From: Scott Worley <scottworley@scottworley.com>
Date: Mon, 1 May 2017 08:20:54 +0000 (-0700)
Subject: Reverse a file
X-Git-Url: http://git.scottworley.com/overonion/commitdiff_plain/9571ed09bf5c97e104eae36ed323c2cd2d4c09b7

Reverse a file
---

9571ed09bf5c97e104eae36ed323c2cd2d4c09b7
diff --git a/reverse.c b/reverse.c
new file mode 100644
index 0000000..ac55a10
--- /dev/null
+++ b/reverse.c
@@ -0,0 +1,70 @@
+#define _FILE_OFFSET_BITS 64
+
+#define BUFFER_SIZE 4096
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <err.h>
+#include <sysexits.h>
+
+void write_all(int fd, const void *buf, size_t count) {
+  const char* cbuf = buf;
+  size_t written = 0;
+  while (written < count) {
+    int ret = write(fd, &cbuf[written], count - written);
+    if (ret == -1) {
+      err(EX_IOERR, "Could not write");
+    }
+    written += ret;
+  }
+}
+
+
+int main(int argc, char** argv) {
+  if (argc != 2) {
+    errx(EX_USAGE, "Usage: reverse filename");
+  }
+
+  int fd = open(argv[1], O_RDONLY);
+  if (fd == -1) {
+    err(EX_NOINPUT, "Could not open specified file");
+  }
+
+  struct stat stats;
+  if (fstat(fd, &stats) == -1) {
+    err(EX_NOINPUT, "Could not stat input");
+  }
+
+  long page_size = sysconf(_SC_PAGE_SIZE);
+  off_t pages = (stats.st_size - 1) / page_size + 1;
+  long map_size = pages * page_size;
+  char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
+  if (m == MAP_FAILED) {
+    err(EX_NOINPUT, "Could not mmap input");
+  }
+
+  char buf[BUFFER_SIZE];
+  off_t buf_offset = 0;
+  for (off_t p = stats.st_size - 1; p >= 0; p--) {
+    buf[buf_offset++] = m[p];
+    if (buf_offset >= BUFFER_SIZE) {
+      write_all(1, buf, buf_offset);
+      buf_offset = 0;
+    }
+  }
+  if (buf_offset) {
+    write_all(1, buf, buf_offset);
+  }
+
+  if (munmap(m, map_size) == -1) {
+    err(EX_IOERR, "Could not unmap input");
+  }
+  if (close(fd) == -1) {
+    err(EX_IOERR, "Could not close input");
+  }
+
+  return 0;
+}