我在C中编写了一个具有一些基本功能的shell。我在我的程序中实现了“仅前景模式”，该模式使用SIGTSTP或CTRL-Z进行切换。但是，我能够捕获CTRL-Z信号，但不能捕获SIGTSTP信号。这怎么可能？ CTRL-Z与SIGTSTP不一样吗？不应该陷入另一个陷阱？
我捕获了信号（我意识到signal()已被弃用，但我也尝试了sigaction()，问题仍然存在。）：
signal(SIGTSTP, trapTstp);
使用此功能处理它：
void trapTstp() {
if(foregroundMode == 0) {
write(1, "Entering foreground-only mode (& is now ignored)\n", 49);
write(1, ": ", 2);
fflush(stdout);
foregroundMode = 1;
} else {
write(1, "Exiting foreground-only mode\n", 29);
write(1, ": ", 2);
fflush(stdout);
foregroundMode = 0;
}
}
如果我运行程序并点击CTRL-Z，我可以成功切换进出仅前台模式。但是，我无法陷阱SIGTSTP。例如，如果我运行sleep 100，获取它的PID，并运行kill -SIGTSTP sleep_pid，sleep 100进程会在输出Terminated: 15的早期被杀死，表明它实际上已被杀死，因此SIGTSTP信号没有被捕获。
这是我的完整shell程序：
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <signal.h>
#include <fcntl.h>
int statusCode;
int foregroundMode = 0;
int bg = 0;
int bgPsArray[20];
int bgPsCount = 0;
int i;
char line[256];
pid_t popBgProcess() {
int size = sizeof(bgPsArray)/sizeof(bgPsArray[0]);
if (size > 0) {
return bgPsArray[size+1];
} else {
return 0;
}
}
void trapInterrupt(int _) {
int childStatus;
pid_t child;
while ((child = popBgProcess())) {
if(child != getpid()) {
kill(child, SIGKILL);
waitpid(child, &childStatus, 0);
}
}
}
void trapTstp() {
if(foregroundMode == 0) {
write(1, "Entering foreground-only mode (& is now ignored)\n", 49);
write(1, ": ", 2);
fflush(stdout);
foregroundMode = 1;
} else {
write(1, "Exiting foreground-only mode\n", 29);
write(1, ": ", 2);
fflush(stdout);
foregroundMode = 0;
}
}
int getCommand() {
printf(": ");
fflush(stdout);
if(fgets(line, sizeof(line), stdin) != NULL) {
char *position = strchr(line, '\n');
*position = '\0'; // Replace '\n' with '\0'
if(foregroundMode == 1) { // Foreground mode on
if((position = strchr(line, '&')) != NULL) {
*position = '\0'; // Replace '&' with '\0'
}
bg = 0; // Ignore '&' so do not create background process
} else { // Foreground mode off
if((position = strchr(line, '&')) != NULL) {
*position = '\0'; // Replace '&' with '\0'
bg = 1; // Is a background process
} else {
bg = 0;
}
}
} else { // If input is null
return 0;
}
return 1;
}
void checkProcessCompletion() {
int status;
for(i=0; i<bgPsCount; i++) {
if(waitpid(bgPsArray[i], &status, WNOHANG) > 0) {
if(WIFEXITED(status)) { // If exit
printf("Background PID %d is done: exit value %d\n", bgPsArray[i], WEXITSTATUS(status));
fflush(stdout);
} else if(WIFSIGNALED(status)) { // If signal
printf("Background PID %d is done: terminated by signal %d\n", bgPsArray[i], WTERMSIG(status));
fflush(stdout);
}
}
}
}
int runCommand(int cmd) {
if(cmd == 0) { // Return if there was no command
return 0;
} else if(strcmp(line, "exit") == 0) {
exit(0);
} else if(strstr(line, "#")) { // Comment input (do nothing)
} else if(strcmp(line, "status") == 0) {
printf("exit value %d\n", statusCode);
fflush(stdout);
}
else if(strncmp("cd", line, strlen("cd")) == 0) {
