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测试代码：

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>#define ERR_EXIT(m) do { perror(m); exit(EXIT_FAILURE); }while(0)
#define TEST_SIZE 68*1024int main(int argc, char *argv[])
{char a[TEST_SIZE];char b[TEST_SIZE];memset(a, 'a', sizeof(a));memset(b, 'b', sizeof(b));int pipefd[2];int ret = pipe(pipefd);if (ret == -1)ERR_EXIT("pipe_errro");pid_t pid;pid = fork();if(pid == 0) {close(pipefd[0]);ret = write(pipefd[1], a, sizeof(a));printf("apid = %d write %d bytes to pipefd\n", getpid(), ret);exit(0);}pid = fork();if(pid == 0) {close(pipefd[0]);ret = write(pipefd[1], b, sizeof(b));printf("bpid = %d write %d bytes to pipefd\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[4096] = %c\n", n++, getpid(), ret, buf[4095]);write(fd, buf, ret);}
}