lkmpg/examples/other/cat_nonblock.c

/*
 *  cat_nonblock.c - open a file and display its contents, but exit rather than
 *  wait for input.
 */
#include <errno.h> /* for errno */
#include <fcntl.h> /* for open */
#include <stdio.h> /* standard I/O */
#include <stdlib.h> /* for exit */
#include <unistd.h> /* for read */

#define MAX_BYTES 1024 * 4

int main(int argc, char *argv[])
{
    int fd; /* The file descriptor for the file to read */
    size_t bytes; /* The number of bytes read */
    char buffer[MAX_BYTES]; /* The buffer for the bytes */

    /* Usage */
    if (argc != 2) {
        printf("Usage: %s <filename>\n", argv[0]);
        puts("Reads the content of a file, but doesn't wait for input");
        exit(-1);
    }

    /* Open the file for reading in non blocking mode */
    fd = open(argv[1], O_RDONLY | O_NONBLOCK);

    /* If open failed */
    if (fd == -1) {
        puts(errno == EAGAIN ? "Open would block" : "Open failed");
        exit(-1);
    }

    /* Read the file and output its contents */
    do {
        /* Read characters from the file */
        bytes = read(fd, buffer, MAX_BYTES);

        /* If there's an error, report it and die */
        if (bytes == -1) {
            if (errno == EAGAIN)
                puts("Normally I'd block, but you told me not to");
            else
                puts("Another read error");
            exit(-1);
        }

        /* Print the characters */
        if (bytes > 0) {
            for (int i = 0; i < bytes; i++)
                putchar(buffer[i]);
        }

        /* While there are no errors and the file isn't over */
    } while (bytes > 0);

    return 0;
}