linux fcntl 非阻塞

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int set_nonblocking(int fd) {
    int flags = fcntl(fd, F_GETFL, 0);
    if (flags == -1) return -1;
    flags |= O_NONBLOCK;
    return fcntl(fd, F_SETFL, flags);
}

int main() {
    int fd = open("test.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    if (set_nonblocking(fd) == -1) {
        perror("fcntl");
        close(fd);
        return 1;
    }

    char buffer[100];
    ssize_t n = read(fd, buffer, sizeof(buffer));
    if (n == -1) {
        if (errno == EAGAIN || errno == EWOULDBLOCK) {
            printf("Read would block, doing other things...\n");
        } else {
            perror("read");
        }
    } else {
        buffer[n] = '\0';
        printf("Read %ld bytes: %s\n", n, buffer);
    }

    close(fd);
    return 0;
}

fd_set readfds;
struct timeval timeout;

FD_ZERO(&readfds);
FD_SET(fd, &readfds);

timeout.tv_sec = 5; // 等待5秒
timeout.tv_usec = 0;

int ret = select(fd + 1, &readfds, NULL, NULL, &timeout);
if (ret == -1) {
    perror("select");
} else if (ret) {
    if (FD_ISSET(fd, &readfds)) {
        // 文件描述符可读，执行 read 操作
    }
} else {
    // 超时
}