mdev是busybox引入的，busybox有其功能描述及源码实现，这是学习mdev的第一手资料，皆来自busybox1.7.0推荐如下：   -------------
 MDEV Primer
------------- For those of us who know how to use mdev, a primer might seem lame.  For
everyone else, mdev is a weird black box that they hear is awesome, but can't
seem to get their head around how it works.  Thus, a primer. -----------
 Basic Use
----------- Mdev has two primary uses: initial population and dynamic updates.  Both
require sysfs support in the kernel and have it mounted at /sys.  For dynamic
updates, you also need to have hotplugging enabled in your kernel. Here's a typical code snippet from the init script:
[1] mount -t sysfs sysfs /sys
[2] echo /bin/mdev > /proc/sys/kernel/hotplug
[3] mdev -s Of course, a more "full" setup would entail executing this before the previous
code snippet:
[4] mount -t tmpfs mdev /dev
[5] mkdir /dev/pts
[6] mount -t devpts devpts /dev/pts The simple explanation here is that [1] you need to have /sys mounted before
executing mdev.  Then you [2] instruct the kernel to execute /bin/mdev whenever
a device is added or removed so that the device node can be created or
destroyed.  Then you [3] seed /dev with all the device nodes that were created
while the system was booting. For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
(assuming you're running out of flash).  Then you want to [5] create the
/dev/pts mount point and finally [6] mount the devpts filesystem on it. -------------
 MDEV Config   (/etc/mdev.conf)
------------- Mdev has an optional config file for controlling ownership/permissions of
device nodes if your system needs something more than the default root/root
660 permissions. The file has the format:
  :
For example:
 hd[a-z][0-9]* 0:3 660 The config file parsing stops at the first matching line.  If no line is
matched, then the default of 0:0 660 is used.  To set your own default, simply
create your own total match like so:
 .* 1:1 777 If you also enable support for executing your own commands, then the file has
the format:
  : [<@|$|*> ]
The special characters have the meaning:
 @ Run after creating the device.
 $ Run before removing the device.
 * Run both after creating and before removing the device. The command is executed via the system() function (which means you're giving a
command to the shell), so make sure you have a shell installed at /bin/sh. For your convenience, the shell env var $MDEV is set to the device name.  So if
the device 'hdc' was matched, MDEV would be set to "hdc". ----------
 FIRMWARE
---------- Some kernel device drivers need to request firmware at runtime in order to
properly initialize a device.  Place all such firmware files into the
/lib/firmware/ directory.  At runtime, the kernel will invoke mdev with the
filename of the firmware which mdev will load out of /lib/firmware/ and into
the kernel via the sysfs interface.  The exact filename is hardcoded in the
kernel, so look there if you need to want to know what to name the file in
userspace.     mdev.c源码如下：   /* vi: set sw=4 ts=4: */ /* * * mdev - Mini udev for busybox * * Copyright 2005 Rob Landley <rob@landley.net> * Copyright 2005 Frank Sorenson <frank@tuxrocks.com> * * Licensed under GPL version 2, see file LICENSE in this tarball for details. */ #include "libbb.h" #include "xregex.h" struct globals { int root_major, root_minor; }; #define G (*(struct globals*)&bb_common_bufsiz1) #define root_major (G.root_major) #define root_minor (G.root_minor) #define MAX_SYSFS_DEPTH 3 /* prevent infinite loops in /sys symlinks */ /* mknod in /dev based on a path like "/sys/block/hda/hda1" */ static void make_device(char *path, int delete) { const char *device_name; int major, minor, type, len; int mode = 0660; uid_t uid = 0; gid_t gid = 0; char *temp = path + strlen(path); char *command = NULL; /* Try to read major/minor string. Note that the kernel puts /n after * the data, so we don't need to worry about null terminating the string * because sscanf() will stop at the first nondigit, which /n is. We * also depend on path having writeable space after it. */ if (!delete) { strcat(path, "/dev"); len = open_read_close(path, temp + 1, 64); *temp++ = 0; if (len < 1) return; } /* Determine device name, type, major and minor */ device_name = bb_basename(path); type = path[5]=='c' ? S_IFCHR : S_IFBLK; /* If we have a config file, look up permissions for this device */ if (ENABLE_FEATURE_MDEV_CONF) { char *conf, *pos, *end; int line, fd; /* mmap the config file */ fd = open("/etc/mdev.conf", O_RDONLY); if (fd < 0) goto end_parse; len = xlseek(fd, 0, SEEK_END); conf = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0); close(fd); if (!conf) goto end_parse; line = 0; /* Loop through lines in mmaped file*/ for (pos=conf; pos-conf<len;) { int field; char *end2; line++; /* find end of this line */ for (end=pos; end-conf<len && *end!='/n'; end++) ; /* Three fields: regex, uid:gid, mode */ for (field=0; field < (3 + ENABLE_FEATURE_MDEV_EXEC); field++) { /* Skip whitespace */ while (pos<end && isspace(*pos)) pos++; if (pos==end || *pos=='#') break; for (end2=pos; end2<end && !isspace(*end2) && *end2!