可以在管道上進行非阻塞I / O嗎？ fcntl無法設置O_NONBLOCK.

Linux編程接口的頁面918包括一個表’從管道讀取n個字節(jié)或FIFO(p)’的語義.此表列出了管道和FIFO的行為,其中一列標題為O_NONBLOCK已啟用？這意味著您可以在管道上設置O_NONBLOCK標志.它是否正確？以下代碼無法設置標志,但fcntl(2)不報告錯誤.

#include

#include

#include

#include

#include

#define SLEEP 1

int

main(int argc, char *argv[]) {

pid_t childPid;

int pfd[2];

int nread, flags;

int c = 'a';

setbuf(stdout, NULL);

if (pipe(pfd) == -1) {

printf("error: pipe");

exit(EXIT_FAILURE);

}

switch (childPid = fork()) {

case -1:

printf("error: fork");

exit(EXIT_FAILURE);

case 0: /* child */

if (close(pfd[0]) == -1) {

printf("child: close pfd read");

exit(EXIT_FAILURE);

}

sleep(SLEEP);

_exit(EXIT_SUCCESS);

default:

break;

/* parent falls through */

}

if (close(pfd[1]) == -1) {

printf("parent: close pipe write");

exit(EXIT_FAILURE);

}

flags = fcntl(pfd[0], F_GETFD);

flags |= O_NONBLOCK;

if (fcntl(pfd[0], F_SETFD, flags))

perror("fcntl");

/* verify flags set correctly */

flags = fcntl(pfd[0], F_GETFD);

if (!(flags & O_NONBLOCK)) {

printf("failed to set O_NONBLOCK\n");

exit(EXIT_FAILURE);

}

wait(NULL);

exit(EXIT_SUCCESS);

}

管道和O_NONBLOCK沒有什么特別之處.以下示例按預期工作.我沒有檢查每次調用的每個retval,以使示例更具可讀性.真實世界的應用程序必須進行檢查.

#include

#include

#include

#include

int main()

{

int fds[2];

pid_t pid;

char buf[100];

pipe(fds);

pid = fork();

if ( pid )

{

while (1 )

{

memcpy( buf, "abcdefghi\0",10);

write( fds[1], buf, 10);

sleep(2);

}

}

else

{

int retval = fcntl( fds[0], F_SETFL, fcntl(fds[0], F_GETFL) | O_NONBLOCK);

printf("Ret from fcntl: %d\n", retval);

while (1)

{

ssize_t r=read( fds[0], buf, 10 );

printf("read: %d\n", r);

if ( r > 0 )

{

printf("Buffer: %s\n", buf);

}

else

{

printf("Read nothing\n");

perror("Error was");

sleep(1);

}

}

}

}

寫完我的例子后,我檢查你的代碼,發(fā)現：

flags = fcntl(pfd[0], F_GETFD);

flags |= O_NONBLOCK;

if (fcntl(pfd[0], F_SETFD, flags))

請將F_SETFD更改為F_SETFL以及get操作.您不會更改文件描述符標志,但文件描述符狀態(tài)標志:-)

來自man 3 fcntl：

File descriptor flags

The following commands manipulate the flags associated with a file

descriptor. Currently, only one such flag is defined: FD_CLOEXEC, the

close-on-exec flag. If the FD_CLOEXEC bit is 0, the file descriptor

will remain open across an execve(2), otherwise it will be closed.

File status flags

Each open file description has certain associated status flags, ini‐

tialized by open(2) and possibly modified by fcntl(). Duplicated file

descriptors (made with dup(2), fcntl(F_DUPFD), fork(2), etc.) refer to

the same open file description, and thus share the same file status

flags.

F_SETFL (int)

Set the file status flags to the value specified by arg. File

access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags

(i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored.

On Linux this command can change only the O_APPEND, O_ASYNC,

O_DIRECT, O_NOATIME, and O_NONBLOCK flags. It is not possible to change the O_DSYNC and O_SYNC flags; see BUGS, below.

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