What causes "Error opening output file: %s\n" in DrKLO/Telegram?

Create the output directory first: `mkdir -p $(dirname )`.

Error opening output file: %s\n

Error message

Error opening output file: %s\n

What it means

fopen(argv[argc-1], "w") on the output file returned NULL. The tool opens the output in write mode, which creates or truncates the file. A NULL return typically means the parent directory does not exist, the path points to a directory or special file, the filesystem is read-only, or the user lacks write permission. Notably, the tool correctly closes fin before exiting (line 109).

Source

Thrown at TMessagesProj/jni/opus/src/repacketizer_demo.c:108

      } else if (strcmp(argv[i], "-split")==0)
         split = 1;
      else
      {
         fprintf(stderr, "Unknown option: %s\n", argv[i]);
         usage(argv[0]);
         return EXIT_FAILURE;
      }
   }
   fin = fopen(argv[argc-2], "r");
   if(fin==NULL)
   {
     fprintf(stderr, "Error opening input file: %s\n", argv[argc-2]);
     return EXIT_FAILURE;
   }
   fout = fopen(argv[argc-1], "w");
   if(fout==NULL)
   {
     fprintf(stderr, "Error opening output file: %s\n", argv[argc-1]);
     fclose(fin);
     return EXIT_FAILURE;
   }

   rp = opus_repacketizer_create();
   while (!eof)
   {
      int err;
      int nb_packets=merge;
      opus_repacketizer_init(rp);
      for (i=0;i<nb_packets;i++)
      {
         unsigned char ch[4];
         err = fread(ch, 1, 4, fin);
         len[i] = char_to_int(ch);
         /*fprintf(stderr, "in len = %d\n", len[i]);*/
         if (len[i]>1500 || len[i]<0)
         {

View on GitHub (pinned to 45ab8f4308)

Solutions

Create the output directory first: `mkdir -p $(dirname <output_path>)`.
Verify write permission on the target directory.
Ensure the output path is not an existing directory.
Check available disk space.

Example fix

// before
repacketizer_demo in.oct /nonexistent_dir/out.oct
// after
mkdir -p /output/dir && repacketizer_demo in.oct /output/dir/out.oct

Defensive patterns

Strategy: validation

Validate before calling

// Pre-check output directory writability
#include <unistd.h>
#include <libgen.h>
#include <sys/stat.h>
char *path_copy = strdup(argv[argc-1]);
char *dir = dirname(path_copy);
if (access(dir, W_OK) != 0) {
    fprintf(stderr, "Output directory not writable: %s\n", dir);
    free(path_copy);
    return EXIT_FAILURE;
}
free(path_copy);

Prevention

Create the output directory with `mkdir -p` before running.
Verify the output path does not point to an existing directory.
Check disk space with `df` for large batch operations.

When it happens

Trigger: Output directory does not exist; output path is a directory; read-only filesystem; disk quota exceeded; no write permission on the target directory.

Common situations: Forgetting to create the output directory; writing to /tmp on a system where it is mounted read-only; path typo; trying to overwrite a file owned by root without sudo.

Related errors

AI-assisted analysis of DrKLO/Telegram@45ab8f4308 (2026-08-14). Data as JSON: /api/errors/31d93730f62750a5. Report an issue: GitHub.