以下程序的预期输出是:

received REQUEST
 from client
received REPLY
 from server


例如,可以在GNU / Linux上看到。但是,在Cygwin上,实际输出是这样的:

received REQUEST
 from client
received
 from server


从客户端的流而不是"\n"接收到空行("REPLY\n"字符串)。

问题源于putc和行缓冲的结合。


server函数中,如果将my_puts替换为fprintf,则会产生预期的输出。
server中,如果我们执行setvbuf调用,该调用将流切换到行缓冲,则还会产生预期的输出。
如果更改my_puts使其使用fprintf(f, "%c", ch)而不是putc(ch, f)输出每个字符,则将产生预期的输出。
putc替换fputc没有区别。
在服务器中,根据ISO C的要求在输入和输出之间添加文件定位操作(例如fseek(cli_stream, 0, SEEK_CUR))(因此恰好在my_puts调用之前)没有区别。这包含在下面的代码中。


这个程序做错了什么会导致这个奇怪的问题吗?

码:

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <errno.h>
#include <unistd.h>
#include <signal.h>

int my_puts(FILE *f, const char *str)
{
   int ch;

   while ((ch = *str++))
     if (putc(ch, f) == EOF) {
       perror("putc");
       return EOF;
     }

   return 0;
}

int client(struct sockaddr_in *srv_addr)
{
  int srv_fd = socket(AF_INET, SOCK_STREAM, 0);
  FILE *srv_stream;
  char buf[256];

  if (srv_fd == -1) {
    perror("socket");
    return EXIT_FAILURE;
  }

  if (connect(srv_fd, (struct sockaddr *) srv_addr, sizeof *srv_addr) == -1) {
    perror("connect");
    close(srv_fd);
    return EXIT_FAILURE;
  }

  if ((srv_stream = fdopen(srv_fd, "r+")) == 0) {
    perror("fdopen");
    close(srv_fd);
    return EXIT_FAILURE;
  }

   setvbuf(srv_stream, (char *) NULL, _IOLBF, 0);

  my_puts(srv_stream, "REQUEST\n");
  fflush(srv_stream);
  shutdown(srv_fd, SHUT_WR);

  if (fgets(buf, sizeof buf, srv_stream) == 0) {
    printf("short fread");
    fclose(srv_stream);
    return EXIT_FAILURE;
  }

  printf("received %s from server\n", buf);
  return 0;
}

void server(int acc_fd)
{
  struct sockaddr_in claddr;
  socklen_t claddrlen = sizeof claddr;
  int cli_fd = accept(acc_fd, (struct sockaddr *) &claddr, &claddrlen);
  FILE *cli_stream = fdopen(cli_fd, "r+");;
  char buf[256];

  if (cli_fd == -1) {
    perror("accept");
    if (cli_stream)
      fclose(cli_stream);
    return;
  }

  if (cli_stream == 0) {
    perror("fdopen");
    close(cli_fd);
    return;
  }

  setvbuf(cli_stream, (char *) NULL, _IOLBF, 0);

  if (fgets(buf, sizeof buf, cli_stream) == 0) {
    printf("short fread");
    fclose(cli_stream);
    return;
  }

  printf("received %s from client\n", buf);

  fseek(cli_stream, 0, SEEK_CUR);

  my_puts(cli_stream, "REPLY\n");
  fflush(cli_stream);
  fclose(cli_stream);
}

int main(void)
{
  struct sockaddr_in saddr = { 0 };
  int acc_fd = socket(AF_INET, SOCK_STREAM, 0);
  pid_t child;

  saddr.sin_family = AF_INET;
  saddr.sin_port = htons(12345);
  saddr.sin_addr.s_addr = htons(INADDR_ANY);

  if (acc_fd == -1) {
    perror("socket");
    return EXIT_FAILURE;
  }

  if (bind(acc_fd, (struct sockaddr *) &saddr, sizeof saddr) == -1) {
    perror("bind");
    return EXIT_FAILURE;
  }

  if (listen(acc_fd, 1) == -1) {
    perror("listen");
    return EXIT_FAILURE;
  }

  child = fork();

  if (child == -1) {
    perror("fork");
    return EXIT_FAILURE;
  } else if (child == 0) {
    server(acc_fd);
    _exit(0);
  } else {
    int status;
    int result = client(&saddr);
    kill(child, SIGKILL);
    if (wait(&status) != child) {
      printf("wait problem");
      return EXIT_FAILURE;
    }
    return result;
  }

  return 0;
}

最佳答案

Cygwin git现在has the fix,从OpenBSD向后移植。

较小的repro测试用例为posted to the mailing list

关于c - Cygwin:由于stdio putc +行缓冲导致流数据丢失,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/35928828/

10-10 13:05