fgets

Author: smithAchang

c/fgets/Makefile

@@ -0,0 +1,8 @@
+.PHONY: udpSendTest
+
+CCFLAGS := -g3 -O0 -W -Wall -std=gnu11 -pipe -Wno-int-to-pointer-cast -Wno-pointer-to-int-cast -Wno-unused-variable -Wno-unused-but-set-variable -Wno-unused-parameter
+LDFLAGS := -lpthread
+
+udpSend: main.c
+	@echo "test udp send program ..."
+	gcc $(CCFLAGS) $^ $(LDFLAGS)

c/fgets/main.c

@@ -0,0 +1,152 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <pthread.h>
+#include <signal.h> 
+#include <unistd.h>
+#include <errno.h> 
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/socket.h>
+#include <sys/time.h>
+#include <sys/prctl.h>
+#include <sys/epoll.h>
+#include <netinet/icmp6.h>      // struct nd_neighbor_solicit/advert, which contains icmp6_hdr and ND_NEIGHBOR_ADVERT
+#include <netinet/in.h>         // IPPROTO_IPV6, IPPROTO_ICMPV6, INET6_ADDRSTRLEN
+#include <netinet/ip.h>         // IP_MAXPACKET (65535)
+#include <arpa/inet.h>          // inet_pton() and inet_ntop()
+#include <poll.h>
+#include <fcntl.h>
+
+
+
+
+const char* cnGetSysErrnoDesc()
+{
+ static __thread char   s_acErrorDesc[512];
+ s_acErrorDesc[0] = '\0';
+ strerror_r(errno,s_acErrorDesc,sizeof(s_acErrorDesc));
+ return  s_acErrorDesc;
+}
+
+
+int main(int argc, char* argv[])
+{
+    int nonblockingFlag = 0;
+    
+    if(argc > 1)
+    {
+      nonblockingFlag = 1;        
+    }
+
+    int pipefds[2];
+    int rc;
+    rc = pipe(pipefds);
+    if(rc < 0)
+    {
+        perror("Create pipe in failure");
+        return -1;
+    }
+    
+    pid_t child = fork();
+    
+    if(child == 0)
+    {
+        printf("Child process must exit when parent process exited\n");
+        prctl(PR_SET_PDEATHSIG, SIGTERM);
+
+        printf("Child process set self to be the leader of process group\n");
+        setpgid(0, 0);
+        int blackhole = open("/dev/null", O_RDWR | O_CLOEXEC);
+        if(blackhole < 0)
+        {
+            return -1;
+        }
+        
+        dup2(blackhole, STDIN_FILENO);
+        
+        printf("Redirect the standard out and standard err\n");
+        dup2(pipefds[1], STDOUT_FILENO);
+        dup2(pipefds[1], STDERR_FILENO);
+        
+        printf("Sleep a while to watch the blocking status of fgets API\n"); 
+        sleep(10);
+        
+        printf("fflush the output streams if any\n"); 
+
+        fflush(stdout);
+        fflush(stderr);
+
+        /*immediately exit*/        
+        _exit(0);
+    }
+    
+    printf("Parent process continues to run ...\n");
+    close(pipefds[1]);
+    
+    if(nonblockingFlag)
+    {
+        printf("Set nonblocking flag\n");
+        int oldFlags = fcntl(pipefds[0], F_GETFL);
+        fcntl(pipefds[0], F_SETFL, oldFlags | O_NONBLOCK);
+    }
+    else
+    {
+        printf("Using blocked fd APIs\n");
+    }
+    
+    FILE* fp = fdopen(pipefds[0], "r");
+    
+    if(fp == NULL)
+    {
+        perror("fdopen pipe fd in failure");
+        return -1;
+    }
+    
+    int status;
+    pid_t chckpid;
+    char szLineBuf[512];
+    char* szLine;
+    
+    do
+    {
+        szLine = fgets(szLineBuf, sizeof(szLineBuf), fp);
+        if(szLine == NULL)
+        {
+           continue; 
+        }
+        
+        printf("Child process output: %s\n", szLine);
+        status = 0;
+        chckpid = waitpid(child, &status, WNOHANG | WEXITED);
+    }while(chckpid == 0 /*running*/);
+    
+    printf("Detect child process has exited ...\n");
+    
+    do
+    {
+        szLine = fgets(szLineBuf, sizeof(szLineBuf), fp);
+        if(szLine == NULL)
+        {
+           perror("Read child process's lefted output");
+           break; 
+        }
+        
+        printf("Child process lefted output: %s\n", szLine);
+    }while(szLine != NULL);
+    
+    if(WIFEXITED(status))
+    {
+        printf("Normally exited\n");
+    }
+    
+    if(WIFSIGNALED(status))
+    {
+        printf("exited by signal: %d\n", WTERMSIG(status));
+    }
+    
+    fclose(fp);
+    printf("Will exit !!!\n");
+	return 0;
+}
+

kb/fgets.md

@@ -0,0 +1,18 @@
+虽然笔者认可`不重复发明轮子`的理论，但是，在遇到具体问题的时间，如果你不知道一个具体的`API`，还是会陷入重复制造的麻烦当中。
+
+笔者遇到的问题，是父进程从父子进程之间的`pipe`，按行工作协议读取子进程的行输出。
+
+因为不知道`fdopen`函数的存在，原来曾蹩脚地写过解析行的小函数，正确性还没有办法一下子保证正确 :(
+
+在用`fdopen`将文件句柄转换为文件流后，您立马可以直接使用工业级的`fgets stream API`！
+
+
+
+# 注意事项
++ `fdopen`后句柄所有权将发生转移，后期必须使用`fclose`进行关闭文件流对象
++ `fcntl`设置**pipe**文件句柄的`O_NONBLOCK`选项，对于`fgets`存在影响，建议在非阻塞场景，必须设置
++ + `waitpid(childpid, &status, WNOHANG | WEXITED) == 0`检测子进程是否已退出
+
+
+# 遗留问题
++ 文件流操作在`f* API`体系中单独设置阻塞非阻塞状态？