Userland: Implement nc command

This is a very simple version of the nc (netcat) command. It only
supports outgoing TCP connections, and has no options aside from the
target host and port.

Userland/nc.cpp

@@ -0,0 +1,126 @@
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/select.h>
+#include <sys/socket.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#define max(a, b) ((a > b) ? a : b)
+
+int main(int argc, char** argv)
+{
+    if (argc < 3) {
+        fprintf(stderr, "Usage: nc <host> <port>\n");
+        return -1;
+    }
+
+    const char* addr_str = argv[1];
+    int port = atoi(argv[2]);
+
+    int fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (fd < 0) {
+        perror("socket");
+        return 1;
+    }
+
+    struct timeval timeout {
+        3, 0
+    };
+    int rc = setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
+    if (rc < 0) {
+        perror("setsockopt");
+        return 1;
+    }
+    rc = setsockopt(fd, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof(timeout));
+    if (rc < 0) {
+        perror("setsockopt");
+        return 1;
+    }
+
+    struct sockaddr_in dst_addr;
+    memset(&dst_addr, 0, sizeof(dst_addr));
+
+    dst_addr.sin_family = AF_INET;
+    dst_addr.sin_port = htons(port);
+    rc = inet_pton(AF_INET, addr_str, &dst_addr.sin_addr);
+    if (rc <= 0) {
+        perror("inet_pton");
+        return 1;
+    }
+
+    rc = connect(fd, (struct sockaddr*)&dst_addr, sizeof(dst_addr));
+    if (rc < 0) {
+        perror("connect");
+        return 1;
+    }
+
+    fd_set readfds, writefds, exceptfds;
+
+    for (;;) {
+        FD_ZERO(&readfds);
+        FD_ZERO(&writefds);
+        FD_ZERO(&exceptfds);
+
+        int highest_fd = 0;
+
+        FD_SET(STDIN_FILENO, &readfds);
+        FD_SET(STDIN_FILENO, &exceptfds);
+        highest_fd = max(highest_fd, STDIN_FILENO);
+        FD_SET(fd, &readfds);
+        FD_SET(fd, &exceptfds);
+        highest_fd = max(highest_fd, fd);
+
+        int ready = select(highest_fd + 1, &readfds, &writefds, &exceptfds, NULL);
+        if (ready == -1) {
+            if (errno == EINTR)
+                continue;
+
+            perror("select");
+            return 1;
+        }
+
+        if (FD_ISSET(STDIN_FILENO, &readfds)) {
+            char buf[1024];
+            int nread = read(STDIN_FILENO, buf, sizeof(buf));
+            if (nread < 0) {
+                perror("read(STDIN_FILENO)");
+                return 1;
+            }
+
+            // stdin closed
+            if (nread == 0) {
+                return 0;
+            }
+
+            if (write(fd, buf, nread) < 0) {
+                perror("write(fd)");
+                return 1;
+            }
+        }
+
+        if (FD_ISSET(fd, &readfds)) {
+            char buf[1024];
+            int nread = read(fd, buf, sizeof(buf));
+            if (nread < 0) {
+                perror("read(fd)");
+                return 1;
+            }
+
+            // remote end closed
+            if (nread == 0) {
+                return 0;
+            }
+
+            if (write(STDOUT_FILENO, buf, nread) < 0) {
+                perror("write(STDOUT_FILENO)");
+                return 1;
+            }
+        }
+    }
+
+    return 0;
+}