x80000000, 0x800000); printf( "posix_typed_mem_open" ); p = posix_typed_mem_open( "/memory/ram", O_RDWR, POSIX_TYPED_MEM_ALLOCATE_CONTIG ); printf( "mmapping" ); m = (int) mmap ((void*)0x80000000, 0x800000, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, p, 0); if( m == (int) MAP_FAILED ) { printf( "mmap failed" ) } else { for (i=0; i&lt;(0x800000/4); i++) { printf( "writing a zero at 0x%x", (0x80000000 + i) ); rdram[i]=0; } printf( "done writing zeros" ); } </code></pre> <p>This is being run on one of one of my testers' devices (the memory mapping on my device is not suitable), so I'd like to know this makes sense before I have him run it to see what kind of output he gets.</p>
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    1. USpaulscode
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      1. POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
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      1. VTUpMod
    1. COMaybe I missed something obvious, but why don't you just call malloc and let the c-library handle the dirt stuff?
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      1. POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
      1. USNils Pipenbrinck
    2. COBecause malloc does not let you specify where the allocated memory is located. As indicated in the question, I need to allocate a specific region of memory (0x80000000 in this case).
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      1. POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
      1. USpaulscode
    3. CO@paulcode: The code looks fine now .As for the output of this code, I am a bit skeptical.You are trying to access 0x80000000 (physical memory) via your process (virtual memory). I dont think you can access part of the main memory (unless you write a driver module to get the actual address itself). Even if you are able to get hold of the physical address, you would need to lock it at the kernel level so that it is never given to some other process. Let me know what output you get from your tester.
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      1. POHow do you allocate a specific area of memory using the 'mmap' command in C? (Android NDK)
      1. UStomkaith13
 

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