c - linux管道和缓冲区,不知道如何让循环工作
问题描述
我正在为我正在学习的 Unix 课程用 C 语言编写一个 DB 程序。虽然我已经设法让所有管道、缓冲区和命令正常工作,但 while 循环却让我陷入了循环。该程序将完全执行第一个命令,接受第二个命令,但随后立即退出。我已经用错误检查加载了它,没有抛出任何东西,所以机制很好,但循环中的某个地方存在我无法识别的逻辑问题。
这是存储库:https ://gitlab.com/IrateSwami/c_pipes/tree/dev
示例运行:
1234567 102 08/11/18 4.00
1234567 101 08/14/18 14.00
3456787 9873 08/30/18 100.00
1234567 100 08/16/18 35.00
3456787 9874 09/30/18 4.00
12345 1010 09/01/18 34.00
1001001 905 08/14/18 9.00
1001001 903 08/30/18 11.00
12345 1001 09/14/18 16.00
12345 1111 08/24/18 2.00
12345 1112 08/31/18 44.00
1001001 902 09/25/18 19.00
Enter a command: add,1234567,999,01/01/01,99.99
Enter a command: list
从slave.c文件给我带来麻烦的do while:
// start the do-while loop for command stuff
// read in from the pipe
error_check = read(read_pipe, buffer, 1000);
if(error_check<0){
perror("child process error, reading in from pipe");
exit(-3);
}
// null terminate the end of buffer
buffer[error_check] = '\0';
// here's where the command stuff starts
char *first_command;
while(strcmp(first_command, "exit\n") != 0){
// grab the first thing from the buffer, it'll be the command
char *command = strtok(buffer, ",");
first_command = command;
printf("first command: %s\n", first_command);
// now for the parameters
int parameter_count = 0;
char *parameters[4];
while(command != NULL){
command = strtok(NULL, ",");
parameters[parameter_count] = command;
parameter_count++;
}
// exit appropriately
if(strcmp(first_command, "exit\n") == 0)
return 9;
// add a record
else if(strcmp(first_command, "add") == 0){
Record temp_record;
temp_record.account_number = atoi(parameters[0]);
temp_record.check_number = atoi(parameters[1]);
temp_record.amount = atof(parameters[3]);
strcmp(temp_record.transaction_date, parameters[2]);
records[record_count] = temp_record;
record_count++;
error_check = write(write_pipe, "add completed", strlen(buffer));
if(error_check<0){
perror("error writing in add function");
exit(-6);
}
}
// delete a record
else if(strcmp(first_command, "delete") == 0){
for (int i = 0; i < record_count; i++){
if(
atoi(parameters[0]) == records[i].account_number &&
atoi(parameters[1]) == records[i].check_number){
records[i].account_number = 0;
records[i].check_number = 0;
records[i].amount = 0.0;
strcpy(records[i].transaction_date, "\0");
}
}
}
// list all the records contained
else if(strcmp(first_command, "list\n") == 0){
// write all the records to the buffer
position = 0;
for(int i = 0; i < record_count; i++){
position += sprintf(buffer+position, "%d %d %s %.2f\n",
records[i].account_number,
records[i].check_number,
records[i].transaction_date,
records[i].amount);
}
printf("%s\n", buffer);
// write the buffer to the pipe
error_check = write(write_pipe, buffer, strlen(buffer));
// check for errors
if(error_check<0){
perror("child process write error");
exit(-4);
}
// make sure the length of the buffer was proper
if(error_check!=strlen(buffer)){
printf("child process error, buffer was a weird size\n");
exit(-5);
}
}
else{
printf("you didn't input a correct command\n");
}
// empty out everything for reuse
strcpy(buffer, "\0");
command = "\0";
first_command = "\0";
for(int i = 0; i < 4; i++)
parameters[i] = "\0";
// grab the new command
error_check = read(read_pipe, buffer, 1000);
if(error_check<0){
perror("child process error, reading in from pipe");
exit(-5);
}
// null terminate the end of buffer
buffer[error_check] = '\0';
printf("end of child do while buffer: %s\n", buffer);
}
解决方案
首先,我们可以注意到在第一个命令(add ...)发送给它之后,孩子(从)应该输出的内容没有出现在屏幕上。
在命令的第二个提示符下,如果您停止程序并执行ps
,您会注意到孩子已经死了:
$ ps
PID TTY TIME CMD
27069 pts/29 00:00:00 master
27070 pts/29 00:00:00 slave <defunct>
从站的一个主要问题是您正在比较first_command
未初始化的“退出”。该比较要求first_command
访问地址处的字节。如果该地址为 NULL,或者更一般地在您的程序空间之外,您会得到一个分段错误,该错误会杀死孩子。
父进程如何忽视这一点?因为当你read
从管道中时,你只检查错误,而不是 EOF (0),这意味着管道是空的并且是关闭的(IOW,孩子已经死了)。
修复:初始化你的变量
char *first_command = "";
(或者更好,使用do...while
循环)
您的代码中还有其他小问题,有些已经在评论中报告给您,有些在编译时清楚地出现-Wall
(始终使用该标志并修复警告),还有一些等待您的睿智;-)
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