我正在尝试用字符读取一个数字字符，但我不知道stdin缓冲区是否为空。
我的第一个解决方案是寻找＆＃39; \ n＆＃39;在stdin缓冲区中的字符，但如果我输入多个用＆＃39;分隔的数字，这是不好的。 ＆＃39;（空格）。
我怎么知道在stdin缓冲区中我是否有字符？
我需要用C语言进行操作并且可移植。
答案 0 :(得分：3)
有几个词：
超时为0的 poll或select - 这些将立即返回，如果没有可用数据或带有数据的描述符数量，则结果为-1，错误为EAGAIN（一，因为你只检查标准输入。）
ioctl是使用描述符的瑞士军刀。您需要的是I_NREAD：
if (ioctl(0, I_NREAD, &n) == 0 && n > 0)
// we have exactly n bytes to read
然而，正确的解决方案是将您获得的所有内容（使用scanf）作为一行阅读，然后处理结果 - 这对sscanf来说效果很好：
char buf[80]; // large enough
scanf("%79s", buf); // read everything we have in stdin
if (sscanf(buf, "%d", &number) == 1)
// we have a number
...只要你正确处理重读，比你的缓冲区长的字符串，以及其他现实生活中的复杂情况。
编辑：删除feof，因为它实际上用于其他事情。
答案 1 :(得分：1)
对于任何来自谷歌的人 - 简单select解决方案来检查stdin空虚：
fd_set readfds;
FD_ZERO(&readfds);
FD_SET(STDIN_FILENO, &readfds);
fd_set savefds = readfds;
struct timeval timeout;
timeout.tv_sec = 0;
timeout.tv_usec = 0;
int chr;
int sel_rv = select(1, &readfds, NULL, NULL, &timeout);
if (sel_rv > 0) {
puts("Input:");
while ((chr = getchar()) != EOF) putchar(chr);
} else if (sel_rv == -1) {
perror("select failed");
}
readfds = savefds;
需要unistd.h，stdlib.h和stdio.h。
可以找到here的解释。
UPD： 感谢DrBeco注意到select在错误时返回-1 - 添加了错误处理。
实际上，选择返回：
答案 2 :(得分：1)
我在本页的this引用了@stek29's post的灵感，并准备了一个简单的示例如下：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(void)
{
fd_set readfds;
FD_ZERO(&readfds);
struct timeval timeout;
timeout.tv_sec = 0;
timeout.tv_usec = 0;
char message[50];
while(1)
{
FD_SET(STDIN_FILENO, &readfds);
if (select(1, &readfds, NULL, NULL, &timeout))
{
scanf("%s", message);
printf("Message: %s\n", message);
}
printf("...\n");
sleep(1);
}
return(0);
}
答案 3 :(得分：0)
int number=-1; //-1 is default std for error;
int success=0; //will serve as boolean (0==FALSE;1==TRUE)
char buf[BUFFER_SIZE]; // Define this as convinient (e.g. #define BUFFER_SIZE 100)
char *p=buf; //we'll use a pointer in order to preserve input, in case you want to use it later
fgets(buf,BUFFER_SIZE,stdin); //use fgets() for security AND to grab EVERYTHING from stdin, including whitespaces
while(*p!='\0'){ //parse the buf
if(sscanf(p,"%d",&number)==1){ //at each char position try to grab a valid number format,
success=1; //if you succeed, then flag it.
break;
}
p++; //if you don't succeed, advance the pointer to the next char position
} //repeat the cycle until the end of buf (string end char =='\0')
if (success)
printf(">> Number=%d at position nº %d.",number,(int)(p-buf)); //you get the position by calculating the diff
//between the current position of the p and the
//beginning position of the buf
else {
// do whatever you want in case of failure at grabbing a number
}
答案 4 :(得分：-1)
有许多方法可以检查stdin是否有可用的输入。按照以下顺序，最方便携带的是：select，fcntl和poll。
根据具体情况，介绍了一些操作步骤。
#include <stdio.h> /* same old */
#include <stdlib.h> /* same old */
#include <time.h> /* struct timeval for select() */
#include <unistd.h> /* select() */
#include <poll.h> /* poll() */
#include <sys/ioctl.h> /* FIONREAD ioctl() */
#include <termios.h> /* tcgetattr() and tcsetattr() */
#include <fcntl.h> /* fnctl() */
#define BUFF 256
int chkin_select(void);
int chkin_poll(void);
int chkin_ioctl(void);
int chkin_fcntl(void);
int chkin_termios(void);
/*
Simple loops to test varios options of non-blocking test for stdin
*/
int main(void)
{
char sin[BUFF]="r";
printf("\nType 'q' to advance\nTesting select()\n");
while(sin[0]++ != 'q')
{
while(!chkin_select())
{
printf("nothing to read on select()\n");
sleep(2);
}
fgets(sin, BUFF, stdin);
printf("\nInput select(): %s\n", sin);
}
printf("\nType 'q' to advance\nTesting poll()\n");
while(sin[0]++ != 'q')
{
while(!chkin_poll())
{
printf("nothing to read poll()\n");
sleep(2);
}
fgets(sin, BUFF, stdin);
printf("\nInput poll(): %s\n", sin);
}
printf("\nType 'q' to advance\nTesting ioctl()\n");
while(sin[0]++ != 'q')
{
while(!chkin_ioctl())
{
printf("nothing to read ioctl()\n");
sleep(2);
}
fgets(sin, BUFF, stdin);
printf("\nInput ioctl(): %s\n", sin);
}
