Initial import

libc/calls/pwrite.c

@@ -0,0 +1,60 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ This program is free software; you can redistribute it and/or modify         │
+│ it under the terms of the GNU General Public License as published by         │
+│ the Free Software Foundation; version 2 of the License.                      │
+│                                                                              │
+│ This program is distributed in the hope that it will be useful, but          │
+│ WITHOUT ANY WARRANTY; without even the implied warranty of                   │
+│ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU             │
+│ General Public License for more details.                                     │
+│                                                                              │
+│ You should have received a copy of the GNU General Public License            │
+│ along with this program; if not, write to the Free Software                  │
+│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA                │
+│ 02110-1301 USA                                                               │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/assert.h"
+#include "libc/calls/calls.h"
+#include "libc/calls/internal.h"
+#include "libc/calls/struct/iovec.h"
+#include "libc/dce.h"
+#include "libc/macros.h"
+#include "libc/sysv/errfuns.h"
+
+/**
+ * Writes to file at offset, thus avoiding superfluous lseek().
+ *
+ * @param fd is something open()'d earlier, noting pipes might not work
+ * @param buf is copied from, cf. copy_file_range(), sendfile(), etc.
+ * @param size in range [1..0x7ffff000] is reasonable
+ * @param offset is bytes from start of file at which write begins
+ * @return [1..size] bytes on success, or -1 w/ errno; noting zero is
+ *     impossible unless size was passed as zero to do an error check
+ * @see pread(), write()
+ */
+ssize_t pwrite(int fd, const void *buf, size_t size, int64_t offset) {
+  ssize_t rc;
+  size_t wrote;
+  if (fd == -1 || offset < 0) return einval();
+  size = MIN(size, 0x7ffff000);
+  if (!IsWindows()) {
+    rc = pwrite$sysv(fd, buf, size, offset);
+  } else if (isfdkind(fd, kFdFile)) {
+    rc = write$nt(&g_fds.p[fd], (struct iovec[]){{buf, size}}, 1, offset);
+  } else {
+    return ebadf();
+  }
+  if (rc != -1) {
+    wrote = (size_t)rc;
+    if (wrote == 0) {
+      assert(size == 0);
+    } else {
+      assert(wrote <= size);
+    }
+  }
+  return rc;
+}