我正在尝试为操作系统类创建命令行shell。我们的任务之一是创建一个内置的＆＃34;历史＆＃34;命令打印出shell中执行的最后10个命令。这是我为＆＃34;历史＆＃34;编写的代码。命令：
char* cmd_hsitory[10]; // This is a global variable
int add_history(char **args) {
cmd_history[9] = NULL;
for(int i = 8; i >= 0; i--) {
cmd_history[i+1] = cmd_history[i];
}
cmd_history[0] = *args;
return 1;
}
其中char ** args参数是最后一个命令。这是打印历史记录的函数：
int lsh_history(char **args) {
printf("Last 10 commands: \n");
for(int i = 0; i < 10; i++) {
printf("%s\n", cmd_history[i]);
}
return 1;
}
此代码发生了一些奇怪的行为。例如，当我连续运行命令[cd，cd，ls，history]时，这是打印输出：
Last 10 commands:
ls
ls
cd
(null)
(null)
(null)
(null)
(null)
(null)
(null)
这里的第一个问题是我运行cd命令两次，而ls命令只执行一次。如果我运行＆＃34;历史＆＃34;命令我得到：
Last 10 commands:
history
ls
ls
cd
(null)
(null)
(null)
(null)
(null)
(null)
除了2 ls命令与1 cd命令之外，这似乎是正确的。
但是，这并不是非常一致，因为有时我会得到混合命令和历史记录＆＃34;命令会出现几次。如果有人告诉我代码的明显错误，那将会有很大帮助。谢谢！
编辑：这是完整的源代码： 附：大部分代码都来自互联网（斯蒂芬布伦南），我在此基础上进行学习。我不会将此代码作为我的作业提交。
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
/*
Function Declaration for history queue
*/
int add_history(char **args);
/*
Function Declarations for builtin shell commands:
*/
int lsh_cd(char **args);
int lsh_help(char **args);
int lsh_exit(char **args);
int lsh_history(char **args);
/*
List of builtin commands, followed by their corresponding functions.
*/
char *builtin_str[] = {
"cd",
"help",
"exit",
"history"
};
int (*builtin_func[]) (char **) = {
&lsh_cd,
&lsh_help,
&lsh_exit,
&lsh_history
};
char *cmd_history[10];
int lsh_num_builtins() {
return sizeof(builtin_str) / sizeof(char *);
}
int add_history(char **args) {
cmd_history[9] = NULL;
for(int i = 8; i >= 0; --i) {
cmd_history[i+1] = cmd_history[i];
}
cmd_history[0] = NULL;
cmd_history[0] = *args;
return 1;
}
/*
Builtin function implementations.
*/
/**
@brief Builtin command: command history.
@param args List of args. args[0] is "history".
@return Always returns 1 to continue executing.
*/
int lsh_history(char **args) {
printf("Last 10 commands: \n");
for(int i = 0; i < 10; i++) {
printf("%s\n", cmd_history[i]);
}
return 1;
}
/**
@brief Bultin command: change directory.
@param args List of args. args[0] is "cd". args[1] is the directory.
@return Always returns 1, to continue executing.
*/
int lsh_cd(char **args)
{
if (args[1] == NULL) {
chdir("/Users/Landon/");
} else {
if (chdir(args[1]) != 0) {
perror("lsh");
}
}
return 1;
}
/**
@brief Builtin command: print help.
@param args List of args. Not examined.
@return Always returns 1, to continue executing.
*/
int lsh_help(char **args)
{
int i;
printf("Stephen Brennan's LSH\n");
printf("Type program names and arguments, and hit enter.\n");
printf("The following are built in:\n");
for (i = 0; i < lsh_num_builtins(); i++) {
printf(" %s\n", builtin_str[i]);
}
printf("Use the man command for information on other programs.\n");
return 1;
}
/**
@brief Builtin command: exit.
@param args List of args. Not examined.
@return Always returns 0, to terminate execution.
*/
int lsh_exit(char **args)
{
return 0;
}
/**
@brief Launch a program and wait for it to terminate.
@param args Null terminated list of arguments (including program).
@return Always returns 1, to continue execution.
