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A whole bunch of Segmentation Faults

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Join Date: Nov 2009

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thure

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A whole bunch of Segmentation Faults

19 Days Ago

Hi there,

This is my first time working with an unsafe language, so naturally my first code would be full of memory-related bugs. I still have no idea how I ought to approach some of the problems I've been getting, and I've been resisting asking for help as much as I could.

The assignment I've been given is a buttload of work, so I'd rather not bog anyone down with too much of it. At the moment, I'm trying to work out how to design a menu() function that gives main() both the function choice the user wants and the arguments for that function. Here's what I've got so far:

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struct REPLY {
	char use[256];
	char pass[256];
	char type[64];
	char nuse[256];
	char npass[256];
	char ntype[64];
	int option;
};
 
struct REPLY *reply;
 
int menu (void)
{
	int o;
	printf("Please type a number corresponding to the following options:\n");
	printf("1. Add\n");
	printf("2. Delete\n");
	printf("3. Edit\n");
	printf("4. Purge\n");
	printf("5. Quit\n");
	scanf("Selection: %i\n", o);
	reply->option=o;
	printf("\n\n%i\n\n\n", reply->option);
	switch(reply->option){
		case '1':
		{
			printf("Please provide the username.\n");
			scanf("%s\n", &reply->use);
			printf("Please provide the password.\n");
			scanf("%s\n", &reply->pass);
			printf("Please provide the user type.\n");
			scanf("%s\n", &reply->type);
			break;
		}
		case '2':
		{
			printf("Please provide the username.\n");
			scanf("%s\n",&reply->use);
			break;
		}
		case '3':
		{
			printf("Please provide the username.\n");
			scanf("%s\n",&reply->use);
			printf("Please provide the password.\n");
			scanf("%s\n",&reply->use);
			printf("Please provide the new username.\n");
			scanf("%s\n",&reply->nuse);
			printf("Please provide the new password.\n");
			scanf("%s\n",&reply->npass);
			printf("Please provide the new user type.\n");
			scanf("%s\n",&reply->ntype);
			break;
		}
		case '4':
		{
			break;
		}
		case '5':
		{
			break;
		}
	}
 
	return 0;
}
struct REPLY {
	char use[256];
	char pass[256];
	char type[64];
	char nuse[256];
	char npass[256];
	char ntype[64];
	int option;
};

struct REPLY *reply;

int menu (void)
{
	int o;
	printf("Please type a number corresponding to the following options:\n");
	printf("1. Add\n");
	printf("2. Delete\n");
	printf("3. Edit\n");
	printf("4. Purge\n");
	printf("5. Quit\n");
	scanf("Selection: %i\n", o);
	reply->option=o;
	printf("\n\n%i\n\n\n", reply->option);
	switch(reply->option){
		case '1':
		{
			printf("Please provide the username.\n");
			scanf("%s\n", &reply->use);
			printf("Please provide the password.\n");
			scanf("%s\n", &reply->pass);
			printf("Please provide the user type.\n");
			scanf("%s\n", &reply->type);
			break;
		}
		case '2':
		{
			printf("Please provide the username.\n");
			scanf("%s\n",&reply->use);
			break;
		}
		case '3':
		{
			printf("Please provide the username.\n");
			scanf("%s\n",&reply->use);
			printf("Please provide the password.\n");
			scanf("%s\n",&reply->use);
			printf("Please provide the new username.\n");
			scanf("%s\n",&reply->nuse);
			printf("Please provide the new password.\n");
			scanf("%s\n",&reply->npass);
			printf("Please provide the new user type.\n");
			scanf("%s\n",&reply->ntype);
			break;
		}
		case '4':
		{
			break;
		}
		case '5':
		{
			break;
		}
	}
	
	return 0;
}

This code is saved as menu.c, which is included in the inclusion statements of the main function. I'm getting a segmentation fault at

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	reply->option=o;

and I haven't been able to find out why. I also get the sense main won't be able to access reply for some reason.

I just need a push, and it's probably so simple it went over my head, but can someone help me out?

