为什么下面打印“资源暂时不可用”?
为什么以下代码 80% 的时间会打印“read(): 资源暂时不可用”? 这是 EAGAIN 代码,与 WOULD BLOCK 相同,表示没有数据等待读取,但 select 返回 1 表示有数据(在 Linux 中测试):
#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;
}
Why does the following code print ‘read(): Resource temporarily unavailable’ 80% of the time? That is the EAGAIN code, which is the same as WOULD BLOCK which means there is no data waiting to be read, but select is returning 1 saying there is data (tested in Linux):
#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;
}
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如果没有可用数据,您的 select 调用将在 5 秒超时后返回 0。 您的代码将忽略这一点并尝试从设备读取数据。 检查 ret == 0 这将解决你的问题。
Your select call will return 0 after the 5 second timeout elapses if no data is available. Your code will ignore this and try to read from the device anyways. Check for ret == 0 and that will fix your problem.