linux系统编程之管道（二）：管道读写规则
一，管道读写规则
当没有数据可读时
- O_NONBLOCK disable：read调用阻塞，即进程暂停执行，一直等到有数据来到为止。
- O_NONBLOCK enable：read调用返回-1，errno值为EAGAIN。
当管道满的时候
- O_NONBLOCK disable： write调用阻塞，直到有进程读走数据
- O_NONBLOCK enable：调用返回-1，errno值为EAGAIN
如果所有管道写端对应的文件描述符被关闭，则read返回0
如果所有管道读端对应的文件描述符被关闭，则write操作会产生信号SIGPIPE
当要写入的数据量不大于PIPE_BUF时，linux将保证写入的原子性。
当要写入的数据量大于PIPE_BUF时，linux将不再保证写入的原子性。
二，验证示例
示例一：O_NONBLOCK disable：read调用阻塞，即进程暂停执行，一直等到有数据来到为止。
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int main(void) { int fds[2]; if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } pid_t pid; pid = fork(); if(pid == -1){ perror("fork error"); exit(EXIT_FAILURE); } if(pid == 0){ close(fds[0]);//子进程关闭读端 sleep(10); write(fds[1],"hello",5); exit(EXIT_SUCCESS); } close(fds[1]);//父进程关闭写端 char buf[10] = {0}; read(fds[0],buf,10); printf("receive datas = %s\n",buf); return 0; }
结果：
说明：管道创建时默认打开了文件描述符，且默认是阻塞（block）模式打开
所以这里，我们让子进程先睡眠10s，父进程因为没有数据从管道中读出，被阻塞了，直到子进程睡眠结束，向管道中写入数据后，父进程才读到数据
示例二：O_NONBLOCK enable：read调用返回-1，errno值为EAGAIN。
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int main(void) { int fds[2]; if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } pid_t pid; pid = fork(); if(pid == -1){ perror("fork error"); exit(EXIT_FAILURE); } if(pid == 0){ close(fds[0]);//子进程关闭读端 sleep(10); write(fds[1],"hello",5); exit(EXIT_SUCCESS); } close(fds[1]);//父进程关闭写端 char buf[10] = {0}; int flags = fcntl(fds[0], F_GETFL);//先获取原先的flags fcntl(fds[0],F_SETFL,flags | O_NONBLOCK);//设置fd为阻塞模式 int ret; ret = read(fds[0],buf,10); if(ret == -1){ perror("read error"); exit(EXIT_FAILURE); } printf("receive datas = %s\n",buf); return 0; }
结果：
示例三：如果所有管道写端对应的文件描述符被关闭，则read返回0
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int main(void) { int fds[2]; if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } pid_t pid; pid = fork(); if(pid == -1){ perror("fork error"); exit(EXIT_FAILURE); } if(pid == 0){ close(fds[1]);//子进程关闭写端 exit(EXIT_SUCCESS); } close(fds[1]);//父进程关闭写端 char buf[10] = {0}; int ret; ret = read(fds[0],buf,10); printf("ret = %d\n", ret); return 0; }
结果：
可知确实返回0，表示读到了文件末尾，并不表示出错
示例四：如果所有管道读端对应的文件描述符被关闭，则write操作会产生信号SIGPIPE
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> #include <signal.h> void sighandler(int signo); int main(void) { int fds[2]; if(signal(SIGPIPE,sighandler) == SIG_ERR) { perror("signal error"); exit(EXIT_FAILURE); } if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } pid_t pid; pid = fork(); if(pid == -1){ perror("fork error"); exit(EXIT_FAILURE); } if(pid == 0){ close(fds[0]);//子进程关闭读端 exit(EXIT_SUCCESS); } close(fds[0]);//父进程关闭读端 sleep(1);//确保子进程也将读端关闭 int ret; ret = write(fds[1],"hello",5); if(ret == -1){ printf("write error\n"); } return 0; } void sighandler(int signo) { printf("catch a SIGPIPE signal and signum = %d\n",signo); }
结果：
可知当所有读端都关闭时，write时确实产生SIGPIPE信号
示例五：O_NONBLOCK disable： write调用阻塞，直到有进程读走数据
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int main(void) { int fds[2]; if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } int ret; int count = 0; while(1){ ret = write(fds[1],"A",1);//fds[1]默认是阻塞模式 if(ret == -1){ perror("write error"); break; } count++; } return 0; }
结果：
说明：fd打开时默认是阻塞模式，当pipe缓冲区满时，write操作确实阻塞了，等待其他进程将数据从管道中取走
示例六：O_NONBLOCK enable：调用返回-1，errno值为EAGAIN
#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <fcntl.h> int main(void) { int fds[2]; if(pipe(fds) == -1){ perror("pipe error"); exit(EXIT_FAILURE); } int ret; int count = 0; int flags = fcntl(fds[1],F_GETFL); fcntl(fds[1],F_SETFL,flags|O_NONBLOCK); while(1){ ret = write(fds[1],"A",1);//fds[1]默认是阻塞模式 if(ret == -1){ perror("write error"); break; } count++; } printf("the pipe capcity is = %d\n",count); return 0; }
结果：
可知也出现EGIN错误，管道容量是65536字节
man 7 pipe说明：
Pipe capacity
A pipe has a limited capacity. If the pipe is full, then a write(2) will block or fail, depending on whether the O_NONBLOCK flag is set (see below). Different implementations have different limits for the pipe capacity. Applications should not rely on a particular capacity: an application should be designed so that a reading process consumes data as soon as it is available, so that a writing process does not remain blocked. In Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on i386). Since Linux 2.6.11, the pipe capacity is 65536 bytes.