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Ancient Dragon Ancient Dragon is offline

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-7

19 Days Ago

line 22: >> scanf("Selection: %i\n", o);

You can't combine scanf() and printf() like that, and the parameter must be a pointer to an integer

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printf("Selction: ");
scanf("%i", &o);
printf("Selction: ");
scanf("%i", &o);

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.

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Join Date: Nov 2009

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thure

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19 Days Ago

That dismissed one of the warnings, but I still get a segmentation fault at

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reply->option=o;

Why is that even a problem?

All of the other warnings look like this:

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menu.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[256]’
menu.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[256]’

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Aia

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19 Days Ago

•

Originally Posted by thure

That dismissed one of the warnings, but I still get a segmentation fault at
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reply->option=o;
reply->option=o;
Why is that even a problem?

struct REPLY *reply; is a pointer, anything you try to write to it it will produce a segmentation fault, since there's no memory allocated for it.

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thure

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19 Days Ago

I'm never sure when to use malloc and when C allocates the memory for me. Thank you, I'll try that.

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Aia

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19 Days Ago

•

Originally Posted by thure

I'm never sure when to use malloc and when C allocates the memory for me.

For some, memory management in C, is a curse, nevertheless it is one of the strength of the language.

•

Originally Posted by thure

All of the other warnings look like this:
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menu.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[256]’
menu.c:30: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[256]’

After allocating memory for it, remove any & in front of those pointers to string like

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scanf("%s\n", &reply->pass);

and the '\n' is not going to give you the intended result, neither.

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thure

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19 Days Ago

Success! This was the sort of push I needed. I'm not getting any warnings, and as far as I can tell the menu is working properly.

I might have questions later about how to get menu()'s reply back to main() since C's scoping is so complex. This is what I've done:

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...
 
struct REPL *reply;
 
int menu (void)
{
	reply=(struct REPL*)malloc(sizeof(struct REPL));
 
...
...

struct REPL *reply;

int menu (void)
{
	reply=(struct REPL*)malloc(sizeof(struct REPL));

...

Since reply is declared globally, will main() be able to see and use it after menu is finished?

@Aia, thanks for your help - I really appreciate it.

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thure

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19 Days Ago

Alright, more segmentation faults...

This time I'm trying to parse a file into a linked list. The intent is to decrypt and read the file line-by-line, separating a line into parts and storing those parts into a struct.

I'm only getting the segmentation fault when the file, password.csv, already exists, for some reason. If it's starting anew the program behaves normally.

GDB says the problem is in this line. The whole function is pasted below.

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while(fgets(c, 600, fin)!=NULL)
while(fgets(c, 600, fin)!=NULL)

It happens in this function:

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int parse (void){
	cipher(0);
	FILE *fin=fopen("password.csv", "rt");
	//processing
	char *c;
	while(fgets(c, 600, fin)!=NULL){
		char use[256];
		char pass[256];
		char type[64];
		strcpy(use, strtok(c,", "));
		strcpy(pass, strtok(NULL,", "));
		strcpy(type, strtok(NULL,", "));
		push(use, pass, type);
	}
	fclose(fin);
	return 1;
}
int parse (void){
	cipher(0);
	FILE *fin=fopen("password.csv", "rt");
	//processing
	char *c;
	while(fgets(c, 600, fin)!=NULL){
		char use[256];
		char pass[256];
		char type[64];
		strcpy(use, strtok(c,", "));
		strcpy(pass, strtok(NULL,", "));
		strcpy(type, strtok(NULL,", "));
		push(use, pass, type);
	}
	fclose(fin);
	return 1;
}

I'd really appreciate your thoughts.

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Aia

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19 Days Ago

char *c; is a pointer, again, it doesn't point to any proper memory allocated to it.
while(fgets(c, 600, fin)!=NULL) Guess what you are trying to do there? Yeap! Trying to write to poor char *c that it doesn't have any proper memory available for it.

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thure

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#10

18 Days Ago

Aw man, I thought C handled chars! It's just one byte! Okay, thanks again for your help.

As a general rule, do segmentation faults happen because memory isn't allocated? I'd guess with beginner C it wouldn't be anything fancier.