"relative executor" program

Makefile

CFLAGS = -pedantic -Wall -Wextra -std=c11 -O2

rel: rel.o

rel.o:

clean:
	-rm *.o rel

rel.c

#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);
}

See https://www.reddit.com/r/unix/comments/152hr8c/shebangs_cant_be_used_with_relative_paths_so_i/

Someone else's implementation: https://gist.github.com/rusek/c5e09dd12cc4c57c9660