One example is the spng_set_png_file() function which is essentially spng_set_png_stream(ctx, file_read_fn, file) where file_read_fn looks like this:

libspng/spng/spng.c

Lines 5024 to 5036 in e5c1fc4

    
           static int file_read_fn(spng_ctx *ctx, void *user, void *data, size_t n) 
        
           { 
        
               FILE *file = user; 
        
               (void)ctx; 
        
               if(fread(data, n, 1, file) != 1) 
        
               { 
        
                   if(feof(file)) return SPNG_IO_EOF; 
        
                   else return SPNG_IO_ERROR; 
        
               } 
        
               return 0; 
        
           }

For writing file_write_fn is used, it's right after file_read_fn in the same file.

There is documentation on the read and write callbacks: https://libspng.org/docs/context/#spng_read_fn, https://libspng.org/docs/context/#spng_write_fn

	static int file_read_fn(spng_ctx ctx, void user, void *data, size_t n)
	{
	FILE *file = user;
	(void)ctx;

	if(fread(data, n, 1, file) != 1)
	{
	if(feof(file)) return SPNG_IO_EOF;
	else return SPNG_IO_ERROR;
	}

	return 0;
	}

Uh oh!

Example for spng_set_png_stream() #258

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