[英]How am I supposed to use *status in waitpid(pid_t pid, int *status, int options)?
[英]waitpid(pid, status, options) not always setting status
我正在為任務復制外殼管道。 我使管道全部正常工作(此后沒有更改管道代碼,因此可以正常工作),但是在管道的某些步驟中執行失敗時,仍然需要終止管道。 在實現該代碼的某個時刻,管道開始表現出意外的行為。
長度為2的管道正常工作。 長度大於2的管道以下列方式失敗:
這是使用waitpid(無調試代碼)的部分代碼:
for (int i = 0 ; i < num_cmds ; i++) {
if (waitpid(cmd_pids[i], &cmd_status[i], NULL) != cmd_pids[i]) {
printf("Error waiting for command %s\n", cmds[i]);
perror("Error");
}
if ((cmd_status[i] > 0) && (cmd_status[i-1] == 0)) {
killPipeline(SIGINT); //Kill pipeline; something failed
}
fflush(logfp);
}
現在,這就是為什么我認為waitpid部分要怪的症結所在:由於賦值需要一些奇怪的控制流,我發現將cmd_status的值初始化為-2很方便。 日志記錄功能將子進程的返回狀態打印到日志中,文件顯示第二個函數的退出狀態為-2,這當然意味着它沒有在程序中設置存在狀態。 似乎管道根本沒有等待這些程序執行。
即
輸入“ ls | grep管道| wc”
輸出:
You entered : list of pipe commands ls | grep pipe | wc
Creating process ls
Creating process grep pipe
Creating process wc
Waiting for command ls
0 0 0 //wc received no input, but somehow an EOF
Command ls finished
Waiting for command grep
^C //Terminate grep because it's waiting for input from a terminated process
Error waiting for command grep
Error: Interrupted system call
Command grep finished
Waiting for command wc
Command wc finished
要提取通過進程通過waitpid()
返回的值傳遞給exit()
的狀態,請使用WEXITSTATUS()
宏。
同樣,對於waitpid()
失敗的測試是錯誤的。 waitpid() return -1 on failure and in this case set
errno , which then would make sense to be interpreted by a call to
perror() , which then would make sense to be interpreted by a call to
。
perror()
讀取errno
的值。 因此,僅在已知已設置errno
情況下,調用perror()
才有意義。 如下所示,在我對代碼的修改中刪除的perror("Error")
並非如此。
for (int i = 0 ; i < num_cmds ; i++)
{
int status = -1;
int result = waitpid(cmd_pids[i], &status, NULL);
if (-1 == result)
{
perror("waitpid()"); /* added call to perror() here as errno is know to have been set */
}
else
{
if (result != cmd_pids[i])
{
fprintf(stderr, "Error waiting for command '%s'\n", cmds[i]);
/* removed call to perror() here as errno had not been set here. As this however should
log an error, I modded the code to printf to stderr, but stdout. */
}
else
{
cmd_status[i] = WEXITSTATUS(status);
if ((cmd_status[i] > 0) && ((0 == i) || (cmd_status[i-1] == 0))) /* This mod keeps
the app from dereferencing cmd_status in case i == 0, as this would lead to
cmd_status[-1], which is undefined beaviour. */
{
killPipeline(SIGINT); //Kill pipeline; something failed
}
}
}
...
向所有試圖通過waitpid查找任何問題的人致歉,因為它們只是表面上的錯誤。 實際的錯誤確實在我的管道代碼中,這顯然使我一路崩潰。
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