Example of how to use the newly proposed cgroup_cgxget() and cgroup_cgxset() C APIs.

libcgroup-cgxget-example.c

#include <libcgroup.h>
#include <stdlib.h>
 
#define CGNAME      "cgx_example"
#define CONTROLLER  "cpu"
#define SETTING     "cpu.shares"
#define VALUE       "999"
 
static int other_interesting_calls(void)
{
    enum cg_version_t version;
    struct cgroup_library_version const *lib_version;
    int ret;
 
    ret = cgroup_get_controller_version(CONTROLLER, &version);
    if (ret != 0) {
        fprintf(stderr, "cgroup_get_controller_version() failed: %d\n",
            ret);
        goto out;
    }
    fprintf(stdout, "Controller %s is mounted as cgroup version %d\n\n",
        CONTROLLER, version);
 
    lib_version = cgroup_version();
    fprintf(stdout, "Currently running libcgroup version %d.%d.%d\n\n",
        lib_version->major, lib_version->minor, lib_version->release);
 
out:
    return ret;
}
 
static int read_setting(struct cgroup **cg, char *setting)
{
    struct cgroup_controller *cgc;
    char *value = NULL;
    int ret;
 
    ret = cgroup_cgxget(cg, CGROUP_V1, false);
    if (ret != 0) {
        fprintf(stderr, "cgroup_cgxget() failed: %d\n", ret);
        goto out;
    }
 
    cgc = cgroup_get_controller(*cg, CONTROLLER);
    if (cgc == NULL) {
        fprintf(stderr, "cgroup_get_controller() failed\n");
        ret = 4;
        goto out;
    }
 
    ret = cgroup_get_value_string(cgc, setting, &value);
    if (ret != 0) {
        fprintf(stderr, "cgroup_get_value_string() failed: %d\n", ret);
        goto out;
    }
    fprintf(stdout, "cgxget received: %s = %s\n", setting, value);
 
out:
    if (value)
        free(value);
 
    return ret;
}
 
int main(void)
{
    struct cgroup_controller *cgc;
    struct cgroup *cg;
    int ret;
 
    /*----------------------------------------------------------------
     * Set up the structures
     ****************************************************************/
    ret = cgroup_init();
    if (ret != 0) {
        fprintf(stderr, "cgroup_init() failed: %d\n", ret);
        return ret;
    }
 
    cg = cgroup_new_cgroup(CGNAME);
    if (cg == NULL) {
        fprintf(stderr, "cgroup_new_cgroup() failed\n");
        return 2;
    }
 
    cgc = cgroup_add_controller(cg, CONTROLLER);
    if (cgc == NULL) {
        fprintf(stderr, "cgroup_add_controller() failed\n");
        ret = 3;
        goto out;
    }
 
    ret = cgroup_add_value_string(cgc, SETTING, NULL);
    if (ret != 0) {
        fprintf(stderr, "cgroup_add_value_string() failed: %d\n", ret);
        goto out;
    }
 
    ret = other_interesting_calls();
    if (ret != 0)
        goto out;
 
    /*----------------------------------------------------------------
     * Read what's currently in sysfs
     ****************************************************************/
    ret = read_setting(&cg, SETTING);
    if (ret != 0)
        goto out;
 
    /*----------------------------------------------------------------
     * Modify the in-ram copy of the cgroup and then set in sysfs
     * via cgxset()
     ****************************************************************/
    cgc = cgroup_get_controller(cg, CONTROLLER);
    if (cgc == NULL) {
        fprintf(stderr, "cgroup_get_controller() failed\n");
        ret = 5;
        goto out;
    }
 
    ret = cgroup_set_value_string(cgc, SETTING, VALUE);
    if (ret != 0) {
        fprintf(stderr, "cgroup_set_value_string() failed: %d\n", ret);
        goto out;
    }
 
    ret = cgroup_cgxset(cg, CGROUP_V1, false);
    if (ret != 0) {
        fprintf(stderr, "cgroup_cgxset() failed: %d\n", ret);
        goto out;
    }
 
    /*----------------------------------------------------------------
     * Read what's currently in sysfs
     ****************************************************************/
    ret = read_setting(&cg, SETTING);
    if (ret != 0)
        goto out;
 
out:
    if (cg != NULL)
        cgroup_free(&cg);
 
    return ret;
}

Here's one way you could run the above code:

$ export SRCPATH=/path/to/libcgroupsrc
$ gcc libcgroup-cgxget-example.c -lcgroup -I $SRCPATH/include -L $SRCPATH/src/.libs -o example
$ sudo $SRCPATH/src/tools/cgcreate -g cpu:cgx_example
$ sudo LD_LIBRARY_PATH=$SRCPATH/src/.libs ./example

Controller cpu is mounted as cgroup version 2
Currently running libcgroup version 1.0.42
cgxget received: cpu.shares = 1024
cgxget received: cpu.shares = 993