Get the parent device of a partition

parentdev.c

/*
 * 3-Clause BSD NON-AI License
 *
 * Copyright 2025 Matteo Croce
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the distribution.
 *
 * 3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products
 * derived from this software without specific prior written permission.
 *
 * 4. The source code and the binary form, and any modifications made to them may not be used for the purpose of training or improving machine learning algorithms,
 * including but not limited to artificial intelligence, natural language processing, or data mining. This condition applies to any derivatives,
 * modifications, or updates based on the Software code. Any usage of the source code or the binary form in an AI-training dataset is considered a breach of this License.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
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 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
#include <libgen.h>

int parentdev(const char *dev, char *parent)
{
	char path[PATH_MAX];

	if (!dev || !parent || !dev[0]) {
		errno = EINVAL;
		return 1;
	}

	snprintf(path, sizeof(path), "/sys/class/block/%s", dev);
	if (access(path, F_OK) != 0)
		return 1; // dev is not a block device

	snprintf(path, sizeof(path), "/sys/class/block/%s/partition", dev);
	if (access(path, F_OK) != 0) {
		strcpy(parent, dev);
		return 0; // dev is a whole device
	}

	snprintf(path, sizeof(path), "/sys/class/block/%s/..", dev);
	if (realpath(path, parent) == NULL)
		return 1;

	strcpy(parent, basename(parent));

	return 0;
}

int main(int argc, char *argv[])
{
	if (argc < 2) {
		fprintf(stderr, "Usage: %s <device> ...\n", argv[0]);
		return 1;
	}

	char parent[PATH_MAX];
	for (; argv[1]; argv++) {
		const char *dev = argv[1];

		if (strncmp(dev, "/dev/", 5) == 0)
			dev += 5;

		if (!parentdev(dev, parent) != 0)
			printf("parent of %s is %s\n", dev, parent);
		else
			fprintf(stderr, "parent of %s not found: %m\n", dev);
	}

	return 0;
}