Change inode ctime in Linux - c

Question

utimensat() updates ctime when restoring atime and/or mtime, but I need to restore all 3 timestamps. I solved this in a tricky way, by saving the current clock, moving system clock to the needed ctime, calling utimensat(), restoring current clock.
It does the job, but I understand this is a bad solution, so I'm looking for a better one, all in (root) userspace without kernel mods.
Here is the code I wrote to verify utimensat() behaviour and test the solution.
/* Test utimensat() behaviour, restore also Change timestamp
*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define MYFILE "/home/user1/Work/myfile.txt"
static int get_timestamps(struct stat *sb, const char *descr) {
sleep(1);
if (stat(MYFILE, sb) == -1) {
perror("stat(" MYFILE ") ERROR");
return(errno);
}
char *a = ctime(&sb->st_atime);
char *m = ctime(&sb->st_mtime);
char *c = ctime(&sb->st_ctime);
printf("%s\n", descr);
printf("Last file access: %.*s.%lu\n", (int)strlen(a)-1, a, sb->st_atim.tv_nsec);
printf("Last file modification: %.*s.%lu\n", (int)strlen(m)-1, m, sb->st_mtim.tv_nsec);
printf("Last status change: %.*s.%lu\n\n", (int)strlen(c)-1, c, sb->st_ctim.tv_nsec);
return(0);
}
int main(int argc, char **argv) {
int rc = 0;
struct stat sb_original;
struct stat sb;
// Get file stats
rc = get_timestamps(&sb_original, "Initial");
if (rc) return(rc);
// Read file
FILE *myfile = fopen(MYFILE, "r");
if (myfile == NULL) {
perror("fopen(" MYFILE ") ERROR");
return(errno);
}
char buffer[10];
(void)fread(buffer, 1, 1, myfile);
(void)fclose(myfile);
rc = get_timestamps(&sb, "After fopen(); fread(); fclose()");
if (rc) return(rc);
system("touch " MYFILE);
rc = get_timestamps(&sb, "After touch");
if (rc) return(rc);
// Restore original Last Access timestamp updated after fopen(); fread(); fclose()
if (sb.st_ino) {
int have_sudo = 0;
struct timespec ts[2], tsctime, tsnow;
ts[0].tv_sec = sb_original.st_atim.tv_sec;
ts[0].tv_nsec = sb_original.st_atim.tv_nsec;
ts[1].tv_sec = sb_original.st_mtim.tv_sec;
ts[1].tv_nsec = sb_original.st_mtim.tv_nsec;
tsctime.tv_sec = sb_original.st_mtim.tv_sec;
tsctime.tv_nsec = sb_original.st_mtim.tv_nsec;
if (clock_gettime(CLOCK_REALTIME, &tsnow) < 0) {
perror("clock_gettime(CLOCK_REALTIME) ERROR");
return(errno);
}
/* ##### Fixme: is there a better way to restore all 3 timestamps? */
// Bad solution but does the job ;)
if (clock_settime(CLOCK_REALTIME, &tsctime) < 0) {
perror("clock_settime(CLOCK_REALTIME,ctime) ERROR");
if (errno != EPERM)
return(errno);
have_sudo = 1;
}
if (utimensat(0, MYFILE, ts, 0) < 0) { // This updates Change timestamp!
perror("utimensat(" MYFILE ") ERROR");
return(errno);
}
if (have_sudo && clock_settime(CLOCK_REALTIME, &tsnow) < 0) {
perror("clock_settime(CLOCK_REALTIME,now) ERROR");
return(errno);
}
rc = get_timestamps(&sb, "After utimensat()");
if (rc) return(rc);
}
return(0);
}