How to get rid of segmentation fault? - c

Question

#include <stdio.h>
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>
int getCharNum(int a) { return a / (8 * sizeof(char)); }
int getBitNum(int a) { return a % (8 * sizeof(char)); }
int fromCharNum(int a) { return a * 8 * sizeof(char); }
int get2DimI(int a) { return getCharNum(a) / 187500; }
int get2DimJ(int a) { return getCharNum(a) % 187500; }
void rle_compress(char *src, char *dst, int ls, int *ld) {
uint8_t t[129];
int i, j = 0, k = 0, keep;
char out[187500];
t[0] = src[j];
while (j++ < ls) {
t[1] = src[j];
if (t[0] != t[1]) {
i = 1;
if (j < ls)
do
t[++i] = src[++j];
while (j < ls && i < 128 && t[i] != t[i - 1]);
if ((keep = t[i] == t[i - 1]))
--i;
out[k++] = (char)i;
t[0] = t[i];
if (!keep)
continue;
}
i = 2;
do
t[1] = src[++j];
while (++i < 130 && t[0] == t[1]);
out[k++] = i + 125;
out[k++] = t[0];
t[0] = t[1];
}
ld = &k;
dst = out;
}
void rle_extract(char *src, char *dst, int ls) {
int i, j, l = 0, k = 0, max;
char out[187500];
j = 0;
while (k + 2 < ls) {
i = src[k++]; //segfault
j = src[k++];
max = i + (i < 128 ? 1 : -126);
while (max--)
out[l++] = j;
}
dst = out;
return 0;
}
int main(void) {
int32_t n = 0;
scanf("%d", &n);
int32_t a[n];
int32_t b[] = { -1, -1, -1 };
char **count;
count = (char**)malloc(1000 * sizeof(char*));
int count_l[] = { [999] = 0 };
for (int i = 0; i < 1000; ++i) {
count[i] = (char*)malloc(187500 * sizeof(char));
char *temp = NULL;
rle_compress(count[i], temp, 187500, &count_l[i]);
free(count[i]);
count[i] = temp;
}
for (int i = 0; i < n; i++)
scanf("%d", &a[i]);
for (int i = 0; i < n; i++) {
char *src = count[get2DimI(a[i]) / 187500];
char dst[187500];
rle_extract(src, dst, count_l[i / 187500]);
dst[get2DimJ(a[i])] ^= 1 << (getBitNum(a[i]));
rle_compress(dst, count[get2DimI(a[i]) / 187500], 187500, &count_l[i]);
}
int32_t mv = 187500000 / (8 * sizeof(char));
int j = 0;
for (int i = 0; i < mv; i++) {
char *src = count[i / 187500];
char dst[187500];
rle_extract(src, dst, count_l[i / 187500]);
int32_t x = dst[i % 187500];
if (x == 0)
continue;
for (int k = 0; k < 8 * sizeof(char); k++) {
if ((x >> (k)) & 1) {
b[j++] = fromCharNum(i) + k;
}
}
//free(dst);
}
int m1 = min(b[0], min(b[1], b[2])),
m3 = max(b[0], max(b[1], b[2])),
m2 = b[0] + b[1] + b[2] - m1 - m3;
printf("%d %d %d", m1, m2, m3);
for (int i = 0; i < 1000; ++i)
free(count[i]);
free(count);
return 0;
}
How to fix this code?
I'm trying to compress byte array (which should compress greatly as n should be <=1500000 and numbers are from 0 to 1.5*10^9), but code gives me segfault on all testing inputs which I've tried. Without compress everything worked like a charm, but needed a lot of memory (and limits are 64MiB).

Answer 1

The code is obscure but there are some major problems:
void rle_extract(char *src, char *dst, int ls) does not take the output buffer from its caller, nor does it return a pointer to it: dst = out; just updates the argument value, not the caller's variable passed as an argument. Furthermore return 0; from a void function is incorrect too.
in any case, rle_extract should not return its local out buffer because it is only defined during the execution of the function and is discarded as soon as the function returns.
You should either pass the buffer as an argument or allocate it locally and return the pointer to the caller.
There might be other problems, there is no explanation for what the code is supposed to do.