What causes "%s: can't open stdout\n" in DrKLO/Telegram?

Redirect stdout to a real file: `wrjpgcom ... > out.jpg`.

%s: can't open stdout\n

Error message

%s: can't open stdout\n

What it means

In the Unix-style build with USE_FDOPEN, when no output file is named wrjpgcom writes to stdout and re-opens fd 1 in binary mode via fdopen(fileno(stdout), WRITE_BINARY). If fdopen returns NULL it prints `%s: can't open stdout` and exits EXIT_FAILURE. This is a stdout re-attach failure, not a disk error.

Source

Thrown at TMessagesProj/jni/mozjpeg/wrjpgcom.c:536

    usage();
  }
  if ((outfile = fopen(argv[argn + 1], WRITE_BINARY)) == NULL) {
    fprintf(stderr, "%s: can't open %s\n", progname, argv[argn + 1]);
    exit(EXIT_FAILURE);
  }
#else
  /* Unix style: expect zero or one file name */
  if (argn < argc - 1) {
    fprintf(stderr, "%s: only one input file\n", progname);
    usage();
  }
  /* default output file is stdout */
#ifdef USE_SETMODE              /* need to hack file mode? */
  setmode(fileno(stdout), O_BINARY);
#endif
#ifdef USE_FDOPEN               /* need to re-open in binary mode? */
  if ((outfile = fdopen(fileno(stdout), WRITE_BINARY)) == NULL) {
    fprintf(stderr, "%s: can't open stdout\n", progname);
    exit(EXIT_FAILURE);
  }
#else
  outfile = stdout;
#endif
#endif /* TWO_FILE_COMMANDLINE */

  /* Collect comment text from comment_file or stdin, if necessary */
  if (comment_arg == NULL) {
    FILE *src_file;
    int c;

    comment_arg = (char *)malloc((size_t)MAX_COM_LENGTH);
    if (comment_arg == NULL)
      ERREXIT("Insufficient memory");
    comment_length = 0;
    src_file = (comment_file != NULL ? comment_file : stdin);
    while ((c = getc(src_file)) != EOF) {

View on GitHub (pinned to 45ab8f4308)

Solutions

Redirect stdout to a real file: `wrjpgcom ... > out.jpg`.
Use a TWO_FILE_COMMANDLINE build and name an explicit output file.
Ensure fd 1 is a valid open writable descriptor before launching.

Example fix

// before
wrjpgcom -comment hi in.jpg   # stdout closed
// after
wrjpgcom -comment hi in.jpg > out.jpg

Defensive patterns

Strategy: validation

Validate before calling

#include <unistd.h>
/* ensure stdout (fd 1) is a valid writable descriptor */
if (fcntl(1, F_GETFD) == -1) { /* redirect or name an output file */ }

Prevention

Redirect stdout to a file: wrjpgcom ... > out.jpg.
Use a TWO_FILE_COMMANDLINE build with a named output.
Never launch with fd 1 closed.

When it happens

Trigger: Running wrjpgcom with stdout closed or an invalid fd 1 (e.g. launched with stdout>&- or from a process that closed fd 1), under a USE_FDOPEN build.

Common situations: Daemon/JNI spawn that did not wire stdout, `>&-` redirection in a shell wrapper, or piping into a command that immediately closes its stdin.

Related errors

AI-assisted analysis of DrKLO/Telegram@45ab8f4308 (2026-08-14). Data as JSON: /api/errors/4e393ce43ce96126. Report an issue: GitHub.