printf("\nType 'q' to advance\nTesting fcntl()\n");
while(sin[0]++ != 'q')
{
while(!chkin_fcntl())
{
printf("nothing to read fcntl()\n");
sleep(2);
}
fgets(sin, BUFF, stdin);
printf("\nInput fcntl: %s\n", sin);
}
printf("\nType 'q' to advance\nTesting termios()\n");
while(sin[0]++ != 'q')
{
while(!chkin_termios())
{
printf("nothing to read termios()\n");
sleep(2);
}
fgets(sin, BUFF, stdin);
printf("\nInput termios: %s\n", sin);
}
return EXIT_SUCCESS;
}
/*
select() and pselect() allow a program to monitor multiple file
descriptors, waiting until one or more of the file descriptors become
"ready" for some class of I/O operation (e.g., input possible). A
file descriptor is considered ready if it is possible to perform a
corresponding I/O operation (e.g., read(2) without blocking, or a
sufficiently small write(2)).
*/
int chkin_select(void)
{
fd_set rd;
struct timeval tv={0};
int ret;
FD_ZERO(&rd);
FD_SET(STDIN_FILENO, &rd);
ret=select(1, &rd, NULL, NULL, &tv);
return (ret>0);
}
/* poll() performs a similar task to select(2): it waits for one of a
set of file descriptors to become ready to perform I/O.
The set of file descriptors to be monitored is specified in the fds
argument, which is an array of structures of the following form:
struct pollfd {
int fd; // file descriptor //
short events; // requested events //
short revents; // returned events //
};
The caller should specify the number of items in the fds array in
nfds.
*/
int chkin_poll(void)
{
int ret;
struct pollfd pfd[1] = {0};
pfd[0].fd = STDIN_FILENO;
pfd[0].events = POLLIN;
ret = poll(pfd, 1, 0);
return (ret>0);
}
/*
The ioctl(2) call for terminals and serial ports accepts many
possible command arguments. Most require a third argument, of
varying type, here called argp or arg.
Use of ioctl makes for nonportable programs. Use the POSIX interface
described in termios(3) whenever possible.
*/
int chkin_ioctl(void)
{
int n;
ioctl(STDIN_FILENO, FIONREAD, &n);
return (n>0);
}
/*
fcntl() performs one of the operations described below on the open
file descriptor fd. The operation is determined by cmd.
fcntl() can take an optional third argument. Whether or not this
argument is required is determined by cmd. The required argument
type is indicated in parentheses after each cmd name (in most cases,
the required type is int, and we identify the argument using the name
arg), or void is specified if the argument is not required.
Certain of the operations below are supported only since a particular
Linux kernel version. The preferred method of checking whether the
host kernel supports a particular operation is to invoke fcntl() with
the desired cmd value and then test whether the call failed with
EINVAL, indicating that the kernel does not recognize this value.
*/
int chkin_fcntl(void)
{
int flag, ch;
flag = fcntl(STDIN_FILENO, F_GETFL, 0); /* save old flags */
fcntl(STDIN_FILENO, F_SETFL, flag|O_NONBLOCK); /* set non-block */
ch = ungetc(getc(stdin), stdin);
fcntl(STDIN_FILENO, F_SETFL, flag); /* return old state */
return (ch!=EOF);
}
/*
The termios functions describe a general terminal interface that is provided to control asynchronous communications ports.
This function doesn't wait for '\n' to return!
*/
int chkin_termios(void)
{
struct termios old, new;
int ch;
tcgetattr(STDIN_FILENO, &old); /* save settings */
new = old;
new.c_lflag &= ~ICANON; /* non-canonical mode: inputs by char, not lines */
new.c_cc[VMIN] = 0; /* wait for no bytes at all */
new.c_cc[VTIME] = 0; /* timeout */
tcsetattr(STDIN_FILENO, TCSANOW, &new); /* new settings */
ch = ungetc(getc(stdin), stdin); /* check by reading and puking it back */
tcsetattr(STDIN_FILENO, TCSANOW, &old); /* restore old settings */
return (ch!=EOF);
}
尝试避免使用ioctl和termios太具体或太低的水平。另外，对于这种情况，您不能真正将feof与stdin或任何FIFO一起有意义地使用。您可以保证指针的位置，如果尝试使用ftell或fseek，则会出现错误（询问perror）。
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