三，管道写与PIPE_BUF关系
man帮助说明：
PIPE_BUF
POSIX.1-2001 says that write(2)s of less than PIPE_BUF bytes must be atomic: the output data is written to the pipe as a contiguous sequence. Writes of more than PIPE_BUF bytes may be nonatomic: the kernel may interleave the data with data written by other processes. POSIX.1-2001 requires PIPE_BUF to be at least 512 bytes. (On Linux, PIPE_BUF is 4096 bytes.) The precise semantics depend on whether the file descriptor is nonblocking (O_NONBLOCK), whether there are multiple writers to the pipe, and on n, the number of bytes to be written: O_NONBLOCK disabled, n <= PIPE_BUF All n bytes are written atomically; write(2) may block if there is not room for n bytes to be written immediately 阻塞模式时且n<PIPE_BUF:写入具有原子性，如果没有足够的空间供n个字节全部写入，则阻塞直到有足够空间将n个字节全部写入管道 O_NONBLOCK enabled, n <= PIPE_BUF If there is room to write n bytes to the pipe, then write(2) succeeds immediately, writing all n bytes; otherwise write(2) fails, with errno set to EAGAIN. 非阻塞模式时且n<PIPE_BUF：写入具有原子性，立即全部成功写入，否则一个都不写入，返回错误 O_NONBLOCK disabled, n > PIPE_BUF The write is nonatomic: the data given to write(2) may be interleaved with write(2)s by other process; the write(2) blocks until n bytes have been written. 阻塞模式时且n>PIPE_BUF：不具有原子性，可能中间有其他进程穿插写入，直到将n字节全部写入才返回，否则阻塞等待写入 O_NONBLOCK enabled, n > PIPE_BUF If the pipe is full, then write(2) fails, with errno set to EAGAIN. Otherwise, from 1 to n bytes may be written (i.e., a "partial write" may occur; the caller should check the return value from write(2) to see how many bytes were actually written), and these bytes may be interleaved with writes by other processes.
非阻塞模式时且N>PIPE_BUF：如果管道满的，则立即失败，一个都不写入，返回错误，如果不满，则返回写入的字节数为1~n，即部分写入，写入时可能有其他进程穿插写入
- 当要写入的数据量不大于PIPE_BUF时，linux将保证写入的原子性。
- 当要写入的数据量大于PIPE_BUF时，linux将不再保证写入的原子性。
注：管道容量不一定等于PIPE_BUF
示例：当写入数据大于PIPE_BUF时
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <errno.h> #include <fcntl.h> #define ERR_EXIT(m) \ do \ { \ perror(m); \ exit(EXIT_FAILURE); \ } while(0) #define TEST_SIZE 68*1024 int main(void) { char a[TEST_SIZE]; char b[TEST_SIZE]; char c[TEST_SIZE]; memset(a, 'A', sizeof(a)); memset(b, 'B', sizeof(b)); memset(c, 'C', sizeof(c)); int pipefd[2]; int ret = pipe(pipefd); if (ret == -1) ERR_EXIT("pipe error"); pid_t pid; pid = fork(); if (pid == 0)//第一个子进程 { close(pipefd[0]); ret = write(pipefd[1], a, sizeof(a)); printf("apid=%d write %d bytes to pipe\n", getpid(), ret); exit(0); } pid = fork(); if (pid == 0)//第二个子进程 { close(pipefd[0]); ret = write(pipefd[1], b, sizeof(b)); printf("bpid=%d write %d bytes to pipe\n", getpid(), ret); exit(0); } pid = fork(); if (pid == 0)//第三个子进程 { close(pipefd[0]); ret = write(pipefd[1], c, sizeof(c)); printf("bpid=%d write %d bytes to pipe\n", getpid(), ret); exit(0); } close(pipefd[1]); sleep(1); int fd = open("test.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644); char buf[1024*4] = {0}; int n = 1; while (1) { ret = read(pipefd[0], buf, sizeof(buf)); if (ret == 0) break; printf("n=%02d pid=%d read %d bytes from pipe buf[4095]=%c\n", n++, getpid(), ret, buf[4095]); write(fd, buf, ret); } return 0; }
结果：
可见各子进程间出现穿插写入，并没保证原子性写入，且父进程在子进程编写时边读。