text: #define _GNU_SOURCE
  #include <stdio.h>
  #include <stdarg.h>
  #include <stdlib.h>
  #include <dlfcn.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  #include <limits.h>
  #include <string.h>
  #include <spawn.h>
  
  #define MAX_REDIRECTS 128
  
  static int nrRedirects = 0;
  static char * from[MAX_REDIRECTS];
  static char * to[MAX_REDIRECTS];
  
  // FIXME: might run too late.
  static void init() __attribute__((constructor));
  
  static void init()
  {
      char * spec = getenv("NIX_REDIRECTS");
      if (!spec) return;
  
      unsetenv("NIX_REDIRECTS");
  
      char * spec2 = malloc(strlen(spec) + 1);
      strcpy(spec2, spec);
  
      char * pos = spec2, * eq;
      while ((eq = strchr(pos, '='))) {
          *eq = 0;
          from[nrRedirects] = pos;
          pos = eq + 1;
          to[nrRedirects] = pos;
          nrRedirects++;
          if (nrRedirects == MAX_REDIRECTS) break;
          char * end = strchr(pos, ':');
          if (!end) break;
          *end = 0;
          pos = end + 1;
      }
  
  }
  
  static const char * rewrite(const char * path, char * buf)
  {
      if (path == NULL) return path;
      for (int n = 0; n < nrRedirects; ++n) {
          int len = strlen(from[n]);
          if (strncmp(path, from[n], len) != 0) continue;
          if (snprintf(buf, PATH_MAX, "%s%s", to[n], path + len) >= PATH_MAX)
              abort();
          return buf;
      }
  
      return path;
  }
  
  /* The following set of Glibc library functions is very incomplete -
     it contains only what we needed for programs in Nixpkgs. Just add
     more functions as needed. */
  
  int open(const char * path, int flags, ...)
  {
      int (*open_real) (const char *, int, mode_t) = dlsym(RTLD_NEXT, "open");
      mode_t mode = 0;
      if (flags & O_CREAT) {
          va_list ap;
          va_start(ap, flags);
          mode = va_arg(ap, mode_t);
          va_end(ap);
      }
      char buf[PATH_MAX];
      return open_real(rewrite(path, buf), flags, mode);
  }
  
  int open64(const char * path, int flags, ...)
  {
      int (*open64_real) (const char *, int, mode_t) = dlsym(RTLD_NEXT, "open64");
      mode_t mode = 0;
      if (flags & O_CREAT) {
          va_list ap;
          va_start(ap, flags);
          mode = va_arg(ap, mode_t);
          va_end(ap);
      }
      char buf[PATH_MAX];
      return open64_real(rewrite(path, buf), flags, mode);
  }
  
  FILE * fopen(const char * path, const char * mode)
  {
      FILE * (*fopen_real) (const char *, const char *) = dlsym(RTLD_NEXT, "fopen");
      char buf[PATH_MAX];
      return fopen_real(rewrite(path, buf), mode);
  }
  
  FILE * fopen64(const char * path, const char * mode)
  {
      FILE * (*fopen64_real) (const char *, const char *) = dlsym(RTLD_NEXT, "fopen64");
      char buf[PATH_MAX];
      return fopen64_real(rewrite(path, buf), mode);
  }
  
  int __xstat(int ver, const char * path, struct stat * st)
  {
      int (*__xstat_real) (int ver, const char *, struct stat *) = dlsym(RTLD_NEXT, "__xstat");
      char buf[PATH_MAX];
      return __xstat_real(ver, rewrite(path, buf), st);
  }
  
  int __xstat64(int ver, const char * path, struct stat64 * st)
  {
      int (*__xstat64_real) (int ver, const char *, struct stat64 *) = dlsym(RTLD_NEXT, "__xstat64");
      char buf[PATH_MAX];
      return __xstat64_real(ver, rewrite(path, buf), st);
  }
  
  int * access(const char * path, int mode)
  {
      int * (*access_real) (const char *, int mode) = dlsym(RTLD_NEXT, "access");
      char buf[PATH_MAX];
      return access_real(rewrite(path, buf), mode);
  }
  
  int posix_spawn(pid_t * pid, const char * path,
      const posix_spawn_file_actions_t * file_actions,
      const posix_spawnattr_t * attrp,
      char * const argv[], char * const envp[])
  {
      int (*posix_spawn_real) (pid_t *, const char *,
          const posix_spawn_file_actions_t *,
          const posix_spawnattr_t *,
          char * const argv[], char * const envp[]) = dlsym(RTLD_NEXT, "posix_spawn");
      char buf[PATH_MAX];
      return posix_spawn_real(pid, rewrite(path, buf), file_actions, attrp, argv, envp);
  }
  
  int execv(const char *path, char *const argv[])
  {
      int (*execv_real) (const char *path, char *const argv[]) = dlsym(RTLD_NEXT, "execv");
      char buf[PATH_MAX];
      return execv_real(rewrite(path, buf), argv);
  }
  