Differences between debug mode and release mode in

I. essential differences between debug and release compilation methods

Debug is usually called a debug version. It contains debugging information without any optimization, so that programmers can debug programs easily. Release is called a release version. It is often optimized to optimize the code size and running speed, so that users can use it well.

The real secret of debug and release lies in a set of compilation options. The options for the two are listed below (of course, there are other options, such as/FD/Fo, but the difference is not important. Usually they will not cause the release version error and will not be discussed here)

Debug version:

/MDD/MLD or/MTD use the debug Runtime Library (the runtime function library of the debug version)

/OD turn off the optimization Switch

/D "_ debug" is equivalent to # DEFINE _ Debug. Enable the compile and debug code switch (mainly

Assert function)

/Zi creates the edit and continue (Edit to continue) database, so that the database has been debugged

If the source code is modified, you do not need to re-compile it.

/GZ helps capture Memory Errors

/GM enables the minimize reconnection switch to reduce the link time

Release Version:

/MD/ml or/MT use the runtime function library of the released version

/O1 or/O2 optimization switch to minimize or minimize the program

/D "ndebug" Disable the Conditional compilation and debugging code switch (that is, do not compile the assert function)

/GF combines repeated strings and puts string constants in read-only memory to prevent

Modified

In fact, there is no essential limit between debug and release. They are just a set of compilation options, and the compiler only acts according to the predefined options. In fact, we can even modify these options to get optimized debugging versions or release versions with trace statements.

　Ii. under which circumstances will the release version go wrong?

With the above introduction, let's compare these options one by one to see how release errors are generated.

1. Runtime Library: Specifies the runtime function library to be linked, which usually only affects the performance of the program. The Runtime Library of the debug version contains debugging information and uses some protection mechanisms to help identify errors. Therefore, the performance is inferior to that of the release version. The Runtime Library provided by the compiler is usually stable and does not cause release errors. However, the debug Runtime Library enhances error detection, such as heap memory allocation, sometimes there may be debug errors but release is normal. It should be pointed out that, if the debug is wrong, even if the release is normal, the program must have a bug, but it may be that a certain run of the release version is not shown.

2. Optimization: This is the main cause of errors, because the source program is basically translated directly when optimization is disabled, and the compiler will make a series of assumptions after optimization is enabled. There are several types of errors:

(1) frame pointer (FPO for short): All call information (return address, parameters) and automatic variables are stored in the stack during function call. If the declaration and implementation of a function are different (parameters, return values, and call methods), an error is generated. However, in debug mode, stack access is implemented through the address saved by the EBP register, if no errors such as array out-of-bounds (or "not many" out-of-bounds) occur, the function can normally be executed. In the release mode, optimization will omit the EBP stack base address pointer, in this way, accessing the stack through a global pointer will cause the program to crash if the returned address is incorrect. The strong type feature of C ++ can check the majority of such errors, but if forced type conversion is used, it will not work. You can force the/Oy-compilation option in the release version to disable frame pointer omitting to determine whether such errors are possible. Common Errors include:

● An error occurred while writing the message response function of MFC. Correct should be

Afx_msg lresult onmessageown (wparam, lparam );

On_message macro contains forced type conversion. One of the methods to prevent this error is to redefine the on_message macro and add the following code to stdafx. H (after # include "afxwin. H"). An error will be reported during compilation of the function prototype errors.

# UNDEF on_message

# Define on_message (message, memberfxn) {message, 0, 0, 0, afxsig_lwl, (afx_pmsg) (afx_pmsgw) (static_cast <lresult (afx_msg_call cwnd: *) (wparam, lparam)> (& memberfxn )},

(2) Volatile variable: Volatile tells the compiler that the variable may be modified by unknown methods (such as systems, other processes and threads) outside the program ). To improve program performance, the optimizer usually places some variables in registers (similar to the Register keyword), while other processes can only modify the memory where the variable is located, but the value in the Register remains unchanged. If your program is multi-threaded, or you find that the value of a variable is inconsistent with the expected value and you are sure that the setting is correct, you may encounter such a problem. This type of error is sometimes manifested as the program has the fastest optimization error and the smallest optimization is normal. Add volatile to the variables you think are suspicious.

