Не работает переименование файла

#include <pwd.h>
#include <stdio.h>
#include <errno.h>
#include <dirent.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
using namespace std;
 
void usage(void)
{
    printf("Use program with following options:\n");
    printf("\n");
    printf("-p PATH    set path to directory\n");
    printf("-u USER    set user name\n");
    printf("-h     help\n");
    printf("\n");
}
 
int main(int argc, char** argv) {
DIR *dir;
FILE *file;
int rez = 0;
int path_length = 0;
int status = 0;
struct stat file_stats;
struct passwd *user_info;
char file_path[NAME_MAX];
struct dirent *dir_content;
char *path, *user, *buf_string, *result;
 
if(argc < 3)
{
    usage();
    exit(EXIT_FAILURE);
}
 
while ( (rez = getopt(argc,argv,"hp:u:")) != -1)
{    
    switch (rez)
    {
        //Taking arguments from argv[]
        case 'h': usage();break;
        case 'p': path=optarg; path_length = strlen(path); break;
        case 'u': user=optarg; break;
        case '?': usage(); break;
    }
}
 
if ( (dir=opendir(path)) == NULL )
{
    fprintf(stderr, "Error while opening directory\n");
    exit(EXIT_FAILURE);
}
 
if ( (user_info = getpwnam(user)) == NULL )
{
    fprintf(stderr, "Error, there's no specified user in system\n");
    closedir(dir);
    exit(EXIT_FAILURE);
}
printf("%s\n", user);
printf("%s\n", path);
printf("%d\n", path_length);
printf("%s\n", user_info->pw_name);
while ( (dir_content = readdir(dir)) != NULL )
{
    if ( dir_content->d_type != 4 )
    {
        sprintf(file_path, "%s%s", path, dir_content->d_name);
        printf("%s\n", file_path);
        if ( (file=fopen(file_path, "r")) == NULL )
            perror("File cannot be opened");
        stat(file_path, &file_stats);
        fclose(file);
        if ( (file_stats.st_uid == user_info->pw_uid) )  
                                    //&& (S_IXUSR )
        {
            buf_string = strcat(file_path, ".sh");
            printf("%s\n", buf_string);
            if ( rename(file_path, buf_string) != 0 )
                perror("File wan not renamed");
        }
    }
}
 
closedir(dir);
    return 0;
}