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POsegfault on write() with ~8MB buffer (OSX, Linux)
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copied!<p>I was curious what kind of buffer sizes write() and read() could handle on Linux/OSX/FreeBSD, so I started playing around with dumb programs like the following:</p> <pre><code>#include <unistd.h> #include <fcntl.h> #include <stdio.h> #include <string.h> #include <sys/stat.h> int main( void ) { size_t s = 8*1024*1024 - 16*1024; while( 1 ) { s += 1024; int f = open( "test.txt", O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR | S_IXUSR ); char mem[s]; size_t written = write( f, &mem[0], s ); close( f ); printf( "(%ld) %lu\n", sizeof(size_t), written ); } return 0; } </code></pre> <p>This allowed me to test how close to a seeming "8MB barrier" I could get before segfaulting. Somewhere around the 8MB mark, my program dies, here's an example output:</p> <pre><code>(8) 8373248 (8) 8374272 (8) 8375296 (8) 8376320 (8) 8377344 (8) 8378368 (8) 8379392 (8) 8380416 (8) 8381440 (8) 8382464 Segmentation fault: 11 </code></pre> <p>This is the same on OSX and Linux, however my FreeBSD VM is not only much faster at running this test, it also can go on for quite a ways! I've successfully tested it up to 511MB, which is just a ridiculous amount of data to write in one call.</p> <p>What is it that makes the write() call segfault, and how can I figure out the maximum amount that I can possibly write() in a single call, without doing something ridiculous like I'm doing right now?</p> <p>(Note, all three operating systems are 64-bit, OSX 10.7.3, Ubuntu 11.10, FreeBSD 9.0)</p>

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