Add file descriptors and basic cat

src/user/app/cat/cat.c

@@ -0,0 +1,45 @@
+#include "src/user/syscall.h"
+
+#define BLOCK_SIZE 65536
+
+#define PRINT_S(s) write(1, s, sizeof(s) - 1);
+#define PRINT_D(s) write(1, s, string_length(s));
+
+uint64_t cat(const char *path) {
+  uint64_t fd = open(path);
+
+  if (fd == (uint64_t)-1) {
+    PRINT_S("cat: path does not exist\n");
+    return (uint64_t)-1;
+  }
+
+  static char buffer[BLOCK_SIZE];
+  uint64_t bytes;
+  while ((bytes = read(fd, BLOCK_SIZE, buffer))) {
+    if (bytes == (uint64_t)-1) {
+      PRINT_S("cat: could not read file\n");
+      close(fd);
+      return (uint64_t)-1;
+    }
+    write(1, buffer, bytes);
+  }
+
+  close(fd);
+
+  return 0;
+}
+
+uint64_t main(uint64_t argc, const char **argv) {
+  if (argc == 1) {
+    return cat(".");
+  }
+
+  uint64_t code = 0;
+  for (uint64_t i = 1; i < argc; i++) {
+    if (cat(argv[i]) == (uint64_t)-1) {
+      code = (uint64_t)-1;
+    }
+  }
+
+  return code;
+}