Highlight some words in PDF using c++ MuPDF - c++

Question

Iam trying to highlight some words inside a PDF, I searched on a good C++ library for doing this, I found MuPDF, I download the last version and compiled it.
Now iam starting to write some codes to highlight the text in the PDF, there is no examples for this task in c++, so I start to try myself.
fz_document *doc;
fz_context *ctx;
ctx = fz_new_context(NULL, NULL, FZ_STORE_UNLIMITED);
fz_register_document_handlers(ctx);
doc = fz_open_document(ctx, "D:/b.pdf");
cout << fz_count_pages(ctx, doc) << endl;
fz_page *page = fz_load_page(ctx, doc, 0);
fz_quad *q;
fz_search_page(ctx, page, "more", q, 1);
fz_rect rec = fz_rect_from_quad((*q));
fz_stext_page *pp = fz_new_stext_page(ctx, rec);
fz_point point;
point.x = 0;
point.y = 0;
fz_highlight_selection(ctx, pp, point, point, q, 16);
fz_buffer *buffer = fz_new_buffer_from_stext_page(ctx, pp);
fz_save_buffer(ctx, buffer, "D:/Final.pdf");
That is what i tried so far iam not sure it crash at a point, iam using it with Qt 5.13 MSVC 2017, so what i did wrong, or if some one has a good useful example to do this or for the library in general as it leak examples so far from my search, all the examples are in python, java, and other put for c++ there is few examples just 2 examples coming with the library.
Even if there is another good c++ library has this function please share it.
Thanks in advance.

Answer 1

So it seems you are making the common newbie error of thinking that just because an API uses a pointer, you must declare a pointer. But that is not correct, instead you should declare an object and pass the address of that object. So for example this
fz_quad *q;
fz_search_page(ctx, page, "more", q, 1);
fz_rect rec = fz_rect_from_quad((*q));
should actually be this
fz_quad q; // object not pointer
fz_search_page(ctx, page, "more", &q, 1); // address of the object to get the pointer
fz_rect rec = fz_rect_from_quad(q);
The idea is that fz_search_page will fill in the fz_quad object. Your version fails because you gave an uninitialised pointer to fz_search_page which will result in memory corruption when fz_search_page tries to use that pointer.
You should also definitely add the sanity check
doc = fz_open_document(ctx, "D:/b.pdf");
if (doc == nullptr) // check if we can open the document
{
std::cerr << "cannot open document\n"; // or whatever error handling you prefer
exit(1);
}
Opening files or documents can fail for all sorts of reasons and you should always check that it works.
There's probably lots else that needs improving but those issues stood out for me.

Answer 2

To elaborate on john's answer, you need to allocate space for the results, not just a pointer. This can be stack allocated.
fz_quad q[100]; // stack allocate array of 100 quads
int n = fz_search_page(ctx, page, "more", q, 100);
However, there seem to be some more areas of confusion as to what the APIs actually do.
The fz_search_page function returns a list of quads covering the search hits. fz_highlight_selection also returns a list of quads but this time based on the location on a page a user has dragged using the start and end coordinates of the selection.
fz_new_buffer_from_stext_page returns a plain text version of the structured text data. This is NOT in PDF format.
If you want to add a highlight annotation, then you should create a highlight annotation covering the area you want highlighted:
n = pdf_search_page(ctx, page, "more", q, 100);
if (n > 0) {
pdf_annot *annot = pdf_create_annot(ctx, page, PDF_ANNOT_HIGHLIGHT);
for (i = 0; i < n; ++i)
pdf_add_annot_quad_point(ctx, annot, q[i]);
pdf_update_annot(ctx, annot);
}
Then you can save the new modified PDF document:
pdf_save_document(ctx, doc, "out.pdf", NULL);