SegFault when reading a file

I keep getting a seg fault at the end of a file read. Each line of the file is around 500+ characters so MAX_LEN is 600. Any ideas?

Code:

//~ _________________________________________________
//~ Read GIS data
void read_gis(char *gis,GooCanvasItem *canvas)
{	
	FILE * gis_file;
	//~ gchar *path_data;
	char path[MAX_LEN + 1];
	int count = 0;
	size_t i;
    
	gis_file = fopen(gis,"r");
	if (gis_file != NULL)
	{
		while ( fgets(path, MAX_LEN+1, gis_file) != NULL )
		{
			count++;
			printf("%d %s\n\n",count,path);
			i = strlen(path);
			printf("strlen %d\n",i);
			path[i+1] = '\0';
			
		
			//~ path_data = path;
			//~ goo_canvas_path_new(canvas,path_data,"stroke-color","white","line-width",5.0,NULL);
			//~ goo_canvas_path_new(canvas,"M 20,20 L 4000,4000","stroke-color","white","line-width",5.0,NULL);
		}
	}
	fclose (gis_file);
}

So how do you know this function is to blame?

> i = strlen(path);
> path[i+1] = '\0';
What exactly are you trying to do here?

p[ strlen(p) ] indexes the \0 character at the end of a string.
If fgets() FILLED the buffer completely, there is still a \0 at the end (the very end), and all you managed to do here was trash someone else's memory.

Oh, and your fclose() call should be inside the if NULL test

You're right. I does not occur in this subroutine. It actually occurs in the one below, right below the printf statement. It seems the fscanf is causing it. Any ideas?

Code:

//~ _________________________________________________
//~ Read RADAR data

void read_radar(char *radar,GooCanvasItem *group)

{	
	FILE * radar_file;

	//~ Open File
	if ((radar_file = fopen(radar,"r")) != NULL)
	{
		char *call;
		float lat;
		float lon;
		char *name;

		while(!feof(radar_file))
		{
			printf("HERE3\n");
			fscanf(radar_file,"%s %f %f %s",call,&lat,&lon,name);
			printf("%s %f %f %s\n",call,lat,lon,name);

			lon = (top_long - lon) / -ps;
			lat = (top_lat - lat) / ps;

			printf("%s %f %f %s\n",call,lat,lon,name);
			goo_canvas_ellipse_new(group,lon,lat,5.0,5.0,"fill-color","yellow","stroke-color","black",NULL);
               goo_canvas_text_new(group,name,lat,lon,-1.0,GTK_ANCHOR_CENTER,"font","Sans Bold 15","fill-color","black",NULL);
		}
		fclose (radar_file);
	}
	else
	{
		printf("Radar File Not Opened\n");
		exit(1);
	}
}

You need to allocate memory for name and call before you can read data into them. All you have there are pointers to Dog knows where.

Hi.

fscanf fails because call and name pointers point to "only-god-knows-where". Try replacing them with character arrays instead like this:

Code:

		char call[512];
		float lat;
		float lon;
		char name[512];

		while(!feof(radar_file))

That worked! And finally goo is displaying things. But I have suddenly started to get seg faults again. this time I cant really pin point the source but I think it is the while causing it since all of my printf's show up except the 4th one.

Code:

//~ _________________________________________________
//~ Read RADAR data
void read_radar(char *radar,GooCanvasItem *group)
{	
	FILE * radar_file;
	