*/
int lsh_launch(char **args)
{
pid_t pid;
int status;
pid = fork();
if (pid == 0) {
// Child process
if (execvp(args[0], args) == -1) {
perror("lsh");
}
exit(EXIT_FAILURE);
} else if (pid < 0) {
// Error forking
perror("lsh");
} else {
// Parent process
do {
waitpid(pid, &status, WUNTRACED);
} while (!WIFEXITED(status) && !WIFSIGNALED(status));
}
return 1;
}
/**
@brief Execute shell built-in or launch program.
@param args Null terminated list of arguments.
@return 1 if the shell should continue running, 0 if it should terminate
*/
int lsh_execute(char **args)
{
int i;
if (args[0] == NULL) {
// An empty command was entered.
return 1;
}
for (i = 0; i < lsh_num_builtins(); i++) {
if (strcmp(args[0], builtin_str[i]) == 0) {
return (*builtin_func[i])(args);
}
}
return lsh_launch(args);
}
#define LSH_RL_BUFSIZE 1024
/**
@brief Read a line of input from stdin.
@return The line from stdin.
*/
char *lsh_read_line(void)
{
int bufsize = LSH_RL_BUFSIZE;
int position = 0;
char *buffer = malloc(sizeof(char) * bufsize);
int c;
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
while (1) {
// Read a character
c = getchar();
if (c == EOF) {
exit(EXIT_SUCCESS);
} else if (c == '\n') {
buffer[position] = '\0';
return buffer;
} else {
buffer[position] = c;
}
position++;
// If we have exceeded the buffer, reallocate.
if (position >= bufsize) {
bufsize += LSH_RL_BUFSIZE;
buffer = realloc(buffer, bufsize);
if (!buffer) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
}
}
#define LSH_TOK_BUFSIZE 64
#define LSH_TOK_DELIM " \t\r\n\a"
/**
@brief Split a line into tokens (very naively).
@param line The line.
@return Null-terminated array of tokens.
*/
char **lsh_split_line(char *line)
{
int bufsize = LSH_TOK_BUFSIZE, position = 0;
char **tokens = malloc(bufsize * sizeof(char*));
char *token, **tokens_backup;
if (!tokens) {
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
token = strtok(line, LSH_TOK_DELIM);
while (token != NULL) {
tokens[position] = token;
position++;
if (position >= bufsize) {
bufsize += LSH_TOK_BUFSIZE;
tokens_backup = tokens;
tokens = realloc(tokens, bufsize * sizeof(char*));
if (!tokens) {
free(tokens_backup);
fprintf(stderr, "lsh: allocation error\n");
exit(EXIT_FAILURE);
}
}
token = strtok(NULL, LSH_TOK_DELIM);
}
tokens[position] = NULL;
return tokens;
}
/**
@brief Loop getting input and executing it.
*/
void lsh_loop(void)
{
char *line;
char **args;
int status;
do {
printf("> ");
line = lsh_read_line();
args = lsh_split_line(line);
status = lsh_execute(args);
add_history(args);
free(line);
free(args);
} while (status);
}
/**
@brief Main entry point.
@param argc Argument count.
@param argv Argument vector.
@return status code
*/
int main(int argc, char **argv)
{
// Load config files, if any.
// Run command loop.
lsh_loop();
// Perform any shutdown/cleanup.
return EXIT_SUCCESS;
}
答案 0 :(得分：0)
您需要将arg字符串strdup（）到cmd_history指针上，而不仅仅是让cmd_history指针引用原始命令字符串。当你释放（行）时，你将cmd_history指针引用的内存放回到空闲池中。在循环的下一次迭代中，您可能会也可能不会覆盖该数据。
答案 1 :(得分：0)
你写的（（评论已添加））：
void lsh_loop(void)
{
...
line = lsh_read_line(); //mallocs line
args = lsh_split_line(line); //mallocs args array; consisting of references into line[]
status = lsh_execute(args); //no promblem here
add_history(args); // ***PERMANETNLY*** stores args[0] into cmd_history[]!!!
free(line); //frees line[]
free(args); //frees the pointer array into freed line[], which was copied to cmd_history[] --> cmd_history[] points to unallocated memory!!!
}
主要问题与strtok（）有关：它在原始字符串中返回指向spitted字符串的指针：如果稍后释放令牌指针数组或原始字符串，稍后使用 - ＆GT; UB。 另外lsh_history（）也会杀死最后一个会留下内存泄漏的条目，如果你会坚定地分配该条目。
<强>解决方案：
建立cmd_history[]您自己的字符串存储，复制并在lsh_history()中正确释放它们。