如何等待 sysfs 的输出？

Question

我尝试了明显的（见下文），但它没有捕捉到 /var/log/syslog 的新输出。我确定某个地方已经有答案了，但我一直没能找到它。

这是我的代码（我猜怎么做）：

#include <stdio.h>
#include <stdlib.h>  // provides fopen()
#include <unistd.h>  // provides sleep()


int main() {

    // *** This is failing to pick up new output on /var/log/syslog. I'm not sure
    // how to do this properly.

    // Open a read file handle on /sys/kernel/tracing/trace and wait for data to
    // appear there. When it does, echo it to the screen. This is essentially an
    // implementation of "tail -f /sys/kernel/tracing/trace".

    //FILE *fp = fopen("/sys/kernel/tracing/trace", "r");
    FILE *fp = fopen("/var/log/syslog", "r");
    char c;

    if (fp != NULL) {
        printf("Opened the file successfully. Waiting...\n");
    } else {
        printf("Failed to open the file.\n");
      exit(1);
    }

    // Check every second and output whatever is in the buffer.
    while(1) {
        c = fgetc(fp);

        // We get back -1 when there is nothing to read.
        if (c != -1) {
            printf("%c", c);
        } else {
            printf(".");  fflush(stdout);
            sleep(1);
        }
    }

    fclose(fp);
    return 0;
}

Answer 1

需要调用 clearerr(fp) 来清除 feof 指标：

// Check every second and output whatever is in the buffer.
while(1) {
    c = fgetc(fp);

    // We get back -1 when there is nothing to read.
    if (c != -1) {
        printf("%c", c);
    } else {
        printf(".");  fflush(stdout); 
        if(feof(fp)) clearerr(fp);  // <-- clear the feof indicator
        sleep(1);
    }
}