	//~ Open File
	if ((radar_file = fopen(radar,"r")) != NULL)
	{
		char call[4];
		float lat;
		float lon;
		char name[25];

		while(!feof(radar_file))
		{
			fscanf(radar_file,"%s %f %f %s",call,&lat,&lon,name);
			printf("%s %f %f %s\n",call,lat,lon,name);
			
			lon = (top_long - lon) / -ps;
			lat = (top_lat - lat) / ps;
			
			printf("%s %f %f %s\n",call,lat,lon,name);
			
			printf("HERE1\n");
			
			goo_canvas_ellipse_new(group,lon,lat,5.0,5.0,"fill-color","yellow","stroke-color","black",NULL);
			printf("HERE2\n");
			goo_canvas_text_new(group,name,lat,lon,-1.0,GTK_ANCHOR_CENTER,"fill-color","green",NULL);
			printf("HERE3\n");
		}
		printf("HERE4\n");
		fclose (radar_file);
	}
	else
	{
		printf("Radar File Not Opened\n");
		exit(1);
	}
}

> void read_gis(char *gis,GooCanvasItem *canvas)

Then you post
> void read_radar(char *radar,GooCanvasItem *group)

> You're right. I does not occur in this subroutine. It actually occurs in the one below
Thanks

But there are more errors in this function, besides those already mentioned. And they're answered in the FAQ.

Originally Posted by deadpickle

But I have suddenly started to get seg faults again.

A little tip: when using printf for debugging like this, place a fflush() call afterwards:

Code:

printf("HERE2"); fflush(stdout);

because with a segfault, execution may have already passed this point but not yet flushed output to the console (and it will never happen after the fault), so you can get some misleading results. But with the fflush(), the printf statement will appear immediately.

You might also want to try a real debugger like "gdb" for this. It is one thing they are REALLY good at -- it will give you the exact line that caused the fault 80% of the time.

Finally got GDB to work after installing a train of debugs. It is as I expected:
0xb75d70a1 in _IO_feof (fp=0x88b6600) at feof.c:37
37 _IO_flockfile (fp);

The error seems to be in the while loop. Any ideas on how to fix it?

Stop using feof to control your loop.


Quzah.

At the segfault, you then use the 'bt' command to tell you all the nested functions which have been called to reach that point.

You then look at the innermost one of that list which is YOUR code, and start looking really hard at all the parameters you pass to whatever library function you happen to be calling.

It might not be the answer, but it's a good (and necessary) place to start looking.

Thank you for all of your help. Its good to learn how to debug a program.

From what this sounds like to me is that there is an error in reading the radars.gis file at the end. The line it points to looks like such:

Code:

while(!feof(radar_file))

#1 0x080492fd in read_radar (radar=0x8049b68 "/mnt/share/uas/GRRUVI2_uc/radars.gis",
group=0x88aeae8) at goo_0.011.c:68
#2 0x080498f0 in main (argc=Cannot access memory at address 0x6
) at goo_0.011.c:193

Remember when I told you to stop using feof to control your loop?


Quzah.

Post read_radar() again, and put a comment next to line 68.
You did look at line 68 to try and figure out why it would barf?

Code:

//~ _________________________________________________
//~ Read RADAR data
void read_radar(char *radar,GooCanvasItem *group)
{	
	FILE * radar_file;

	//~ Open File
	if ((radar_file = fopen(radar,"r")) != NULL)
	{
		char call[4];
		float lat;
		float lon;
		char name[25];

		while(!feof(radar_file))    //~ the line causing the pain
		{
			fscanf(radar_file,"%s %f %f %s",call,&lat,&lon,name);
			printf("%s %f %f %s\n",call,lat,lon,name);
			
			lon = (top_long - lon) / -ps;
			lat = (top_lat - lat) / ps;
			
			printf("%s %f %f %s\n",call,lat,lon,name);
			
			goo_canvas_ellipse_new(group,lon,lat,5.0,5.0,"fill-color","yellow","stroke-color","black",NULL);
			goo_canvas_text_new(group,name,lat,lon,-1.0,GTK_ANCHOR_CENTER,"fill-color","green",NULL);
		}
		fclose (radar_file);
	}
	else
	{
		printf("Radar File Not Opened\n");
		exit(1);
	}
}

THanks for all the help.

If the way I do the while loop is wrong, then what are my options for looping each line of a file and reading its contents using fscanf?