Thread: Function for getting configuration causes segfault

I'm sort of a self-taught beginner programmer here. One thing I always tried to avoid was pointers. After this last project, I figured I had to start learning them, as well as improving what other skills I might have.

This program is supposed to read a line from a configuration file and pass it back to whatever requests it, be it a variable or another function. It always causes a segfault.

It's probably something stupidly simple that I'm overlooking here, but I'm really new to these things. Any help would be greatly appreciated!

Code:

int main()
{
	char *blah;

	blah = getconf(false,"test");

	printf("%s\n",blah);

	return(0);
}

char *getconf(bool writelog,char get[])
{
	char line[256];
	FILE *config;
	int a = 0;
	char setting[25];
	char *value;
	
	config = fopen("test.conf","r");
	
	while(fgets(line,sizeof(line),config) != NULL) {
		a++;
		
		if(line[0] == '#') continue; //If the line is a comment, skip it
		if(sscanf(line,"%s = %s",setting,value) != 2) {
			if(writelog == true) {
				printf("%i: syntax error!\n",a);
			}
			continue;
		}
		puts("return if");
		if(strcmp(setting,get)) return(value); //GDB backtrace always shows the segfault around this area.
		puts("val not found");
	}

	if(writelog == true) {
		printf("%s: value not found\n",get);
	}
	
	return("no value");
}

Code:

char *value;

You have a pointer which does not point to any memory. Instant segfault.

I see. But why does

Code:

char *blah;

work in main?

Thanks for your reply!
Would my pointers have to point to a char array? Or should I use malloc()?

These things always confuse me, no matter how many tutorials I read...

Now, when you say prompting for the size of the input, does that mean I can wait and allocate based on how big the final variable needs to be?

For example, getconf gets a setting which equals "asdf", could I have it allocate so it *just* fits "asdf"? But then if it gets another setting which equals "something", it allocates to fit "something"?

If so, then that solves the #1 problem I have with C: having to know the maximum size of variables before the program even runs.

Well if you read in the line, and it's not a comment, then apparently the format is

Code:

name = value

. That line is read into your temporary buffer named line. You can then find the = sign and determine the length of the value string based on the length from that = to the end of the line string, and dynamically allocate your value pointer to the appropriate size and copy it in.

Don't forget: fgets reads the newline character also into the buffer, so you will need to remove that before you copy the value into the allocated memory.

Okay. So I have the function set to realloc "value" to the size of the string to be returned.

Back to main; if I declare the pointer as:

Code:

char *blah = getconf(false,"test");

will it automatically allocate the necessary memory? If not, is there some way I can get the size of the returned string before I actually return the string?

You don't need to realloc memory. Allocate the memory in the function, return the pointer to the caller, the caller frees the memory when it's done with it.

Okay. The function now is able to read a line from config, and return it to the caller.
The problem now, is that it always returns the first line of configuration.

The config file looks like this:

Code:

test = abc
something = somethingelse
this = that

I request a line and print it out with:

Code:

char *blah = getconf(true,"something");

For some reason, getconf always returns "abc" -- the first line of the config file -- when it should be equaling "somethingelse", because it's something's value.

Here is the code I'm working with:

Code:

int main()
{
	//This isn't the full main function, but it's the part I'm working with
	char *blah = getconf(true,"something");
	printf("%s\n",blah);
	return(0);
}

char *getconf(bool writelog,char get[])
{
	char line[256];
	FILE *config;
	int a = 0;
	char setting[25];
	char val[150];
	
	config = fopen("test.conf","r");
	
	while(fgets(line,sizeof(line),config) != NULL) {
		a++;
		
		if(line[0] == '#') continue; //If the line is a comment, skip it
		if(sscanf(line,"%s = %s",setting,val) != 2) {
			if(writelog == true) {
				mlog = fopen("test.log","a");
				fprintf(mlog,"%s config: %i: syntax error!\n",gettime(),a);
				fclose(mlog);
			}
			continue;
		}
		if(strcmp(get,setting)) {
			char *value = (char*)malloc(strlen(val) * sizeof(char));
			strcpy(value,val);
			return(value);
		}
	}

	if(writelog == true) {
		mlog = fopen("test.log","a");
		fprintf(mlog,"%s config: %s: value not found!\n",gettime(),get);
		fclose(mlog);
	}
	
	return("no value");
}

Thanks again!

Code:

if(strcmp(get,setting))

strcmp returns 0 on a match, as it's measuring the difference between the two strings. 0 is false, your test is for true.