由 perror scanf 通过管道引起的无限循环

Question

我一直在编写一个 c 程序，它将数据从一个文件传输到下一个文件，但是它一直是无限循环的。

到目前为止我发现了什么。无限循环是在文件 c1.c 上引起的，其中 perror（或 stderr）跳过了 scanf。如果 scanf 确实有效。程序无限循环进一步沿着轨道打印出 perror，即使它超过了那个部分！

我的代码在下面

控制器.c

#include<stdio.h>
#include<stdlib.h>
#include<sys/wait.h>
#include<unistd.h>

int main(int ac, char**av)
{
int pipez[2];
int piped[2];
int status;
pid_t pid;

if (pipe (pipez) == -1){
perror("could not make pipe");
return 1;
}

if ((pid = fork()) == -1){
perror("fork");
return 1;
}

if(pid == 0){
close(pipez[1]);
dup2(pipez[0],0);
close(pipez[0]);
execvp("./c1",av);
perror("demo);
_exit(1);
}
else{
close(pipez[0]);
dup2(pipez[1],1);
close(pipez[1]);
execvp("./c2",av);
perror("demo");
exit(1);
}

waitpid(pid,&status,0);
if(WIFEXITED(status)){
printf("[%d] TERMINATED (Status: %d)\n", pid, WEXITSTATUS(status));
}

if(pipe (piped) == -1){
perror("could not make pipe");
return 1;
}

if((pid = fork()) == -1){
perror("fork");
return 1;
}

if (pid == 0){
close(piped[1]);
dup2(piped[0],0);
close(piped[0]);
execvp("./c2", av);
perror("demo");
_exit(1);
}
else{
close(piped[0]);
dup2(piped(piped[1],1);
close(piped[1]);
execvp("./c3",av);
perror("demo");
exit(1);
}

waitpid(pid, &status, 0);
if (WIFEXITED(status)){
printf("[%d] TERMINATED (Status: %d)\n", pid, WEXITSTATUS(status));
}

return 0;
}

c1.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFSIZE 256

//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe

int main(int a, char**av){
char store[BUFSIZE];
memset(store, '\0', sizeof(store));
while(strcmp(store, "exit!") != 0){

perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
fgets(store, BUFSIZE, stdin);
printf("%s",store); // This also repeats itself dependant on where i put 
//fflush or another printf. Repeated outputs occur in blocks
}

return 0;
}

c2.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFSIZE 256

char *strlwr(char *str);

int main(int ac, char**av){
char store[BUFSIZE];

while(strcmp(store, "exit!") != 0){
scanf("%s", store);
printf("%s", strlwr(store));
}

return 0;
}

char *strlwr(char *str){
unsigned char *p = (unsigned char *)str;

while(*p){
*p = tolower((unsigned char)*p);
p++;
}

return str;

}

c3.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFSIZE 256

int main(int ac, char**av){
char store[BUFSIZE];
int n = 0;

while(strcmp(store, "exit!") != 0{
scanf("%s",store);
printf("Line %d: %s\n",n,store);
n++;

}

return 0;
}

Answer 1

fgets 将'\n' 字符留在store 缓冲区中，因此，最简单的解决方法是将'\n' 添加到比较字符串中：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFSIZE 256

//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe

int main(int a, char**av)
{
    char store[BUFSIZE];
    memset(store, '\0', sizeof(store));
    while (strcmp(store, "exit!\n") != 0)
    {

        perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
        fgets(store, BUFSIZE, stdin);
        printf("%s", store); // This also repeats itself dependant on where i put
                             // fflush or another printf. Repeated outputs occur in blocks
    }

    return 0;
}

此外，如您所见，init value '/0' 是不可接受的；它必须是“\0”。您的编译器可能会告诉您类似

test.c: In function ‘main’:
test.c:242:16: warning: multi-character character constant [-Wmultichar]
  memset(store, '/0', sizeof(store));
                ^

顺便说一句，memset，在这种情况下是无用的，因为fgets 完成了所有工作，但无论如何，如果您希望将缓冲区设置为 nul，只需声明：

char store[BUFSIZE] = {0};

使用fgets 的最佳解决方案是（显然）始终管理左边的换行符：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFSIZE 256

//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe

int main(void)
{
    char store[BUFSIZE] = {0};
    char *pos;
    while (strcmp(store, "exit!") != 0)
    {

        perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
        fgets(store, BUFSIZE, stdin);

        if ((pos = strchr(store, '\n')) != NULL)
            *pos = '\0';
        else
            fprintf(stderr, "String too long!\n");

        printf("%s", store); // This also repeats itself dependant on where i put
                             // fflush or another printf. Repeated outputs occur in blocks
    }

    return 0;
}