Byte alignment Summary

 

In modern computers, memory space is divided by byte. Theoretically, it seems that access to any type of variables can start from any address, however, the actual situation is that access to specific variables is often performed at specific memory addresses, which requires various types of data to be arranged in space according to certain rules, instead of sequential emissions, this is alignment.

Alignment functions and causes: the processing of storage space varies greatly by hardware platform. Some platforms can only access certain types of data from some specific addresses. This may not be the case for other platforms, but the most common problem is that alignment of data storage according to the requirements of their platforms may cause a loss of access efficiency. For example, some platforms start from the even address each time they read data. If an int type (assuming a 32-bit System) is stored at the beginning of the even address, a read cycle can be read, if the data is stored at the beginning of the odd address, it may take two read cycles and splice the high and low bytes of the two read results to obtain the int data. Obviously, reading efficiency is greatly reduced. This is also a game of space and time.

Alignment implementation
Usually, we do not need to consider alignment when writing a program. The compiler will select the alignment policy of the target platform for us. Of course, we can also notify the compiler to pass the pre-compilation command to change the Alignment Method for the specified data.
However, because we generally do not need to care about this issue, the editor is aligned with the data storage. If we do not know it, we are often confused about some problems. The most common result is the sizeof result of the struct data structure, which is unexpected. Therefore, we need to understand Alignment Algorithms.

 

Among all the pre-processing commands, the # pragma command may be the most complex. It is used to set the compiler status or to instruct the compiler to complete some specific actions. # The Pragma command provides a method for each compiler to provide the unique features of the host or operating system while maintaining full compatibility with C and C ++ languages. According to the definition, the compilation instructions are proprietary to machines or operating systems and are different for each compiler.

The format is generally: # pragma para
Here, para is a parameter. Below are some common parameters.

(1) Message parameter. Message parameter, which can be used in the compilation information output window
Output the corresponding information, which is very important for controlling the source code information. The usage is as follows:
# Pragma message ("message text ")
When the compiler encounters this instruction, it prints the message text in the compilation output window.
When we define many Macros in the program to control the source code version, we may forget whether these macros are correctly set, in this case, we can use this command to check it during compilation. Suppose we want to determine whether we have defined the _ x86 macro in the source code. The following method can be used:

# Ifdef _ x86
# Pragma message ("_ x86 macro activated !")
# Endif
After we define the _ x86 macro, the application will display "_
X86 macro activated !". We won't be scratching our heads because we don't remember some specific macros we defined.
.

(2) The other Pragma parameter that is used more frequently is code_seg. Format:
# Pragma code_seg (["section-name" [, "section-class"])
It can set the code segment where function code is stored in the program. It is used when we develop the driver.

(3) # pragma once (commonly used)
You only need to add this command at the beginning of the header file to ensure that the header file is compiled once. This command is actually available in vc6, but it is not widely used in consideration of compatibility.

(4) # pragma hdrstop indicates that the pre-compiled header file ends here, and the subsequent header files are not pre-compiled. BCB can pre-compile the header file to speed up the link, but if all header files are pre-compiled, it may occupy too much disk space. Therefore, this option is used to exclude some header files.

Sometimes there is a dependency between units. For example, unit a depends on unit B. Therefore, Unit B must be compiled before unit. You can use # pragma startup to specify the compilation priority. If # pragma package (smart_init) is used, BCB will be compiled based on the priority.

(5) # pragma resource "*. DFM" indicates adding resources in the *. DFM file to the project. * DFM includes the form
The definition of the appearance.

(6) # pragma warning (Disable: 4507 34; once: 4385; error: 164)

It is equivalent:
# Pragma warning (Disable: 4507 34) // do not display the 4507 and 34 Warnings
# Pragma warning (once: 4385) // only one warning message is reported once
# Pragma warning (error: 164) // the error message 164 is used as an error.
This pragma warning also supports the following formats:
# Pragma warning (push [, N])
# Pragma warning (POP)
Here N represents a warning level (1---4 ).
# Pragma warning (push) saves the existing warning status of all warning information.
# Pragma warning (push, n) saves the existing warning status of all warning information, and sets global warning
The level is set to n.
# Pragma warning (POP) pops up the last warning message to the stack.
All changes are canceled. For example:
# Pragma warning (push)
# Pragma warning (Disable: 4705)
# Pragma warning (Disable: 4706)
# Pragma warning (Disable: 4707)
//.......
# Pragma warning (POP)
At the end of the Code, save all warning information (including 4707, and ).
(7) Pragma comment (...)
This command puts a comment record into an object file or executable file.
Common lib keywords can help us to connect to a library file.

Statement: The above is copied from the document library.