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README_HEAP_DEBUG.md

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Ref: curl#12327

This document shows how to build libcurl with CURLDEBUG (libcurl heap memory tracking) or CRTDBG (Windows CRT heap memory tracking). The latter will track libcurl and the application.

The example commands use %CL_CRTDBG% for CRTDBG, but can be changed to %CL_CURLDEBUG% for CURLDEBUG.

To force a memory leak in both libcurl and the application (curl.exe or simple.exe) add /DCURL_FORCEMEMLEAK to the CL options. This is useful to see what the output of a memory leak looks like. It should cause 7 bytes to leak from libcurl and 5 bytes to leak from the application.

Open a Visual Studio command prompt and build curl:

set "CURL_SRCPATH=x:\j\curl\curl"
set "CL_CRTDBG=cl /DCURL_FORCEMEMLEAK /FC /FIcrtdbg.h /D_CRTDBG_MAP_ALLOC /D_CRT_SECURE_NO_WARNINGS /D_CRT_NONSTDC_NO_DEPRECATE"
set "CL_CURLDEBUG=cl /DCURL_FORCEMEMLEAK /FC /DCURLDEBUG /DMEMDEBUG_LOG_SYNC"

cd /d "%CURL_SRCPATH%\winbuild"
if not exist ..\src\tool_hugehelp.c (..\buildconf.bat)
nmake /f Makefile.vc mode=dll VC=10 DEBUG=yes clean
nmake /f Makefile.vc mode=dll VC=10 DEBUG=yes CC="%CL_CRTDBG%"

The build output will show an object directory ending in -obj-curl. Remove that suffix and switch to the bin output subdir. For example if the object directory is libcurl-vc10-x86-debug-dll-ipv6-sspi-schannel-obj-curl:

cd ..\builds\libcurl-vc10-x86-debug-dll-ipv6-sspi-schannel\bin

If the curl tool (curl.exe) is not the application then build the application:

%CL_CRTDBG% /MDd /I..\include "%CURL_SRCPATH%\docs\examples\simple.c" /link /LIBPATH:..\lib libcurl_debug.lib

(CRTDBG) Run the application. Heap memory allocations in the CRT are logged internally. Heap memory leaks are printed to stderr on exit.

simple

(CRTDBG) Examine the output. If CURL_FORCEMEMLEAK was defined then the last lines of output should show a memory leak in libcurl and the application:

Detected memory leaks!
Dumping objects ->
x:\j\curl\curl\lib\easy.c(169) : {88} normal block at 0x006A8E38, 7 bytes long.
 Data: <       > CD CD CD CD CD CD CD
x:\j\curl\curl\docs\examples\simple.c(59) : {87} normal block at 0x006E8FB0, 5 bytes long.
 Data: <     > CD CD CD CD CD
Object dump complete.

(CURLDEBUG) Run the application. Heap memory allocations in libcurl are logged to curldbg.log. Heap memory leaks can be printed by running memanalyze.pl.

set "CURL_MEMDEBUG=curldbg.log"
simple
if not exist "%CURL_MEMDEBUG%" echo problem: missing "%CURL_MEMDEBUG%"
..\..\..\tests\memanalyze.pl curldbg.log

(CURLDEBUG) Examine the output. If CURL_FORCEMEMLEAK was defined then memanalyze should show a memory leak in libcurl:

Leak detected: memory still allocated: 7 bytes
At 4e7a58, there's 7 bytes.
 allocated by x:\j\curl\curl\lib\easy.c:169