setjmp and longjmp

-
In short, functions setjmp and longjmp just work like label&goto instruction, except that the combination of label&goto only can be used within the same function which means you cannot goto the label outside the function. But the combination of setjmp and longjmp can be used anywhere in the whole program.

For example, we use label&goto to implement a jump function:

main()
{
     int a=1, b=2, c=0;
label:
     c=a+b;
     goto label;
     
}

Now we implement the same function using setjmp and longjmp:

typedef long jmp_buf[16];

main()
{
     int a=1, b=2, c=0;
     jmp_buff environment;
     setjmp (evironment);
     c=a+b;
     longjmp(evironment, 1);
}

So from above, we can see the jump buffer environment contains all the context around the setjmp function, so it works like label. And then the longjmp function will jump to the position where environment assigned.

The prototype of setjmp and longjmp:

int setjmp(jmp_buff env);
void longjmp(jmp_buff env, int val);

So function longjmp never return any value, because it will just jump the execution flow to setjmp position, but once jump to setjmp position, the setjmp function will return the value assigned by val which is the second parameter of longjmp function.

The first time setjmp function is called, it will always return 0. But after longjmp is executed, and jump to setjmp function, setjmp will never return 0. (Even if you set val to 0, setjmp will also return 1).

To make every thing more clearer, let's see an example from reference [1]:

  1. #include<stdio.h>  
  2. #include<setjmp.h>  
  3. jmp_buf ebuf;  
  4. int f2();  
  5. int main()  
  6.  {  
  7.        int i;  
  8.        printf("1");  
  9.        i=setjmp(ebuf);  
  10.        if(i==0)  
  11.        {  
  12.            f2();  
  13.            printf("This will not beprinted.");  
  14.        }  
  15.        printf("%d",i);  
  16.        return 0;  
  17. }  
  18. int f2()  
  19. {  
  20.        printf("2");  
  21.        longjmp(ebuf,3);  
  22. }  

The output of this example will print "1 2 3"


Reference: 
[1] http://blog.csdn.net/sunlylorn/article/details/7178575 

Comments

Post a Comment

Popular posts from this blog

Basic understanding of TLS-PSK protocol

-
Image
This post will talk about the basic understanding of TLS-PSK protocol. It will not cover TLS's history and how its mathematical algorithms work. Instead, what is TLS-PSK protocol and how it works will be discussed here. I prefer using diagrams to help explain things. Hopefully, it can give readers a very basic insights of TLS-PSK. What is TLS-PSK Before talking about what is TLS-PSK, I would like to introduce what is TLS first. TLS, Transport Layer Security, is a cryptographic protocol, aims to provide protection to end-to-end communication over the internet. It helps to protect network users from eavesdropping, tempering and so on. Clear data transmission is dangerous, given that technical hacker can listen to the communication between two computers and steal sensitive data like user name, password and so on. This is situation where TLS can step in and protect you. TLS has two steps: TLS handshake and data transmission. During TLS handshake, the communicating parties mus...
Read more

Differences between ASIC, ASSP and ASIP

-
ASIC (Application Specified Integrated Circuit) is a customized integrated circuit. It is usually used by a person or company for a very limited usage. So when it is developed, only the person or company who orders it can use it. It is not useful for other usages, for example, an IC designed for a specific line of cellular phones of a company, whereby no other products can use it except the cell phones belonging to that product line. - ASIC is just built for one and only one customer. - ASIC is used only in one product line - Only volume production of ASICs for one product can make sense which means low unit cost for high volume products, otherwise the cost is not efficient. - Can exploit parallelism to achieve high performance - Low power consumption ASSP (Application Specified Standard Processor) is an integrated circuit that implements a specific function that appeals to a wide market, which means its function is specified for example a motor drive chip. But it is used widel...
Read more

Orthogonal instruction set

-
Definition: "In computer engineering, an orthogonal instruction set is an instruction set architecture where  all instruction types  can use  all addressing modes . It is 'orthogonal' in the sense that the instruction type and the addressing mode vary independently." The story of orthogonal instruction set: In the past, because the  memory is very precious , so the processor designer tends to  make instructions as powerful as possible . So they hope one instruction can perform  full function  like get data from memory and then perform the calculation. They make  all instructions can have all the addressing modes . By this way,  one instruction can fetch data and do the calculation at the same time .  But this will  inscrease the complexity  of the processor design. Also, accessing memory directly is very time-consuming, so some companies choose to use RISC processor (like ARM) which has a  LOAD-STORE a...
Read more