- Process Termination
There are eight ways for a process to terminate. Normal termination occurs in file ways:
- Return from main
- Calling exit
- Calling _exit or _Exit
- Return of the last thread from its start routine
- Calling pthread_exit from the last thread
Abnormal termination occurs in three ways:
- Calling abort
- Receipt of a signal
- Response of the last thread to a cancellation request
2. Exit Functions
#include<stdlib.h>void exit(int status);void _Exit(int status);#include<unistd.h>void _exit(int status);- _exit and _Exit, which return to the kernel immediately, and exit, which performs certain cleanup processing (use fclose function for all open streams) and then return to kernel.
- If any of these functions is called without an exit status, main does a return without a return value, or the main function is not declared to return a integer, the exit status of the process is undefined.
3. Memory layout of a C program
- Test segment, the machine instructions that the CPU executes. Usually, the test segment is read-only and sharable.
- Initialized data segment, containing variables that are specifically initialized in the program(ex: int data=100).
- Uninitialized data segment, data in this segment is initialized by the kernel to arithmetic 0 or NULL pointers before the program starts executing.
- Stack: where automatic variables are stored, along with information that is saved each time a function is called.
- Heap: where dynamic memory allocation usually takes place. Historically, the heap has been located between the uninitialized data and the stack.
4. Memory Allocation
void * malloc(size_t size);void * calloc(size _t nobj, size_t size);void * realloc(void *ptr, size_t newsize);- malloc, which allocates a specified number of bytes of memory. The initial value of the memory is indeterminate.
- calloc, which allocates space for a specified number of objects of a specified size. The space is initialized to all 0 bits.
- realloc, which increase or decrease the size of a previously allovated area. The initial value of the space between the old contents and the end of the new area is indeterminate.
- One additional function is also worth mentioning. The function alloca has the same calling sequence as malloc; however, instead of allocating memory from the heap, the memory is allocated from the stack frame of the current function. The advantage is that we don't have to free the space; it goes away automatically when the function returns. The disadvantage is that some system don't support alloca , it it's impossible to increase the size of the stack frame after the function has been called.
5. setjmp and longjmp functions
- In C, we can not goto a lablel that is in another function. Instead, we must use the setjmp and longjmp functions to perform this type of branching. These two functions are useful for handling error conditions that occur in a deeply rested function call.
#include<setjmp.h>int setjmp(jmp_buf env);int longjmp(jmp_buf env, int val);Argument val decide return value on setjmp, we can use this to check which function call longjmp.
- The setjmp(3) manual page on one system states that variables stored in memory will have values as of the time of the longjmp, whereas variables in the CPU and floating-point registers are restored to their values when setjmp was called. If you have an automatic variable that you don't want to rolled back, define it with the volatile attribute.