Yalantis/uCrop · error · CImgIOException

load_pnm(): PNM header not found in file '%s'.

Error message

load_pnm(): PNM header not found in file '%s'.

What it means

CImg's load_pnm() parses Portable Anymap (PNM/PBM/PGM/PPM) headers. It first scans the leading line and requires it to match the magic number 'P<digit>' (e.g. P2, P6). If the first non-comment line does not parse as a PNM magic token, it throws CImgIOException, meaning the file is not a recognizable PNM image.

Source

Thrown at ucrop/src/main/jni/CImg.h:57572

    static CImg<T> get_load_pnm(std::FILE *const file) {
      return CImg<T>().load_pnm(file);
    }

    CImg<T>& _load_pnm(std::FILE *const file, const char *const filename) {
      if (!file && !filename)
        throw CImgArgumentException(_cimg_instance
                                    "load_pnm(): Specified filename is (null).",
                                    cimg_instance);

      std::FILE *const nfile = file?file:cimg::fopen(filename,"rb");
      unsigned int ppm_type, W, H, D = 1, colormax = 255;
      CImg<charT> item(16384,1,1,1,0);
      int err, rval, gval, bval;
      const longT cimg_iobuffer = (longT)24*1024*1024;
      while ((err=std::fscanf(nfile,"%16383[^\n]",item.data()))!=EOF && (*item=='#' || !err)) std::fgetc(nfile);
      if (cimg_sscanf(item," P%u",&ppm_type)!=1) {
        if (!file) cimg::fclose(nfile);
        throw CImgIOException(_cimg_instance
                              "load_pnm(): PNM header not found in file '%s'.",
                              cimg_instance,
                              filename?filename:"(FILE*)");
      }
      while ((err=std::fscanf(nfile," %16383[^\n]",item.data()))!=EOF && (*item=='#' || !err)) std::fgetc(nfile);
      if ((err=cimg_sscanf(item," %u %u %u %u",&W,&H,&D,&colormax))<2) {
        if (!file) cimg::fclose(nfile);
        throw CImgIOException(_cimg_instance
                              "load_pnm(): WIDTH and HEIGHT fields undefined in file '%s'.",
                              cimg_instance,
                              filename?filename:"(FILE*)");
      }
      if (ppm_type!=1 && ppm_type!=4) {
        if (err==2 || (err==3 && (ppm_type==5 || ppm_type==7 || ppm_type==8 || ppm_type==9))) {
          while ((err=std::fscanf(nfile," %16383[^\n]",item.data()))!=EOF && (*item=='#' || !err)) std::fgetc(nfile);
          if (cimg_sscanf(item,"%u",&colormax)!=1)
            cimg::warn(_cimg_instance
                       "load_pnm(): COLORMAX field is undefined in file '%s'.",

View on GitHub (pinned to f788b534b4)

Solutions

  1. Verify the file actually starts with a PNM magic number (P1-P7) by inspecting the first bytes, e.g. 'head -c 2 file.pnm'.
  2. Re-export or re-convert the image to PNM with a known-good tool (e.g. `convert img.jpg img.pnm`).
  3. Remove any leading BOM, whitespace, or junk lines before the 'P<digit>' token.
  4. If the file is a different format, call the matching loader (load_jpeg, load_png, ...) or plain load() instead of load_pnm.
  5. Wrap the load call in try/catch for CImgIOException and report the bad file to the user.

Example fix

// before
img.load_pnm(path); // crashes on non-PNM input
// after
std::ifstream f(path, std::ios::binary);
char magic[2] = {0};
f.read(magic, 2);
if (magic[0] == 'P' && magic[1] >= '1' && magic[1] <= '7') {
  img.load_pnm(path);
} else {
  img.load(path.c_str()); // fall back to format auto-detection
}
Defensive patterns

Strategy: validation

Validate before calling

bool looksLikePnm(const char* path) {
  std::ifstream f(path, std::ios::binary);
  char m[2] = {0};
  f.read(m, 2);
  return f && m[0]=='P' && m[1]>='1' && m[1]<='6';
}

Type guard

bool isPnmMagic(const unsigned char* b, size_t n) {
  return n >= 2 && b[0]=='P' && b[1]>='1' && b[1]<='6';
}

Try / catch

try {
  img.load_pnm(path);
} catch (cimg_library::CImgIOException& e) {
  std::cerr << "Invalid PNM: " << e.what() << "\n";
}

Prevention

When it happens

Trigger: Calling CImg<T>::load_pnm(filename) (or load() auto-detecting pnm) on a file whose first non-comment line lacks a valid 'P<type>' magic token: a truncated/corrupted PNM, a renamed non-PNM file (e.g. a JPEG renamed to .pnm), an empty file, or a file starting with garbage/extra bytes before the magic number.

Common situations: Files misnamed during download or conversion pipelines; text editors or scripts prepending BOM/whitespace; truncated uploads; passing an SVG/JPEG file path to load_pnm directly.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of Yalantis/uCrop@f788b534b4 (2026-09-08). Data as JSON: /api/errors/c31842276e3064fd. Report an issue: GitHub.