Using Ioprio_set To Control I/O Priority In Linux

Linux doesn’t provide a glibc wrapper for ioprio_set(), so you need to call it directly via syscall(). Here’s the basic approach:

#include <unistd.h>
#include <sys/syscall.h>

syscall(SYS_ioprio_set, IOPRIO_WHO_PROCESS, pid, ioprio_value);

Required definitions

You’ll need to define the I/O priority classes, target types, and helper macros. Add these to your source file or header:

/* I/O priority classes */
enum {
    IOPRIO_CLASS_NONE = 0,
    IOPRIO_CLASS_RT = 1,
    IOPRIO_CLASS_BE = 2,
    IOPRIO_CLASS_IDLE = 3,
};

/* Target types for ioprio_set */
enum {
    IOPRIO_WHO_PROCESS = 1,
    IOPRIO_WHO_PGRP = 2,
    IOPRIO_WHO_USER = 3,
};

/* Macros for priority encoding/decoding */
#define IOPRIO_CLASS_SHIFT 13
#define IOPRIO_PRIO_CLASS(mask) ((mask) >> IOPRIO_CLASS_SHIFT)
#define IOPRIO_PRIO_DATA(mask) ((mask) & ((1 << IOPRIO_CLASS_SHIFT) - 1))
#define IOPRIO_PRIO_VALUE(class, data) (((class) << IOPRIO_CLASS_SHIFT) | (data))

Complete example

Here’s a working program that sets I/O priority for a process:

#include <unistd.h>
#include <sys/syscall.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

enum {
    IOPRIO_CLASS_NONE = 0,
    IOPRIO_CLASS_RT = 1,
    IOPRIO_CLASS_BE = 2,
    IOPRIO_CLASS_IDLE = 3,
};

enum {
    IOPRIO_WHO_PROCESS = 1,
    IOPRIO_WHO_PGRP = 2,
    IOPRIO_WHO_USER = 3,
};

#define IOPRIO_CLASS_SHIFT 13
#define IOPRIO_PRIO_VALUE(class, data) (((class) << IOPRIO_CLASS_SHIFT) | (data))

int main(int argc, char *argv[]) {
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <pid> <class> [priority]\n", argv[0]);
        fprintf(stderr, "Classes: 0=none, 1=realtime, 2=best-effort, 3=idle\n");
        return 1;
    }

    pid_t target_pid = atoi(argv[1]);
    int ioprio_class = atoi(argv[2]);
    int ioprio_data = (argc > 3) ? atoi(argv[3]) : 0;

    if (ioprio_class < 0 || ioprio_class > 3) {
        fprintf(stderr, "Invalid I/O priority class\n");
        return 1;
    }

    int ioprio = IOPRIO_PRIO_VALUE(ioprio_class, ioprio_data);

    if (syscall(SYS_ioprio_set, IOPRIO_WHO_PROCESS, target_pid, ioprio) < 0) {
        perror("ioprio_set");
        return 1;
    }

    printf("Set PID %d to I/O class %d with priority %d\n", 
           target_pid, ioprio_class, ioprio_data);
    return 0;
}

Compile with:

gcc -o ioprio_set ioprio_set.c

Run to set a process to idle I/O priority:

./ioprio_set 1234 3

I/O priority classes explained

IOPRIO_CLASS_NONE: No specific I/O scheduling hint
IOPRIO_CLASS_RT: Real-time; gets highest I/O scheduling priority. Requires CAP_SYS_ADMIN
IOPRIO_CLASS_BE: Best-effort (default for most processes). Priority ranges 0-7, where 0 is highest
IOPRIO_CLASS_IDLE: Idle priority; only executes when no other I/O activity exists. Useful for background tasks

Getting current I/O priority

You can read a process’s current I/O priority using ioprio_get:

int current_ioprio = syscall(SYS_ioprio_get, IOPRIO_WHO_PROCESS, target_pid);
if (current_ioprio < 0) {
    perror("ioprio_get");
} else {
    int class = IOPRIO_PRIO_CLASS(current_ioprio);
    int data = IOPRIO_PRIO_DATA(current_ioprio);
    printf("Class: %d, Priority: %d\n", class, data);
}

Practical notes

You typically need CAP_SYS_ADMIN capability to raise I/O priority or use realtime class
Non-root users can generally lower their own process priority
The ionice command-line tool uses these same syscalls under the hood
I/O priority is per-process; threads inherit the parent’s priority but can be set individually
Not all I/O schedulers support all classes (CFQ does; noop and deadline have limited support)

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Network analysis: tcpdump, wireshark, netstat, ss for connectivity debugging
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Quick Reference Summary

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Using ioprio_set to Control I/O Priority in Linux