30m version

.gitignore

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+bin/

Makefile

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+SRC=main.c
+TARGET=n
+BUILDDIR=bin/
+PREFIX=/usr/local/bin/
+override CFLAGS+=-O3
+
+all:
+	mkdir -p $(BUILDDIR)
+	$(CC) $(CFLAGS) $(SRC) -o $(BUILDDIR)$(TARGET)
+
+clean:
+	rm -r $(BUILDDIR)
+
+install: all
+	install $(BUILDDIR)$(TARGET) $(PREFIX)$(TARGET)
+
+uninstall:
+	rm -rf $(PREFIX)$(TARGET)

README.md

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+# n
+
+A simple NTP client.
+
+## Installation
+
+Don’t use mine, because it will explode, launch missiles, and rain on your
+porch.
+
+If you still want to, `make install` will install this utility to
+`/usr/local/bin`. You can change the installation directory by providing the
+`PREFIX` option to `make install`.
+
+## Usage
+
+```
+Usage: bin/n [options]
+Options:
+  -h HOST, --host=HOST  NTP server to use
+             defaults to 'europe.pool.ntp.org'
+  -p PORT, --port=PORT  NTP server port to use (must be numerical)
+             defaults to '123'
+  --help                Display this message and exit
+```
+
+<hr/>
+
+Have fun!

main.c

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+#include <getopt.h>
+#include <netdb.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+#include <netinet/in.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#define EPOCH 2208988800ull
+
+// assumes either little or big endian. pdp-endian, youre out of luck
+#define NLITTLE 0x41424344UL
+#define NENDIAN  ('ABCD')
+
+typedef struct {
+#if NENDIAN == NLITTLE
+  unsigned int mode: 3;
+  unsigned int vn: 3;
+  unsigned int li: 2;
+#else
+  unsigned int li: 2;
+  unsigned int vn: 3;
+  unsigned int mode: 3;
+#endif
+  uint8_t stratum;
+  uint8_t poll;
+  uint8_t prec;
+  uint32_t delay;
+  uint32_t dispersion;
+  uint32_t refid;
+  uint32_t rtms;
+  uint32_t rtmf;
+  uint32_t otms;
+  uint32_t otmf;
+  uint32_t rctms;
+  uint32_t rctmf;
+  uint32_t ttms;
+  uint32_t ttmf;
+} __attribute__((packed, aligned(1))) ntp;
+
+typedef struct {
+  const char* host;
+  int port;
+} ntp_info;
+
+ntp_info NDEFAULT = {.host="europe.pool.ntp.org", .port = 123};
+
+void err(char* msg) {
+  fprintf(stderr, "Error %s.\n", msg);
+  exit(1);
+}
+
+void usage(char* program, int code) {
+  fprintf(stderr, "Usage: %s [options]\n", program);
+  fputs("Options:\n", stderr);
+  fputs("  -h HOST, --host=HOST  NTP server to use\n", stderr);
+  fputs("             defaults to 'europe.pool.ntp.org'\n", stderr);
+  fputs("  -p PORT, --port=PORT  NTP server port to use (must be numerical)\n", stderr);
+  fputs("             defaults to '123'\n", stderr);
+  fputs("  --help                Display this message and exit\n", stderr);
+  exit(code);
+}
+
+ntp_info args(int argc, char** argv) {
+  char c;
+  ntp_info res = NDEFAULT;
+  static struct option long_options[] = {
+    {"host", required_argument, 0, 'h'},
+    {"port", required_argument, 0, 'p'},
+    {"help", no_argument, 0, 'u'},
+    {NULL, 0, NULL, 0}
+  };
+  while ((c = getopt_long(argc, argv, "t:a:", long_options, 0)) != -1) {
+    switch (c) {
+      case 'h': res.host = optarg; break;
+      case 'p': {
+        char* end;
+        res.port = strtol(optarg, &end, 10);
+        if (end[0] != 0) {
+          fputs("Port is not numerical.\n", stderr);
+          usage(argv[0], 1);
+        }
+        break;
+     }
+     case 'u': usage(argv[0], 0);
+     default: usage(argv[0], 1);
+    }
+  }
+  return res;
+}
+
+int main(int argc, char** argv) {
+  int sock;
+  ntp n;
+  struct sockaddr_in a;
+  struct hostent* s;
+  ntp_info i = args(argc, argv);
+
+  memset(&n, 0, sizeof(ntp));
+  n.vn = 3;
+  n.mode = 3;
+
+  sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+  if (sock < 0) err("opening socket");
+
+  s = gethostbyname(i.host);
+  if (!s) err("resolving host");
+
+  bzero(&a, sizeof(a));
+  a.sin_family = AF_INET;
+  a.sin_port = htons(i.port);
+  bcopy(s->h_addr, &a.sin_addr.s_addr, s->h_length);
+
+  if (connect(sock, (struct sockaddr *) &a, sizeof(a)) < 0) err("connecting");
+  if (write(sock, &n, sizeof(ntp)) < 0) err("writing to network");
+  if (read(sock, &n, sizeof(ntp)) < 0) err("reading from network");
+
+  n.ttms = ntohl(n.ttms);
+
+  time_t tm = n.ttms - EPOCH;
+
+  fputs(ctime(&tm), stdout);
+
+  return 0;
+}