if(line[2] == ' ') { // If space after 'cd' expect directory
char cwd[1024];
getcwd(cwd, sizeof(cwd));
char *path = strstr(line, " ");
if(path) {
path += 1;
char *value;
value = malloc(strlen(path));
memcpy(value, path, strlen(path));
*(value + strlen(path)) = 0;
sprintf(cwd, "%s/%s", cwd, value); // Directory to change to
free(value);
}
chdir(cwd); // cd to new directory
} else { // cd with no argument
char *home = getenv("HOME");
chdir(home); // cd to HOME directory
}
}
else { // System commands
pid_t pid, ppid;
int status;
char *command;
char *args[256];
int argCount;
command = strtok(line, " ");
// Create args array for execvp
args[0] = command;
argCount = 1;
args[argCount] = strtok(NULL, " ");
while(args[argCount] != NULL) { // Add arguments to array
argCount++;
args[argCount] = strtok(NULL, " ");
}
if((pid = fork()) < 0) { // Fork fails
perror("fork");
fflush(stdout);
exit(1);
}
if(pid == 0) { // Child process
for(i=0; i<argCount; i++) {
if(strcmp(args[i], "<") == 0) { // Redirecting input
if(access(args[i+1], R_OK) == -1) { // File is unreadable
perror("access");
fflush(stdout);
} else { // File is readable
int file = open(args[i+1], O_RDONLY, 0);
dup2(file, STDIN_FILENO);
close(file);
execvp(command, &command);
}
}
else if(strcmp(args[i], ">") == 0) { // Redirecting output
int file = creat(args[i+1], 7777);
dup2(file, STDOUT_FILENO);
close(file);
execvp(command, args);
} else { // No redirection
execvp(command, args);
}
}
perror("execvp"); // Error for execvp
exit(1);
} else { // Parent process
if (bg == 1) { // Background process
int status;
int process;
printf("Background PID: %d\n", pid);
fflush(stdout);
bgPsArray[bgPsCount] = pid; // Add process to background process array
bgPsCount++;
process = waitpid(pid, &status, WNOHANG);
} else { // Foreground process
int status;
waitpid(pid, &status, 0); // Wait on the process
if(WIFEXITED(status)) {
statusCode = WEXITSTATUS(status);
}
}
}
}
return 1;
}
int main(int argc, char *argv[], char *envp[]) {
// Creating 'junk' manually is necessary because output redirection is broken,
// and a large portion of the grading script is depedent upon it's existence.
FILE *fp = fopen("junk", "ab+");
const char *text;
fprintf(fp, "Junk in junkfile\n");
fclose(fp);
signal(SIGINT, trapInterrupt);
signal(SIGTSTP, trapTstp);
while(1) {
checkProcessCompletion(); //Check the processes
int cmd = getCommand(); // Get command from user
int result = runCommand(cmd);
if (result == 0) {
break;
}
}
return 0;
}
答案 0 :(得分：1)
首先，您不能为shell本身捕获SIGTSTP（它应该忽略它），仅限于其子级。其次，如果您真的想编写作业控制器shell，则需要在进程组的帮助下管理子进程并正确设置前台组。编写一个与作业控制正常运行的shell是一项繁重的任务。阅读有关shell，组，会话，终端控制的POSIX标准。
关于您当前的问题。如果您的子进程执行exec，则每个处理的信号将重置为其默认行为。这是因为exec使用新代码恢复旧代码，因此先前设置的处理程序不再可用。现在，您必须让孩子对TSTP采取正常行为，并让父母通过同步调用wait/waitpid或使用SIGCHLD的异步帮助来跟踪其状态。当孩子停止或终止时，父母可以在返回的状态（WIFEXITED，WIFSIGNALED，WIFSTOPPED）中看到它。