feat(fcntl): fcntl/open flag handling

src/fcntl/fcntl.c

@@ -0,0 +1,55 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <stdarg.h>
+
+#include <fcntl.h>
+
+#include "../internal/syscall.h"
+
+/*
+ * fcntl over SYS_fcntl (72). The third argument is read from the varargs
+ * only for the commands POSIX defines one for (F_DUPFD, F_DUPFD_CLOEXEC,
+ * F_SETFD, F_SETFL, the lock commands, and the owner/signal commands);
+ * every other command passes 0, which the kernel ignores. Pointers (the
+ * struct flock * of F_GETLK/F_SETLK/F_SETLKW/F_OFD_*) ride the varargs
+ * slot as a long — on x86_64 long and void * share one GPR slot, so the
+ * read is ABI-exact.
+ *
+ * F_DUPFD_CLOEXEC passes straight through: the x86_64 kernel has supported
+ * it as a single native operation since 2.6.24, so no F_DUPFD + F_SETFD
+ * fallback is needed. F_SETFL needs no O_LARGEFILE massaging on x86_64
+ * (that is a 32-bit compat concern only).
+ */
+int
+fcntl(int fildes, int cmd, ...)
+{
+    long arg = 0;
+    va_list ap;
+
+    switch (cmd)
+    {
+    case F_DUPFD:
+    case F_DUPFD_CLOEXEC:
+    case F_SETFD:
+    case F_SETFL:
+    case F_GETLK:
+    case F_SETLK:
+    case F_SETLKW:
+    case F_SETOWN:
+    case F_SETSIG:
+    case F_GETOWN_EX:
+    case F_SETOWN_EX:
+    case F_OFD_GETLK:
+    case F_OFD_SETLK:
+    case F_OFD_SETLKW:
+        va_start(ap, cmd);
+        arg = va_arg(ap, long);
+        va_end(ap);
+        break;
+    default:
+        break;
+    }
+    return syscall_ret(__syscall3(SYS_fcntl, fildes, cmd, arg));
+}