='#'; end2++) ; if (field == 0) { /* Regex to match this device */ char *regex = xstrndup(pos, end2-pos); regex_t match; regmatch_t off; int result; /* Is this it? */ xregcomp(&match,regex, REG_EXTENDED); result = regexec(&match, device_name, 1, &off, 0); regfree(&match); free(regex); /* If not this device, skip rest of line */ if (result || off.rm_so || off.rm_eo != strlen(device_name)) break; } if (field == 1) { /* uid:gid */ char *s, *s2; /* Find : */ for (s=pos; s<end2 && *s!=':'; s++) ; if (s == end2) break; /* Parse UID */ uid = strtoul(pos, &s2, 10); if (s != s2) { struct passwd *pass; char *_unam = xstrndup(pos, s-pos); pass = getpwnam(_unam); free(_unam); if (!pass) break; uid = pass->pw_uid; } s++; /* parse GID */ gid = strtoul(s, &s2, 10); if (end2 != s2) { struct group *grp; char *_grnam = xstrndup(s, end2-s); grp = getgrnam(_grnam); free(_grnam); if (!grp) break; gid = grp->gr_gid; } } if (field == 2) { /* mode */ mode = strtoul(pos, &pos, 8); if (pos != end2) break; } if (ENABLE_FEATURE_MDEV_EXEC && field == 3) { // Command to run const char *s = "@$*"; const char *s2; s2 = strchr(s, *pos++); if (!s2) { // Force error field = 1; break; } if ((s2-s+1) & (1<<delete)) command = xstrndup(pos, end-pos); } pos = end2; } /* Did everything parse happily? */ if (field > 2) break; if (field) bb_error_msg_and_die("bad line %d",line); /* Next line */ pos = ++end; } munmap(conf, len); end_parse: /* nothing */ ; } umask(0); if (!delete) { if (sscanf(temp, "%d:%d", &major, &minor) != 2) return; if (mknod(device_name, mode | type, makedev(major, minor)) && errno != EEXIST) bb_perror_msg_and_die("mknod %s", device_name); if (major == root_major && minor == root_minor) symlink(device_name, "root"); if (ENABLE_FEATURE_MDEV_CONF) chown(device_name, uid, gid); } if (command) { /* setenv will leak memory, so use putenv */ char *s = xasprintf("MDEV=%s", device_name); putenv(s); if (system(command) == -1) bb_perror_msg_and_die("cannot run %s", command); s[4] = '/0'; unsetenv(s); free(s); free(command); } if (delete) unlink(device_name); } /* File callback for /sys/ traversal */ static int fileAction(const char *fileName, struct stat *statbuf, void *userData, int depth) { size_t len = strlen(fileName) - 4; char *scratch = userData; if (strcmp(fileName + len, "/dev")) return FALSE; strcpy(scratch, fileName); scratch[len] = 0; make_device(scratch, 0); return TRUE; } /* Directory callback for /sys/ traversal */ static int dirAction(const char *fileName, struct stat *statbuf, void *userData, int depth) { return (depth >= MAX_SYSFS_DEPTH ? SKIP : TRUE); } /* For the full gory details, see linux/Documentation/firmware_class/README * * Firmware loading works like this: * - kernel sets FIRMWARE env var * - userspace checks /lib/firmware/$FIRMWARE * - userspace waits for /sys/$DEVPATH/loading to appear * - userspace writes "1" to /sys/$DEVPATH/loading * - userspace copies /lib/firmware/$FIRMWARE into /sys/$DEVPATH/data * - userspace writes "0" (worked) or "-1" (failed) to /sys/$DEVPATH/loading * - kernel loads firmware into device */ static void load_firmware(const char *const firmware, const char *const sysfs_path) { int cnt; int firmware_fd, loading_fd, data_fd; /* check for $FIRMWARE from kernel */ /* XXX: dont bother: open(NULL) works same as open("no-such-file") * if (!firmware) * return; */ /* check for /lib/firmware/$FIRMWARE */ xchdir("/lib/firmware"); firmware_fd = xopen(firmware, O_RDONLY); /* in case we goto out ... */ data_fd = -1; /* check for /sys/$DEVPATH/loading ... give 30 seconds to appear */ xchdir(sysfs_path); for (cnt = 0; cnt < 30; ++cnt) { loading_fd = open("loading", O_WRONLY); if (loading_fd == -1) sleep(1); else break; } if (loading_fd == -1) goto out; /* tell kernel we're loading by `echo 1 > /sys/$DEVPATH/loading` */ if (write(loading_fd, "1", 1) != 1) goto out; /* load firmware by `cat /lib/firmware/$FIRMWARE > /sys/$DEVPATH/data */ data_fd = open("data", O_WRONLY); if (data_fd == -1) goto out; cnt = bb_copyfd_eof(firmware_fd, data_fd); /* tell kernel result by `echo [0|-1] > /sys/$DEVPATH/loading` */ if (cnt > 0) write(loading_fd, "0", 1); else write(loading_fd, "-1", 2); out: if (ENABLE_FEATURE_CLEAN_UP) { close(firmware_fd); close(loading_fd); close(data_fd); } } int mdev_main(int argc, char **argv); int mdev_main(int argc, char **argv) { char *action; char *env_path; RESERVE_CONFIG_BUFFER(temp,PATH_MAX); xchdir("/dev"); /* Scan */ if (argc == 2 && !strcmp(argv[1],"-s")) { struct stat st; xstat("/", &st); root_major = major(st.st_dev); root_minor = minor(st.st_dev); recursive_action("/sys/block", ACTION_RECURSE | ACTION_FOLLOWLINKS, fileAction, dirAction, temp, 0); recursive_action("/sys/class", ACTION_RECURSE | ACTION_FOLLOWLINKS, fileAction, dirAction, temp, 0); /* Hotplug */ } else { action = getenv("ACTION"); env_path = getenv("DEVPATH"); if (!action || !env_path) bb_show_usage(); sprintf(temp, "/sys%s", env_path); if (!strcmp(action, "remove")) make_device(temp, 1); else if (!strcmp(action, "add")) { make_device(temp, 0); if (ENABLE_FEATURE_MDEV_LOAD_FIRMWARE) load_firmware(getenv("FIRMWARE"), temp); } } if (ENABLE_FEATURE_CLEAN_UP) RELEASE_CONFIG_BUFFER(temp); return 0; }