Difference between process and thread

reference ： Threads and processes , Do you really understand _ The clouds are deep i Blog somewhere -CSDN Blog

1 process

• A process is the smallest unit of resources allocated by a system
• Process owned >= 1 Threads
• Each process uses a separate address space ; Include ： Code section , Data area , Stack area , The address space between processes is isolated , They don't influence each other .

2 Process switching

• Process switching will bring additional overhead ： namely Context （ perform B Threads , Need to keep A Thread of thread , Take it out when executing ） Switch .

• Context preservation is specifically ：1.CPU Current values in all registers 2. Save all States of the process ： The state of the current process 3. Save the stack of the process .

3 Running process

• Process switching involves Concurrency and parallelism

• Concurrent ： Each process is assigned a Time slice ： Every time CPU The length of time available to this process . This is because there is only one single core CPU, But you need to execute multiple processes . because CPU The switching and execution of processes are very fast , So it looks like multiple processes “ Run in parallel ”
• parallel ： With multicore CPU Appearance , Parallelism enables real simultaneous execution of multiple tasks .

4 Threads

• Threads in the same process share the resources of the process
• Threads have their own stack area , Share the data area of the process , Code section , Heap area .

  5  How the function looks in memory （ Excerpt from ： What functions look like when they run in memory ？ - You know ）

• The function is called in the stack .
• Detailed explanation of function stack during function call _ Qin Ze's blog -CSDN Blog _ Function stack

  5.1.1 Function call and return

1. By function A The corresponding function in B Address , Jump into B perform . Corresponding to the function call written in the code , namely call 0x400540 ( notes ：0x400540 That's the function. B The address of )
2. Will function A After executing the function B() Next instruction address after push To A The stack frame , namely 0x40056a
3. . function B Finished executing the code , Then execute the last instruction ：ret , This function B After execution, jump to the function A Carry on . The address you jump to is 2 Step by step 0x40056a

    5.1.2 function B Parameters passed in

• When the slave function A() Jump to function B() When , If you need to pass some parameters , How is this achieved ？ The incoming parameters of the function originally exist in registers , But when too many parameters are passed in , Parameters will be written to the stack frame .
• function B The parameter passed in is the function A Local variables of , So parameters are existential functions A In the stack frame of .
• There is also an interview point here ： Stack order of function parameters ： The argument to the function is   From right to left On stack

6 Address space of process runtime （ Excerpt from ： How threads share process resources ？ - You know ）

• The stack area

No disorder between threads , It must be private , But if A Thread's local variables a, B Threads can get a,B You can also modify a

• Heap area ： The code passes malloc/new Where the memory comes out

Just know the pointer of the heap , Every thread Can access the data pointed to by the pointer . So threads share the heap

• Code section ： Written code compiled into executable machine instructions .

Threads share code areas , Each thread can use all functions in the code area

• Data area

Empathy ： Sharing between threads .

Sum up ： Threads share process resources

6 What resources are shared by threads

• Threads in the same process share all the contents of the address space of the owning process .
• The essence of thread operation is the execution of functions , And each thread has its own independence , Private stack area . At the same time, function operation requires additional registers to store information such as registers .
• Thread private ： The stack area , Program counter , Stack pointer , Registers required for operation . These can be collectively referred to as Thread context