libpng-1.2.46-2のデバッグFedora Core 16 - .PNG最初の8ビットがlibpngを過ぎていない

私は次のコードをlibpngのドキュメントから入手しました：また、これはgtkmmベースのアプリケーションから呼び出されます。libpng-1.2.46-2のデバッグFedora Core 16 - .PNG最初の8ビットがlibpngを過ぎていない

FILE *fp = fopen(path.c_str(), "rb"); 
if (!fp) 
    return false; 

unsigned char sig[8]; 
fread(sig, 1, 8, fp); 
if (!png_sig_cmp(sig, 0, 8)) 
    return false;

私はdebuginfoをインストールし、コマンドラインからgdbを実行しました。私はコードをステップ実行して、の機能が、私がGIMPで作成したpngの最初の8ビットと、インターネットからダウンロードしたいくつかの.pngファイルがpngによって認識されていないと言っている理由を確認しました。私は、GDBからの出力は、それが私にあったよりあなたにとってより有益であると思います：

... 
_IO_fread (buf=<optimized out>, size=1, count=8, fp=0x848440) at iofread.c:43 
43 _IO_acquire_lock (fp); 
(gdb) 
_IO_acquire_lock_fct (p=<optimized out>) at libioP.h:968 
968 if ((fp->_flags & _IO_USER_LOCK) == 0) 
(gdb) 
969  _IO_funlockfile (fp); 
(gdb) 
_IO_fread (buf=<optimized out>, size=1, count=8, fp=0x848440) at iofread.c:46 
46 return bytes_requested == bytes_read ? count : bytes_read/size; 
(gdb) 
47 } 
(gdb) 
PNGFileReader::read_png (this=0x7ca620, path=<optimized out>) at pngfilereader.cpp:46 
46 if (!png_sig_cmp(sig, 0, 8)) 
(gdb) step 
png_sig_cmp (sig=0x7fffffffd830 "\211PNG\r\n\032\n\236\330\377\377\377\177", start=0, num_to_check=8) at png.c:122 
122 { 
(gdb) 
124 if (num_to_check > 8) 
(gdb) 
123 png_byte png_signature[8] = {137, 80, 78, 71, 13, 10, 26, 10}; 
(gdb) 
124 if (num_to_check > 8) 
(gdb) 
126 else if (num_to_check < 1) 
(gdb) 
127  return (-1); 
(gdb) 
126 else if (num_to_check < 1) 
(gdb) 
129 if (start > 7) 
(gdb) 
130  return (-1); 
(gdb) 
129 if (start > 7) 
(gdb) 
132 if (start + num_to_check > 8) 
(gdb) 
135 return ((int)(png_memcmp(&sig[start], &png_signature[start], num_to_check))); 
(gdb) 
__memcmp_sse4_1() at ../sysdeps/x86_64/multiarch/memcmp-sse4.S:52 
52  pxor %xmm0, %xmm0 
(gdb) 
53  cmp $79, %rdx 
(gdb) 
54  ja L(79bytesormore) 
(gdb) 
56  cmp $1, %rdx 
(gdb) 
57  je L(firstbyte) 
(gdb) 
59  add %rdx, %rsi 
(gdb) 
60  add %rdx, %rdi 
(gdb) 
61  BRANCH_TO_JMPTBL_ENTRY(L(table_64bytes), %rdx, 4) 
(gdb) 
803  mov -8(%rdi), %rax 
(gdb) 
804  mov -8(%rsi), %rcx 
(gdb) 
805  cmp %rax, %rcx 
(gdb) 
806  jne L(diffin8bytes) 
(gdb) 
807  xor %eax, %eax 
(gdb) 
__memcmp_sse4_1() at ../sysdeps/x86_64/multiarch/memcmp-sse4.S:808 
808  ret 
(gdb) 
png_sig_cmp (sig=<optimized out>, start=<optimized out>, num_to_check=<optimized out>) at png.c:136 
136 } 
(gdb) 
PNGFileReader::read_png (this=0x7ca620, path=<optimized out>) at pngfilereader.cpp:47 
47  return false; 
(gdb) 
46 if (!png_sig_cmp(sig, 0, 8))

png_sig_cmp関数からの復帰スタックトレース1は

#0 PNGFileReader::read_png (this=0x7ca620, path=<optimized out>) at pngfilereader.cpp:46 
#1 0x0000000000409017 in DrawingArea::zoom (this=0x7c8360, zoom_factor=<optimized out>) at drawingarea.cpp:97 
#2 0x00000000004090cd in DrawingArea::on_scroll_event (this=<optimized out>, event=<optimized out>) at drawingarea.cpp:70 
#3 0x000000351fd5cb03 in Gtk::Widget_Class::scroll_event_callback (self=0x6df8e0 [gtkmm__GtkDrawingArea], p0=0x7bf4b0) at widget.cc:4474 
#4 0x000000351cf52848 in _gtk_marshal_BOOLEAN__BOXED (closure=0x62d640, return_value=0x7fffffffda80, n_param_values=<optimized out>, param_values=0x82da90, 
    invocation_hint=<optimized out>, marshal_data=<optimized out>) at gtkmarshalers.c:85 
#5 0x0000003515a0ea24 in g_closure_invoke (closure=0x62d640, return_value=0x7fffffffda80, n_param_values=2, param_values=0x82da90, invocation_hint=<optimized out>) 
    at gclosure.c:774 
#6 0x0000003515a20b5c in signal_emit_unlocked_R (node=<optimized out>, detail=0, instance=0x6df8e0, emission_return=0x7fffffffdbe0, instance_and_params=0x82da90) 
    at gsignal.c:3310 
#7 0x0000003515a29f13 in g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=<optimized out>) at gsignal.c:3013 
#8 0x0000003515a2a2e2 in g_signal_emit (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3060 
#9 0x000000351d07eff9 in gtk_widget_event_internal (widget=0x6df8e0 [gtkmm__GtkDrawingArea], event=0x7bf4b0) at gtkwidget.c:6132 
#10 0x000000351cf51fba in gtk_propagate_event (widget=0x6df8e0 [gtkmm__GtkDrawingArea], event=0x7bf4b0) at gtkmain.c:2614 
#11 0x000000351cf523bb in gtk_main_do_event (event=0x7bf4b0) at gtkmain.c:1889 
#12 0x000000351de47572 in gdk_event_source_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at gdkeventsource.c:360 
#13 0x0000003514a44acd in g_main_dispatch (context=0x663aa0) at gmain.c:2441 
#14 g_main_context_dispatch (context=0x663aa0) at gmain.c:3011 
#15 0x0000003514a452c8 in g_main_context_iterate (context=0x663aa0, block=<optimized out>, dispatch=1, self=<optimized out>) at gmain.c:3089 
#16 0x0000003514a45815 in g_main_loop_run (loop=0x8564c0) at gmain.c:3297 
#17 0x000000351cf517fd in gtk_main() at gtkmain.c:1362 
#18 0x000000351fcfd026 in Gtk::Main::run (window=...) at main.cc:384 
#19 0x00000000004075f0 in main (argc=1, argv=0x7fffffffe168) at main.cpp:10

出典

2012-03-22 Matthew Hoggan