[英]Why does the following prints 'Resource temporarily unavailable'?
為什么以下代碼打印'read():資源暫時不可用'80%的時間? 這是EAGAIN代碼,它與WOULD BLOCK相同,這意味着沒有數據等待讀取,但是select返回1表示有數據(在Linux中測試):
#include <time.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/errno.h>
int main(int argc, char** argv)
{
int fd = open("/dev/lp0", O_RDWR | O_NONBLOCK);
int ret = 0;
int status = 0;
char buffer[1024];
char teststr[] = "This is a test\n";
char XMIT_STATUS_OFFLINE[] = {0x10,0x04,0x02};
char XMIT_STATUS_ERROR[] = {0x10,0x04,0x03};
char XMIT_STATUS_ROLL[] = {0x10,0x04,0x04};
char XMIT_STATUS_SLIP[] = {0x10,0x04,0x05};
fd_set rfds;
FD_ZERO( &rfds );
FD_SET( fd, &rfds );
struct timeval sleep;
sleep.tv_sec = 5;
sleep.tv_usec = 0;
/* Offline status */
ret = write(fd, XMIT_STATUS_OFFLINE, sizeof(XMIT_STATUS_OFFLINE));
//printf("write() returned %d\n", ret);
do {
ret = select( fd + 1, &rfds, NULL, NULL, &sleep );
} while (ret < 0 && (errno == EINTR));
ret = read(fd, buffer, 1024);
if(ret == -1) {
perror("read(): ");
} else {
status = buffer[0];
if((status & 0x04) != 0)
{
printf("The cover is open.\n");
} else {
printf("OFFLINE is good.\n");
}
}
close(fd);
return 0;
}
如果沒有可用的數據,在經過5秒超時后,您的選擇調用將返回0。 您的代碼將忽略此操作並嘗試從設備讀取。 檢查ret == 0,這將解決您的問題。
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