Get battery status on a Linux laptop from a C program

linux_get_battery.c

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <dirent.h>
#include <linux/limits.h>
#include <regex.h>
#include <stdbool.h>
#include <stdlib.h>

#define DATADIR "/sys/class/power_supply"
#define CHARGE_NOW "energy_now"
#define CHARGE_FULL "energy_full"
#define CHARGE_STATUS "status"

#define CHARGE_STATUS_CHARGING 0
#define CHARGE_STATUS_DISCHARGING 1
#define CHARGE_STATUS_FULL 2
#define CHARGE_STATUS_CHARGING_STR "Charging"
#define CHARGE_STATUS_DISCHARGING_STR "Discharging"
#define CHARGE_STATUS_FULL_STR "Full"

#define ERROR_FOLDER_NOT_FOUND (-1)
#define ERROR_FILE_NOT_FOUND (-2)
#define ERROR_REGEX_COMPILE (-3)
#define ERROR_REGEX_FIND (-4)
#define ERROR_FILE_READING (-5)
#define ERROR_CHARGE_STATUS_UNKNOWN (-6)

static int read_charge_level(const char* data_dir,
                             const char* sub_dir,
                             char* buffer,
                             float* charge_level) {
    FILE* file_current = NULL;
    FILE* file_full = NULL;

    long current;
    long full;

    int result = 0;

    snprintf(buffer, PATH_MAX, "%s/%s/%s", data_dir, sub_dir, CHARGE_NOW);
    file_current = fopen(buffer, "r");

    snprintf(buffer, PATH_MAX, "%s/%s/%s", data_dir, sub_dir, CHARGE_FULL);
    file_full = fopen(buffer, "r");

    if (file_current == NULL || file_full == NULL) {
        result = ERROR_FILE_NOT_FOUND;
        goto clean;
    }

    if (fscanf(file_current, "%ld", &current) != 1 ||
        fscanf(file_full, "%ld", &full) != 1) {
        result = ERROR_FILE_READING;
        goto clean;
    }

    *charge_level = (current / (float)full);

clean:
    if (file_current != NULL) fclose(file_current);
    if (file_full != NULL) fclose(file_full);

    return result;
}

static int read_charge_status(const char* data_dir,
                              const char* sub_dir,
                              char* buffer,
                              int* charge_status) {
    FILE* file = NULL;

    int result = 0;

    snprintf(buffer, PATH_MAX, "%s/%s/%s", data_dir, sub_dir, CHARGE_STATUS);
    file = fopen(buffer, "r");

    if (file == NULL) {
        result = ERROR_FILE_NOT_FOUND;
        goto clean;
    }

    if (fscanf(file, "%s", buffer) != 1) {
        result = ERROR_FILE_READING;
        goto clean;
    }

    printf("%s\n", buffer);
    if (strcmp(buffer, CHARGE_STATUS_CHARGING_STR) == 0) {
        *charge_status = CHARGE_STATUS_CHARGING;
    } else if (strcmp(buffer, CHARGE_STATUS_DISCHARGING_STR) == 0) {
        *charge_status = CHARGE_STATUS_DISCHARGING;
    } else if (strcmp(buffer, CHARGE_STATUS_FULL_STR) == 0) {
        *charge_status = CHARGE_STATUS_FULL;
    } else {
        result = ERROR_CHARGE_STATUS_UNKNOWN;
        goto clean;
    }

clean:
    if (file != NULL) fclose(file);

    return result;
}

int get_battery_status(float *level, int *charge_status) {
    DIR* directory;
    struct dirent* dirent;
    char buffer[PATH_MAX];
    int result = 0;

    regex_t regex;

    bool data_found = false;

    if ((directory = opendir(DATADIR)) == NULL) {
        return ERROR_FOLDER_NOT_FOUND;
    }

    while ((dirent = readdir(directory)) != NULL) {
        snprintf(buffer, PATH_MAX, "%s/%s", DATADIR, dirent->d_name);

        if (regcomp(&regex, "BAT[0-9]+", REG_EXTENDED) != 0) {
            result = ERROR_REGEX_COMPILE;
            goto clean;
        }

        if (regexec(&regex, dirent->d_name, 0, NULL, 0) != 0) {
            regfree(&regex);
            continue;
        }

        regfree(&regex);

        result = read_charge_level(DATADIR, dirent->d_name,
                                   buffer, level);
        if (result != 0) {
            continue;
        }
        result = read_charge_status(DATADIR, dirent->d_name,
                                    buffer, charge_status);
        if (result == 0) {
            data_found = true;
            break;
        }
    }

    if (!data_found) {
        result = ERROR_REGEX_FIND;
        goto clean;
    }

clean:
    if (directory != NULL) closedir(directory);

    return result;
}

int main(int argc, char** argv) {
    float level;
    int charge_status;
    int result = get_battery_status(&level, &charge_status);
    if (result == 0) {
        char charge_status_str[20] = {0};
        switch (charge_status) {
            case CHARGE_STATUS_CHARGING:
                strcpy(charge_status_str, CHARGE_STATUS_CHARGING_STR);
                break;
            case CHARGE_STATUS_DISCHARGING:
                strcpy(charge_status_str, CHARGE_STATUS_DISCHARGING_STR);
                break;
            case CHARGE_STATUS_FULL:
                strcpy(charge_status_str, CHARGE_STATUS_FULL_STR);
                break;
        }

        printf("Level: %.2f, Charge: %s\n", level, charge_status_str);
    }

    return result;
}

Not sure about consistency of the CHARGE_NOW and CHARGE_FULL files across versions. So might need adaptation.