phlummox · July 26, 2023 13:58 · Jul 18, 2023 · Jul 18, 2023 · Jul 18, 2023
diff --git a/Makefile b/Makefile
@@ -0,0 +1,9 @@
+
+CFLAGS = -pedantic -Wall -Wextra -std=c11 -O2
+
+rel: rel.o
+
+rel.o:
+
+clean:
+	-rm *.o rel
diff --git a/rel.c b/rel.c
@@ -0,0 +1,67 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <libgen.h>
+#include <unistd.h>
+
+// Variable naming:
+//
+// - Lengths of NUL-terminated strings are called <something>_len
+//   (and don't include the NUL in the length)
+// - Sizes of buffers are called <something>_size
+//   (and represent the exact size of the buffer)
+//
+// We assume a Linux with memory overcommit enabled, and lazily don't bother to
+// check the return value of calls to malloc(). (But we should. See
+// <https://ewontfix.com/3/>.)
+
+
+// Wrapper around POSIX dirname which works on a copy
+// of `path` (since `dirname()` may try to alter its
+// argument).
+//
+// `dir` should point to memory which can hold the full result
+// of `dirname(path)`.
+// `dir_len` should point to memory which can hold a size_t; the
+// length of `dir` will be written to it.
+void dirname_(char *dir, size_t *dir_len, const char *path) {
+  const size_t path_len = strlen(path);
+  char *path_cp = malloc(path_len+1);
+  strncpy(path_cp, path, path_len+1);
+
+  char *tmp = dirname(path_cp);
+  *dir_len = strlen(tmp);
+  strncpy(dir, tmp, (*dir_len)+1);
+  free(path_cp);
+}
+
+int main(int argc, char* argv[]) {
+  if (argc < 3) {
+    fprintf(stderr, "expected at least 2 args: <INTERPRETER> <SCRIPT> ...\n");
+    exit(EXIT_FAILURE);
+  }
+  const char * interpreter = argv[1];
+  const char * orig_script = argv[2];
+
+  const size_t interpreter_len = strlen(interpreter);
+
+  // max buf needed: + 1 for a "/" char between the two, +1 more for NUL.
+  const size_t buf_size = strlen(orig_script) + 1 + interpreter_len + 1;
+
+  char * buf = malloc(buf_size);
+  size_t dirname_len;
+  dirname_(buf, &dirname_len, orig_script);
+  buf[dirname_len] = '/';
+  strncpy(buf + dirname_len + 1, interpreter, interpreter_len+1);
+
+  // adjust argv for passing to execv:
+  // argv[0] <- buf, all others shift back one place.
+
+  argv[0] = buf;
+  for (int i = 1; i < argc - 1; i++) {
+    argv[i] = argv[i+1];
+  }
+  argv[argc-1] = NULL;
+
+  execv(buf, argv);
+}