Reading/Writing errno or stack variable causes segfault. Why? - c

Question

I have a really weird problem here, and haven't managed to find an answer online.
It appears after debugging with printf statements that a segfault ocurred when trying to read errno. Commenting problemed lines out one by one as they cause segfault resulted in having to comment out every reference to errno, after a readdir() call reaches the end of the directory stream and returns NULL.
Even then, the code then segfaults later when trying to access another automatic variable, file_count.
What is going on? Is this a stack overflow? How do I make it stop?
The code is below, if you feel the need to wade through it. All the problematic references to errno are removed, and the program segfaults after successfully executing the third second last line: printf("printing file_count\n");.
EDIT1: Here's a GDB backtrace:
#0 0xc95bf881 in strcpy () from /usr/lib/libc.so.1
#1 0x08051543 in dir_get_list (user=0x8047b88 "user1") at maildir.c:231
#2 0x08050f3e in main (argc=4, argv=0x80479f4) at maildir.c:43
END EDIT1
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <dirent.h>
#include <fcntl.h>
#include <errno.h>
#include <pthread.h>
#define MAX_FILENAME_LENGTH 255
#define MAX_USERNAME_LENGTH 40
#define MAX_PASSWORD_LENGTH 20
typedef int bool;
#define true 1
#define false 0
struct files_struct{
/*The number of email messages in a maildir.*/
int count;
/*A pointer to an array of pointers to the strings of the filenames. */
char **FileNames;
/*A pointer to an array of ints that give the corresponding size of the file.*/
int *FileSize;
};
typedef struct files_struct FilesStruct;
void dir_set_path(char* path);
bool check_user(char* username, char* pass);
FilesStruct* dir_get_list(char* user);
void delete_mail(char* user, char* filename);
char* get_file(char* user, char* filename);
FilesStruct* dir_get_list(char* user){
char maildir_name[MAX_FILENAME_LENGTH];
DIR * maildir_fd;
struct dirent *maildir_p;
strcpy(maildir_name,"./");
strncat(maildir_name,user,MAX_USERNAME_LENGTH);
strcat(maildir_name,"/");
if((pthread_mutex_lock(&maildir_root_mutex))<0)
perror("ERROR on locking maildir_root_mutex");
printf("Opening directory ""%s""\n",maildir_name);
if((maildir_fd = opendir(maildir_name))==NULL)
perror("ERROR on opendir");
int file_count = 0;
/* scan over entire directory, counting number of files to that data can be properly malloced */
while(1){
if((maildir_p = readdir(maildir_fd))==NULL){
closedir(maildir_fd);
printf("breaking loop\n");
break;
}
char file[MAX_FILENAME_LENGTH+1];
strcpy(file,maildir_p->d_name);
printf("File %d: '%s'\n",file_count+1,maildir_p->d_name);
/* if the file is a file other than an email */
if(!strcmp(".",file)||!strcmp("..",file)||!strcmp("pass",file)||!strcmp(".svn",file)){
printf("Continuing without incrementing file_count\n");
continue;
}
file_count++;
}
printf("%u\n",maildir_fd);
printf("printing file_count\n");
printf("%d",file_count);
printf("file_count printed successfully");
/* Additional code OMITTED */

Answer 1

I came across this recently. In my instance it was that another module had declared:
int errno = 0;
as a global, instead of #including errno.h. Any code that used the "proper" errno would immediately segfault.