sysfs.c

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#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
 
#include "sysfs.h"
 
#define SYSFS_STRING_BUFFER_LENGTH 16
 
static char string_buffer[SYSFS_STRING_BUFFER_LENGTH];
 
int sysfs_export(char const *const sysfs_file, int value) {
    return sysfs_write_int(sysfs_file, value);
}
 
int sysfs_unexport(char const *const sysfs_file, int value) {
    return sysfs_write_int(sysfs_file, value);
}
 
int sysfs_is_exported(char const *const sysfs_path) {
    // check for existence of file (not permissions)
    int ret = access(sysfs_path, F_OK);
    if (ret == 0) {
        return 1;
    }
    if (ret < 0) {
        // inexistent file is unexported
        if (errno == ENOENT) {
            return 0;
        }
        return -1;
    }
 
    // treat insufficient permissions as failure
    return -1;
}
 
int sysfs_export_unexported(char const *const sysfs_path,
                            char const *const sysfs_export_file, int value) {
    int exported = sysfs_is_exported(sysfs_path);
    if (exported < 0) {
        return -1;
    }
 
    // if exported return immediately, otherwise export
    if (exported == 1) {
        return 1;
    } else {
        return sysfs_export(sysfs_export_file, value);
    }
}
 
int sysfs_unexport_exported(char const *const sysfs_path,
                            char const *const sysfs_unexport_file, int value) {
    int exported = sysfs_is_exported(sysfs_path);
    if (exported < 0) {
        return -1;
    }
 
    // if not exported return immediately, otherwise unexport
    if (exported == 0) {
        return 1;
    } else {
        return sysfs_unexport(sysfs_unexport_file, value);
    }
}
 
int sysfs_write_string(char const *const sysfs_file, char const *const value) {
    int fd = open(sysfs_file, O_WRONLY);
    if (fd < 0) {
        return -1;
    }
 
    int len = strlen(value);
    int ret = write(fd, value, len);
    close(fd);
    if (ret != len) {
        return -1;
    }
    return 0;
}
 
int sysfs_read_string(char const *const sysfs_file, char *const value,
                      int length) {
    int fd = open(sysfs_file, O_RDONLY);
    if (fd < 0) {
        return -1;
    }
 
    int len = read(fd, value, length);
    close(fd);
    if (len < 0) {
        return -1;
    }
    return len;
}
 
int sysfs_write_int(char const *const sysfs_file, int value) {
    string_buffer[0] = 0;
    int ret = snprintf(string_buffer, SYSFS_STRING_BUFFER_LENGTH, "%d", value);
    if (ret <= 0 || ret >= SYSFS_STRING_BUFFER_LENGTH) {
        return -1;
    }
    return sysfs_write_string(sysfs_file, string_buffer);
}
 
int sysfs_read_int(char const *const sysfs_file, int *const value) {
    int ret = sysfs_read_string(sysfs_file, string_buffer,
                                SYSFS_STRING_BUFFER_LENGTH);
    if (ret <= 0) {
        return -1;
    }
    if (strlen(string_buffer) <= 0) {
        return -1;
    }
    *value = atoi(string_buffer);
    return 0;
}