/*
 * rename-log.c - check the size of a given log file in the '/tmp/var/log'
 *                directory. If it is larger then 1024KB, rename it to
 *                a have a '.old' suffix and create a new, empty log file.
 *
 * IMPORTANT NOTE: we intentionally use a fictitious log directory. Do NOT
 *		   use this program against your system's real log directory.
 */

#include <stdio.h>              /* standard input/output routines.    */
#include <unistd.h>             /* access(), etc.                     */
#include <sys/stat.h>           /* stat(), etc.                       */
#include <fcntl.h>              /* open(), close(), etc.              */
#include <malloc.h>             /* malloc(), etc.                     */
#include <string.h>             /* strcpy(), strcat().                */

#define LOG_DIR "/tmp/var/log"	/* full path to the logs directory. */
#define FILE_SIZE 1024*1024	/* size of file to rename - 1024KB. */
#define FILE_SUFFIX ".old"      /* suffix for old log file.         */

/*
 * function: main. 
 * input:    name of a log file.
 * output:   log file is being renamed if it is too big.
 */
void
main(int argc, char* argv[])
{
    char* file_name;	        /* name of the log file.        */
    char* new_file_name;        /* new name for the log file.   */
    struct stat file_status;    /* status info of the log file. */
    int fd;			/* descriptor for new file.     */
    mode_t file_mode;		/* permissions of old log file. */

    /* read command line arguments */
    if (argc != 2 || !argv[1]) {
	fprintf(stderr, "Usage: %s <log file name>\n", argv[0]);
	exit(1);
    }
    file_name = argv[1];
    new_file_name = (char*)malloc(strlen(file_name)+strlen(FILE_SUFFIX)+1);
    if (!new_file_name) {
	fprintf(stderr, "Out of memory\n");
	exit(1);
    }
    strcpy(new_file_name, file_name);
    strcat(new_file_name, FILE_SUFFIX);

    /* check we have read/write permissions to the /tmp/var/log directory. */
    if (access(LOG_DIR, F_OK) == -1) {
	fprintf(stderr, "Directory '%s' does not exist.\n", LOG_DIR);
	exit(1);
    }
    if (access(LOG_DIR, R_OK | W_OK) == -1) {
	fprintf(stderr, "Directory '%s': access denied.\n", LOG_DIR);
	exit(1);
    }

    /* switch current directory to the logs directory. */
    if (chdir(LOG_DIR) == -1) {
	fprintf(stderr, "Cannot change directory to '%s':", LOG_DIR);
	perror("");
	exit(1);
    }

    /* check the size of the file. */
    if (stat(file_name, &file_status) == -1) {
	fprintf(stderr, "Cannot stat file '%s':", file_name);
	perror("");
	exit(1);
    }

    if (file_status.st_size > FILE_SIZE) {
	if (rename(file_name, new_file_name) == -1) {
	    fprintf(stderr, "Cannot rename '%s' to '%s':",
		    file_name, new_file_name);
	    perror("");
	    exit(1);
	}
	/* create a new, empty log file, with the same access permissions */
	/* as the original log file had.                                  */
        file_mode = file_status.st_mode & ~S_IFMT;
	umask(0);
	fd = open(file_name, O_WRONLY | O_CREAT | O_TRUNC, file_mode);
	if (fd < 0) {
	    fprintf(stderr, "Failed creating empty log file:");
	    perror("");
	    exit(1);
        }
	close(fd);
    }
}