Segmentation error due to allocation issues? - c

Question

Hi so im preety new to coding and I have recently hit a brickwall
This program has a segmentation fault debug and I think it has to do with allocating space for the buffer array or the input file , as those kept popping up as solutions in my search for an answer.
If you know what I did wrong I would appreciate if you told me rather than giving me the solution. Also Im completely lost on how to allocate memory to anything , so it would be great if someone explained how to allocate memory in context to the code.
It is supposed to read a file structured like this
2
3 4
3 4
where 2 = how many files are downloading, 3 = the Kb/s of file( each line represents a file) , and 4 = time remaining until done. The program is supposed to output a file out that has the time needed for all the things to download(mind you when a file finishes the speed goes up to others)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
FILE *in = fopen("download.in", "r");
FILE *out = fopen("download.out", "w");
int i, n, j, a, k, o;
char buffer[100];
fgets(buffer, 10, in);
sscanf(buffer, "%d", &n);
int tn[n];
int xn[n];
i = 0;
while (i < n) {
fgets(buffer, 100, in);
sscanf(buffer, "%d %d", &tn[i], &xn[i]);
++i;
}
for (i = 0; i < n; ++i){
for (j = i + 1; j < n; ++j){
if (tn[i] > tn[j]){
a = tn[i];
tn[i] = tn[j];
tn[j] = a;
a = xn[i];
xn[i] = xn[j];
xn[j] = a;
}
}
}
i = 1;
float b ;
k = 0;
o = 1;
while(o < n){
if(xn[o] == xn[o - 1]){
k = tn[o] + tn[o - 1]
;
}
else{
b = (tn[o] * xn[o]) / (tn[o] + tn[o - 1] + k);
k = 0;
}
++o;
}
fprintf( out, "%f", b );
fclose(in); fclose(out);
return 0;
}
I know the answer it gives is not accurate but I want to fix the segmentation fault first then deal with that
gdb r then gdb bt returns this
(gdb) r
Program received signal SIGSEGV, Segmentation fault.
_IO_fgets (buf=0x7fffffffeb30 "", n=10, fp=0x7fffffffeac0) at iofgets.c:50
50 iofgets.c: No such file or directory.
(gdb) bt
#0 _IO_fgets (buf=0x7fffffffeb30 "", n=10, fp=0x7fffffffeac0) at iofgets.c:50
#1 0x0000000000400766 in main () at main.c:11
codeblocks says the code is ok and the site I am trying to give this says there is a segmentation fault aswell.

Answer 1

Here is code with segfault fixed. I had to (1) specify absolute path and (2) change the loop to a for loop.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
FILE *in = fopen("/home/developer/CLionProjects/untitled4/download.in", "r");
FILE *out = fopen("/home/developer/CLionProjects/untitled4/download.out", "w");
int i, n, j, a, k, o;
char buffer[100];
fgets(buffer, 10, in);
sscanf(buffer, "%d", &n);
int tn[n];
int xn[n];
for (i = 0; i < n; i++) {
if (!fgets(buffer, 100, in)) break;
if (sscanf(buffer, "%d %d", &tn[i], &xn[i]) < 2) break;
}
for (i = 0; i < n; ++i) {
for (j = i + 1; j < n; ++j) {
if (tn[i] > tn[j]) {
a = tn[i];
tn[i] = tn[j];
tn[j] = a;
a = xn[i];
xn[i] = xn[j];
xn[j] = a;
}
}
}
i = 1;
float b;
k = 0;
o = 1;
while (o < n) {
if (xn[o] == xn[o - 1]) {
k = tn[o] + tn[o - 1];
fprintf(out, "Some number: %d\n", k);
} else {
b = (tn[o] * xn[o]) / (tn[o] + tn[o - 1] + k);
k = 0;
fprintf(out, "Some number: %d\n", k);
}
++o;
}
fclose(in);
const char *text = "Write this to the file";
fprintf(out, "Some text: %s\n", text);
fclose(out);
int c;
FILE *file;
file = fopen("/home/developer/CLionProjects/untitled4/download.out", "r");
if (file) {
while ((c = getc(file)) != EOF)
putchar(c);
fclose(file);
}
return 0;
}