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    <p>Here's a straight forward implementation of the <code>md5sum</code> command that computes and displays the MD5 of the file specified on the command-line. It needs to be linked against the OpenSSL library (<code>gcc md5.c -o md5 -lssl</code>) to work. It's pure C, but you should be able to adapt it to your C++ application easily enough.</p> <pre><code>#include &lt;sys/types.h&gt; #include &lt;sys/stat.h&gt; #include &lt;sys/mman.h&gt; #include &lt;fcntl.h&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; #include &lt;openssl/md5.h&gt; unsigned char result[MD5_DIGEST_LENGTH]; // Print the MD5 sum as hex-digits. void print_md5_sum(unsigned char* md) { int i; for(i=0; i &lt;MD5_DIGEST_LENGTH; i++) { printf("%02x",md[i]); } } // Get the size of the file by its file descriptor unsigned long get_size_by_fd(int fd) { struct stat statbuf; if(fstat(fd, &amp;statbuf) &lt; 0) exit(-1); return statbuf.st_size; } int main(int argc, char *argv[]) { int file_descript; unsigned long file_size; char* file_buffer; if(argc != 2) { printf("Must specify the file\n"); exit(-1); } printf("using file:\t%s\n", argv[1]); file_descript = open(argv[1], O_RDONLY); if(file_descript &lt; 0) exit(-1); file_size = get_size_by_fd(file_descript); printf("file size:\t%lu\n", file_size); file_buffer = mmap(0, file_size, PROT_READ, MAP_SHARED, file_descript, 0); MD5((unsigned char*) file_buffer, file_size, result); munmap(file_buffer, file_size); print_md5_sum(result); printf(" %s\n", argv[1]); return 0; } </code></pre>
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    1. POHow to get the MD5 hash of a file in C++?
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    1. COon 32bit platforms, your mmap has a limit as to how large the file can be, though it is an elegant solution to the problem. On 32bit Windows, for example, you couldn't MD5 a DVD with this code.
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      1. USChris K
    2. CO@ChrisKaminski you can slide 4GB window of memory-mapped file on 32-bit platform.
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    3. COExcellent answer, it's helped me immensely. However, you don't call munmap afterward. It's no memory leak for you because the program ends immediately afterwards, but if some buffoon like myself copies the code and doesn't put in munmap, we get a memory leak in our program ;) The solution: munmap(file_buffer, file_size);
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