Fully buffered & Line buffered & Unbuffered

Define

1. Fully buffered means that I/O takes place only when the buffer is fully, the process explicitly calls fflush, or the process terminates by calling exit. A common size for the
standard I/O buffer is 8192 bytes; 
 
2. Line buffered means that I/O takes place when a newline is encountered, when the process calls fflush, or when the process terminates by calling exit. 
 
3. Unbuffered means that I/O take place each time a standard I/O output function is called. 
 
 
Most unix implementations of the standard I/O libarary use the following rules. 
 
1. Standard error is always unbuffered. 
 
2. Standard input and standard output are fully buffered, unless they refer to a terminal device, in which case, they are line buffered. 
 
3. All other streams are fully buffered unless they refer to a terminal device, 
in which case, they are line buffered. 

Functions:

使用setbuf()和setvbuf()可以更改緩衝的類型：
setbuf(FILE *fp, char *buf)   buf用malloc()開闢，當buf為NULL時，緩衝區為不帶緩衝
setvbuf(FILE *fp,char *buf, int buf_mode, int size)
        buf_mde有三種：_IOFBF 0 全緩衝， _IOLBF 1 行緩衝， _IONBF 2 不帶緩衝

Default Buffering modes:

• stdin is always buffered
• stderr is always unbuffered
• if stdout is a terminal then buffering is automatically set to line buffered, else it is set to buffered

Default Buffer sizes:

• Buffer size only directly affects buffered mode
• The default size like the kernel is based on the page size (4096 bytes on my system)
• if stdin/stdout are connected to a terminal then default size = 1024; else size = 4096

Case 1 exit:

#include <stdio.h>int main(int argc, char **argv) {    printf("hello world!");    /*    _Exit(0);    _exit(0);    exit(0);    */}

case 2 line buffered:

#include <stdio.h>int main(void){    int c;    do {        c = fgetc(stdin);        if (c == '\n') {            continue;        }        else if (c == 'z') {            break;        }        printf("--------\n");        printf("%c    ", c);        /*        */        /*        printf("%c\n", c);        */        sleep(5);    } while (1);    return 0;}

case 3.1 fork:

~/test$ ./a.out before fork!pid = 2312, flag = 6before fork!pid = 2311, flag = 5~/test$ ./a.out > fork.log~/test$ cat fork.log before fork!pid = 2314, flag = 6before fork!pid = 2313, flag = 5

    printf ("before fork!"); 

case 3.2 fully buffered:

~/test$ ./a.out before fork!pid = 2372, flag = 6pid = 2371, flag = 5~/test$ ./a.out > fullbuffer.log~/test$ cat fullbuffer.log before fork!pid = 2374, flag = 6before fork!pid = 2373, flag = 5~/test$  

  printf ("before fork!\n"); 

case 3.0 source code

#include <stdio.h>int main(void){    /*    printf ("before fork!");    */    printf ("before fork!\n");    int pid = 0;    int flag = 5;    pid = fork();    if (pid < 0) {        printf ("error\n");    }    else if (pid == 0) {        flag++;        }    else {        sleep(2);    }    printf("pid = %d, flag = %d\n", getpid(), flag);    return 0;}