Added a small set of Linux system call examples.

syscall-samples/.gitignore

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+*~
+a.out

syscall-samples/README.md

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+*Note*: Each C source file here is a self-contained Linux program.
+        `gcc` them individually to build and run.

syscall-samples/getcwd.c

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+/**
+ * This program demonstrates invocation of the getcwd
+ * system call (183) using the syscall function.
+ */
+#include <linux/types.h>
+#include <linux/unistd.h>
+#include <string.h>
+
+int main(int argc, char *argv[]) {
+  char result[1024]; // Lots of room!
+
+  // We keep the hardcoded system call numbers to illustrate
+  // that they are still just that: numbers.
+  syscall(183, result);
+
+  // Append a newline for better looks.
+  int originalLength = strlen(result);
+  result[originalLength] = '\n';
+
+  // Must re-terminate the string, of course.
+  result[originalLength + 1] = '\0';
+
+  syscall(4, 0, result, strlen(result));
+}

syscall-samples/mkdir.c

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+/**
+ * This program demonstrates invocation of the mkdir
+ * system call (39) using the syscall function.
+ */
+#include <linux/types.h>
+#include <linux/unistd.h>
+#include <string.h>
+
+int main(int argc, char *argv[]) {
+  // We demonstrate the use of command-line arguments here.
+  // But note the non-existent error handling (all the better
+  // to illustrate the error code below).
+  int result = syscall(39, argv[1], 0777 /* Note: octal */);
+
+  // A result of -1 means that something went wrong.  Otherwise,
+  // check for your new directory!
+  if (result == -1) {
+    // Don't use this error message in "real" programs. O_o
+    char *errorMessage = "Herp derp mkderp\n";
+    syscall(4, 2, errorMessage, strlen(errorMessage));
+  }
+}

syscall-samples/sysinfo-ram.c

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+/**
+ * This program demonstrates invocation of the sysinfo
+ * system call (116) using the syscall function.  Some
+ * of its output is then displayed.
+ */
+#include <linux/kernel.h>
+#include <linux/types.h>
+#include <linux/unistd.h>
+
+// For convenience, we just use the stdio functions to display
+// the output for this one.
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  // The sysinfo structure is in kernel.h.
+  struct sysinfo result;
+
+  // We keep the hardcoded system call numbers to illustrate
+  // that they are still just that: numbers.
+  syscall(116, &result);
+
+  // Display some of the returned RAM information.
+  printf("Total RAM: %ld, free RAM: %ld\n", result.totalram, result.freeram);
+}