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    <blockquote> <p>int* array = malloc(sizeof(int)<em>arraysize); int</em> temp = malloc(sizeof(int)*arraysize);</p> </blockquote> <p>In general, whenever you allocate memory, check that the allocation succeeded:</p> <pre><code>int *array = NULL, *temp = NULL; if (NULL == (array = malloc(sizeof(int)*arraysize))) { fprintf(stderr, "Out of memory allocating %d bytes\n", sizeof(int)*arraysize); abort(); } if (NULL == (temp = malloc(sizeof(int)*arraysize))) { fprintf(stderr, "Out of memory allocating %d bytes\n", sizeof(int)*arraysize); abort(); } </code></pre> <p>Then, a possibility would be to implement mergesorting <em>on disk</em>, using a file of integers (you can mmap() the file, too).</p> <p>But I find it strange that an allocation of 300000 integers on the heap - 4.8 megabytes at the most, using 64-bit integers - can cause an allocation error, so I think this is something in the mergesort implementation; maybe something having to do with a recursive implementation.</p> <p>I'd start with compiling the program with full debug information, and checking the core dump with <code>gdb</code>.</p> <h2>A "simple" malloc problem</h2> <p>Having to handle a very large array of <em>ASCII strings representing numbers</em>, you could start by first converting it to a file of <em>integers</em>.</p> <pre><code>FILE *fi, *fo, *ft; char buffer[20]; int array[4096], b = 0; fi = fopen("DATASET.dat", "r"); if (NULL == fi) { fprintf(stderr, "Cannot open input file\n"); abort(); } fo = fopen("INTEGER.dat", "w"); if (NULL == fo) { fprintf(stderr, "Cannot open output file\n"); abort(); } ft = fopen("TEMP.dat", "w"); if (NULL == ft) { fprintf(stderr, "Cannot open output file\n"); abort(); } for(i=0; i&lt;arraysize; i++){ fgets(buffer, 20, fi); array[b++] = atoi(buffer); if (4096 == b) { if (b != fwrite(buffer, sizeof(int), b, fo)) { fprintf(stderr, "write error\n"); abort(); } if (b != fwrite(buffer, sizeof(int), b, ft)) { fprintf(stderr, "write error\n"); abort(); } b = 0; } } if (b) { if (b != fwrite(buffer, sizeof(int), b, fo)) { fprintf(stderr, "write error\n"); abort(); } if (b != fwrite(buffer, sizeof(int), b, ft)) { fprintf(stderr, "write error\n"); abort(); } } fclose(fi); fi = NULL; fclose(fo); fo = NULL; fclose(ft); ft = NULL; </code></pre> <p>Now you have an <code>INTEGER.dat</code> file which is made of integers of fixed size. It is, to all intents and purposes, a file copy of an array in memory. Same goes for the temporary array.</p> <p>And you <em>can</em> tell the system to treat that file as if it was an array in memory.</p> <pre><code>int *sort = NULL; int *temp = NULL; // Temp is not shown -- identical treatment as sort fd = open ("INTEGERS.dat", O_RDWR); if (fd == -1) { fprintf(stderr, "cannot reopen output\n"); abort(); } if (MAP_FAILED == (sort = mmap (0, arraysize*sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0))) { fprintf(stderr, "mmap error\n"); abort(); } if (-1 == close (fd)) { fprintf(stderr, "error closing output file\n"); return 1; } do_sort(sort, temp, arraysize); if (-1 == munmap (sort, arraysize*sizeof(int))) { fprintf(stderr, "error releasing mmap for %s\n", "sort"); abort(); } </code></pre>
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    1. COThanks for help. I mistyped the value of the number. It should be 300 million.
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    2. COI also applied your suggestion. It gives an error while trying to sort 400 million elements: Out of memory allocating 1600000000 bytes.
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    3. COHmmm. Then you might be asking for 1.2, 2.4 or 4.8 *giga* bytes of memory... it might be too much. Is this a homework of some sort? That is, do you *have* to do it that way, and is that humongous array the professor's way of telling you to *really* implement it as a file-based (or **mmap**) sort instead of using a RAM shortcut? (if it were *my* professor, it would be :-) ).
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