Funny behavior from sscanf - c

Question

I have a piece of code below which basically reads a text file data.txt and prints to the console. The content of data.txt is below the code listing;
#include "stdio.h"
#define BUFFER_SIZE 93
int main(int argc, char *argv[]){
const char *datafile;
char line[BUFFER_SIZE],*string1;
int a,b,c,d,e;
FILE * File_ptr;
datafile = "data.txt";
File_ptr = fopen(datafile,"r");
if(File_ptr == NULL ){
printf("Error opening file %s\n",datafile);
}
while(fgets(line,BUFFER_SIZE,File_ptr) != 0){
puts(line);
sscanf(line, "%d %d %d %d %d %s", &a,&b,&c,&d,&e,string1);
printf("%d, %d, %d, %d, %d, %s\n",a,b,c,d,e,string1);
}
fclose(File_ptr);
}
Content in data.txt:
100 200 888 456 5443 file1.abc
180 670 812 496 5993 file2.abc
160 230 345 546 5123 file3.abc
23 455 342 235 214 file4.abc
233 5455 3142 2435 1214 file5.abc
What I don't understand is: if the BUFFER_SIZE is defined as < 97, the output would be like this:
100 200 888 456 5443 file1.abc
100 200 888 456 5443 (null)
180 670 812 496 5993 file2.abc
180 670 812 496 5993 (null)
160 230 345 546 5123 file3.abc
160 230 345 546 5123 (null)
23 455 342 235 214 file4.abc
23 455 342 235 214 (null)
233 5455 3142 2435 1214 file5.abc
233 5455 3142 2435 1214 (null)
If the BUFFER_SIZE is defined as 97 ~ 120, the output would be OK, like this:
100 200 888 456 5443 file1.abc
100 200 888 456 5443 file1.abc
180 670 812 496 5993 file2.abc
180 670 812 496 5993 file2.abc
160 230 345 546 5123 file3.abc
160 230 345 546 5123 file3.abc
23 455 342 235 214 file4.abc
23 455 342 235 214 file4.abc
233 5455 3142 2435 1214 file5.abc
233 5455 3142 2435 1214 file5.abc
If the BUFFER_SIZE is defined as >120, a segmentation fault will be triggered at the sscanf() call.
Can someone enlighten me of the reason for this behavior?

Answer 1

Your string1 is an uninitialized pointer that points nowhere. Your sscanf attempts to store data into the location pointed by string1, which is nowhere. Your program exhibits undefined behavior. It can segfault, it can output nonsense, it can do anything. The actual behavior of such program can change for no explainable reasons. This is exactly what you observe.