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POHow to read and write with mmap() on a Linux system
 

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  1. POHow to read and write with mmap() on a Linux system
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    <p>I need to make some stream in and out classes using mmap() in Linux. To do so I tried to make some test code that writes some integers to a file, saves it, loads it again and write the data in the file to cout. If that test code works, then It wont be a problem making stream in and out afterwards. </p> <p>When I first started out I got segment faults and If I did not get that nothing happened, so I googled a little. I found this book <a href="http://www.advancedlinuxprogramming.com/alp-folder/alp-ch05-ipc.pdf" rel="nofollow">http://www.advancedlinuxprogramming.com/alp-folder/alp-ch05-ipc.pdf</a> where around page 107 there is some usefull code. I copy pasted that code and made some small changes and got this code:</p> <pre><code>int fd; void* file_memory; /* Prepare a file large enough to hold an unsigned integer. */ fd = open ("mapTester", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR); //Make the file big enough lseek (fd, 4 * 10 + 1, SEEK_SET); write (fd, "", 1); lseek (fd, 0, SEEK_SET); /* Create the memory mapping. */ file_memory = mmap (0, 4 * 10, PROT_WRITE, MAP_SHARED, fd, 0); close (fd); /* Write a random integer to memory-mapped area. */ sprintf((char*) file_memory, "%d\n", 22); /* Release the memory (unnecessary because the program exits). */ munmap (file_memory, 4 * 10); cout &lt;&lt; "Mark" &lt;&lt; endl; //Start the part where I read from the file int integer; /* Open the file. */ fd = open (argv[1], O_RDWR, S_IRUSR | S_IWUSR); /* Create the memory mapping. */ file_memory = mmap (0, 4 * 10, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); close (fd); /* Read the integer, print it out, and double it. */ scanf ((char *) file_memory, "%d", &amp;integer); printf ("value: %d\n", integer); sprintf ((char*) file_memory, "%d\n", 2 * integer); /* Release the memory (unnecessary because the program exits). */ munmap (file_memory, 4 * 10); </code></pre> <p>But I get a segment fults after the "mark" cout. </p> <p>Then I replace the "read part" with this:</p> <pre><code>fd = open("mapTester", O_RDONLY); int* buffer = (int*) malloc (4*10); read(fd, buffer, 4 * 10); for(int i = 0; i &lt; 1; i++) { cout &lt;&lt; buffer[i] &lt;&lt; endl; } </code></pre> <p>That is some working code that shows me that the file is empty. I tries out a couple of ways to write to the mapping without any change in result. </p> <p>So how am I amble to make my code write? And does my mmap read code seem okay (just in case you can see some obvious flaws)? </p> <p>I have found some other resources that did not help me yet, but because I am a new user I may only post max 2 links. </p>
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