(3) Variable Optimization: The optimization program optimizes the variables based on the usage of the variables. For example, a function has an unused variable. In the debug version, it may mask an array out of bounds. In the release version, this variable may be optimized, in this case, the array out-of-bounds will destroy the useful data in the stack. Of course, the actual situation is much more complicated than this. Related errors include:

● Unauthorized access, including array out-of-bounds and pointer errors. For example

 

Void FN (void) {int I; I = 1; int A [4]; {Int J; j = 1;} A [-1] = 1; // Of course the error is not so obvious. For example, the subscript is variable A [4] = 1 ;}

 

J has an out-of-range scope when the array is out of bounds, but its space is not reclaimed, So I and j will cover up the out-of-bounds. The release version may be optimized because I and j are not very useful, so that the stack is damaged.

3. _ debug and ndebug: When _ debug is defined, the assert () function is compiled, but ndebug is not compiled. In addition, there are a series of assertion Macros in VC ++. This includes:

 

Ansi c asserted void assert (INT expression); C Runtime lib asserted _ assert (booleanexpression); _ asserte (booleanexpression); MFC asserted assert (booleanexpression); Verify (booleanexpression ); assert_valid (pobject); assert_kindof (classname, pobject); ATL asserted atlassert (booleanexpression );

 

In addition, the compilation of the trace () macro is also controlled by _ debug.

All these assertions are compiled only in the debug version, but are ignored in the release version. The only exception is verify (). In fact, these macros call the assert () function, except that some library-related debugging code is attached. If you add any program code to these macros, instead of a Boolean expression (for example, a value assignment or a function call that can change the value of a variable), the release version will not perform these operations, this may cause errors. It is easy for beginners to make such mistakes, and the search method is also very simple, because these macros are listed above, you only need to use the find in files function of VC ++ to find the place where these macros are used in all files of the project and then check them one by one. In addition, some experts may also add Conditional compilation such as # ifdef _ Debug. Pay attention to it.

By the way, it is worth mentioning that the verify () macro allows you to put program code in a Boolean expression. This macro is usually used to check the return values of Windows APIs. Some people may abuse verify () For this reason. In fact, this is dangerous because verify () violates the idea of assertions and cannot completely separate program code from debugging code, in the end, it may cause a lot of trouble. Therefore, experts suggest using this macro as little as possible.

4./GZ option: This option will do the following:

(1) initialize memory and variables. Including using 0xcc to initialize all automatic variables, 0xcd (cleared data) to initialize memory allocated in the heap (that is, dynamically allocated memory, such as new), 0xdd (dead data) fill in the released heap memory (such as delete), 0xfd (defencde data), and initialize the protected memory (debug adds the protected memory before and after the dynamically allocated memory to prevent cross-border access ), the words in the brackets are the notes suggested by Microsoft. The advantage of doing so is that these values are very large and cannot be used as pointers (and in 32-bit systems, pointers are rarely odd values, in some systems, odd-number pointers produce runtime errors), which are rarely used as numerical values, and these values are easy to recognize, therefore, it is helpful to find the release version errors. Note that many people think that the compiler will use 0 to initialize the variable, which is wrong (and this is not conducive to finding errors ).

(2) When a function is called through the function pointer, the matching of the function call is verified by checking the stack pointer. (Prevent original form mismatch)

(3) Check the stack pointer before the function returns, and confirm that it has not been modified. (Avoid out-of-bounds access and original form mismatch. When combined with the second item, you can roughly simulate frame pointer omitting FPO)

The/GZ option usually causes errors in the debug version and the release version is normal, because uninitialized variables in the release version are random, this may cause the pointer to point to a valid address and mask illegal access.

In addition, options such as/GM/GF have fewer errors, and their effects are obvious and easy to find.