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In this video we will implement assert function which is especially important in the system. 

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The assertion is used in all the modules of the kernel. When we write different modules in the kernel, 

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we assume that everything in it should run as we expect. 

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A small error will bring the whole system down.

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As you see, this is the structure of our system. The user programs are in the untrusted zone. 

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When they send requests to the kernel by system call, the barricade will check those requests and data 

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to see if they are valid. 

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If not, we will simply return error message to the programs.

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If the check pass, then the kernel will process these requests. 

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And now we are in the trusted area 

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where we will not perform the same checks as we did in the barricade. 

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Instead we will use assertions to find errors within the kernel. 

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If the assumption we made in the assertions failed, the assertion will stop the system 

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and print the file name and line number which causes the error.  So it could help us find some errors as we build up the project

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.

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The assert function in our system is really simple. So let's do it now.

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First off, we create a header file debug.h. 

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As you see, the assert is actually a macro. We define assert and the expression we want to evaluate. 

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If it evaluates to false, then we will call error check function. 

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The function takes two parameters 

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the file name and the line number

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.

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We pass the predefine macro file and line to it. The file will expand to the current file path 

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and line expands to the current line number. 

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Also we use do while loop to wrap up the macro, otherwise it may cause problems when used in if statements.  

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The value is 0 in this case 

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so that if statement executes only once. 

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Ok let’s see the error check function in the file debug.c. 

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Since we print error message on the screen using printk function, we include print header file.

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In the function, there are only two things we will do. Print message and jump to the infinite loop. 

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In this example, we print error check on the screen, 

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here we add lines to make it more readable.

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Then we print assertion failed, the name of the file and the line number. 

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After we print the file name and line number,  we jump to while loop to stop the system.

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Ok right now we can test the assert function in the main file. 

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we include debug.h file and in this test, we simply assert 0. 

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So the assertion will fail when we execute it. 

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In the build script, we add the file debug.c and add debug.o in the linker command. 

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So let's build the project and check it out.

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Ok you see the assertion failed at main.c file. The line number is 14. If we check main.c file, 

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the assert is at this line.

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So the assert is actually working.

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what we are going to do next is we are going to talk about the function in print.c file.  

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If we use assert in the functions of the print file, 

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we could have problems. 

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For example, if we added the simple check at the begnning of the function write screen,

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we cannot simply using assert to do the check.   Because the printk function is used in the assert and we will eventually enter this function again

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.

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And also the data in the structure screen buffer is not correct if the check fails at this point. 

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So we should write another function and print message by copying the characters on the screen buffer.

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For simplicity, we don't implement the simple check in this function.