Question

I compiled the code below using Ideone.com and the following warning popped up.

rtctimer.c: In function 'rtctimer_next_tick':
rtctimer.c:87.7: warning: ignoring return value of 'read', 
                 declared with attribute warn_unused_result [-Wunused-result]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <poll.h>
#include <sched.h>
#include <fcntl.h>
#include <math.h>
#include <sys/ioctl.h>
#include <linux/rtc.h>
#include "rtctimer.h"

struct rtctimer_s
{
    int rtc_fd;
    int current_hz;
    int rtc_running;
    int verbose;
    int usecs;
};

rtctimer_t *rtctimer_new( int verbose )
{
    rtctimer_t *rtctimer = malloc( sizeof( rtctimer_t ) );
    if( !rtctimer ) return 0;

    rtctimer->verbose = verbose;
    if( (( rtctimer->rtc_fd = open( "/dev/rtc", O_RDONLY ) ) < 0) ) {
        if( rtctimer->verbose ) {
            fprintf( stderr, "rtctimer: Cannot open /dev/rtc: %s\n",
                     strerror( errno ) );
        }
        if( ( rtctimer->rtc_fd = open( "/dev/misc/rtc", O_RDONLY ) ) < 0 ) {
            if( rtctimer->verbose ) {
                fprintf( stderr, "rtctimer: Cannot open /dev/misc/rtc: %s\n",
                         strerror( errno ) );
            }
            free( rtctimer );
            return 0;
        }
    }

    if( fcntl( rtctimer->rtc_fd, F_SETOWN, getpid() ) < 0 ) {
        if( rtctimer->verbose ) {
            fprintf( stderr, "rtctimer: Cannot set ownership of "
                             "/dev/rtc: %s\n", strerror( errno ) );
        }
        close( rtctimer->rtc_fd );
        free( rtctimer );
        return 0;
    }

    rtctimer->rtc_running = 0;
    rtctimer->current_hz = 0;
    rtctimer->usecs = 0;
    return rtctimer;
}

void rtctimer_delete( rtctimer_t *rtctimer )
{
    rtctimer_stop_clock( rtctimer );
    close( rtctimer->rtc_fd );
    free( rtctimer );
}

int rtctimer_next_tick( rtctimer_t *rtctimer )
{
    unsigned long rtc_data;
    struct pollfd pfd;
    pfd.fd = rtctimer->rtc_fd;
    pfd.events = POLLIN | POLLERR;

again:
    if( poll( &pfd, 1, 100000 ) < 0 ) {
        if( errno == EINTR ) {
            goto again;
        }

        if( rtctimer->verbose ) {
            fprintf( stderr, "rtctimer: poll call failed: %s\n",
                     strerror( errno ) );
        }
        return 0;
    }

    read( rtctimer->rtc_fd, &rtc_data, sizeof( rtc_data ) );
    return 1;
}

int rtctimer_set_interval( rtctimer_t *rtctimer, int hz )
{
    int restart;

    if( hz == rtctimer->current_hz ) {
        return 1;
    }

    restart = rtctimer_stop_clock( rtctimer );

    if( ioctl( rtctimer->rtc_fd, RTC_IRQP_SET, hz ) < 0 ) {
        if( rtctimer->verbose ) {
            fprintf( stderr, "rtctimer: Cannot set periodic interval: %s\n",
                     strerror( errno ) );
        }
        return 0;
    }

    rtctimer->current_hz = hz;
    rtctimer->usecs = (int) ( ( ( 1000.0 * 1000.0 ) / hz ) + 0.5 );

    if( restart ) {
        rtctimer_start_clock( rtctimer );
    }

    return 1;
}

int rtctimer_start_clock( rtctimer_t *rtctimer )
{
    if( !rtctimer->rtc_running ) {
        if( ioctl( rtctimer->rtc_fd, RTC_PIE_ON, 0 ) < 0 ) {
            if( rtctimer->verbose ) {
                fprintf( stderr, "rtctimer: Cannot start periodic "
                         "interrupts: %s\n", strerror( errno ) );
            }
            return 0;
        }
        rtctimer->rtc_running = 1;
    }
    return rtctimer->rtc_running;
}

int rtctimer_stop_clock( rtctimer_t *rtctimer )
{
    int was_running = rtctimer->rtc_running;

    if( rtctimer->rtc_running ) {
        if( ioctl( rtctimer->rtc_fd, RTC_PIE_OFF, 0 ) < 0 ) {
            if( rtctimer->verbose ) {
                fprintf( stderr, "rtctimer: Cannot stop periodic "
                         "interrupts: %s\n", strerror( errno ) );
            }
            return rtctimer->rtc_running;
        }
        rtctimer->rtc_running = 0;
    }

    return was_running;
}

int rtctimer_get_resolution( rtctimer_t *rtctimer )
{
    return rtctimer->current_hz;
}

int rtctimer_get_usecs( rtctimer_t *rtctimer )
{
    return rtctimer->usecs;
}

int set_realtime_priority( int max )
{
    struct sched_param schp;

    memset( &schp, 0, sizeof( schp ) );
    if( max ) schp.sched_priority = sched_get_priority_max( SCHED_FIFO );
    else schp.sched_priority = sched_get_priority_max( SCHED_FIFO ) - 1;

    if( sched_setscheduler( 0, SCHED_FIFO, &schp ) != 0 ) {
        return 0;
    }

    return 1;
}

int set_default_priority( void )
{
    struct sched_param schp;

    memset( &schp, 0, sizeof( schp ) );
    schp.sched_priority = 0;
    if( sched_setscheduler( 0, SCHED_OTHER, &schp ) != 0 ) {
        return 0;
    }

    return 1;
}

What did I do wrong?

Answer 1

read() returns a value indicating the number of bytes that it read. The warning is indicating that the code is ignoring that. You should check the return value of read() to ensure it was successful otherwise you are assuming that rtc_data has been correctly populated and it may not have been.