segmentation fault with array of string from file

Question

I think I get understand how string works, but some how get a segmentation error when try to run this. I'm trying to create array of string that i read from file f. Also any comments for optimisation and or a better way to code (especially using pointer)is appreciated.

char a[700000][120];
char str[120];
int i=0,l,p;
while (fgets(str,120,f)) {
    strcpy(a[i],str);
    i++;
    }
int n=i;
for (i=0;i<=3;i++) {
    printf("%s\n",a[i]);
}

I think that StackOverflow occured, due to crazy size of first array `a`, use dynamic memory instead. — PcAF, May 13 '16 at 05:44
I realy have a file with near 700000 lines with near 100 char in each line, that I need to sort by string length . — ZeroVash, May 13 '16 at 05:50
how do you know that no line is more than 120 chars? how many loops does it get through before seg fault? maybe you should print some debug as it loops through the fgets? — tofutim, May 13 '16 at 05:51
it is just a strings with long numbers, and the file size is really huge — ZeroVash, May 13 '16 at 05:53
well because I wrote this lines ))) and I know that there is no lines more than 120 chars. — ZeroVash, May 13 '16 at 05:54
yep, i think I will need to do malloc, but do I need malloc to array Or to the string that I will add? — ZeroVash, May 13 '16 at 05:56
Can you think of a good reason you don't just `fgets` directly into the allocated memory (which would also have been appropriate for your posted sample were it not for that oversized `a` array). In other words, `str` is pointless. — WhozCraig, May 13 '16 at 06:05
The problem is not the strings (fgets reads size-1 max then adds the zero), it's definitely the stack size limit. 100MB is a lot in a stack! — Déjà vu, May 13 '16 at 06:26

tofutim · Accepted Answer · 2016-05-13T06:41:52.190

See if this helps

char **a;
int i;

a = malloc(700000 * sizeof(char*));
for (i = 0; i < 700000; i++) {
    a[i] = malloc(120*sizeof(char));
}

// read file here instead
strcpy(a[0],"hello");
strcpy(a[1],"goodbye");
strcpy(a[2],"yes");

for (i=0;i<=3;i++) {
    printf("%s\n",a[i]);
}

Per Michi, remember to free the memory afterwards.

for (i = 0; i < 700000; i++) {
    free(a[i]);
}
free(a);

Appendix Turns out you can check the stack size AND change it. Consider this

struct rlimit rl;
int result;

result = getrlimit(RLIMIT_STACK, &rl);

printf("stack limit %d\n", rl.rlim_cur);
printf("stack limit %d\n", rl.rlim_max);
return 0;

It gives me

stack limit 8388608
stack limit -1

(there is the 8MB).

You should call free or at least to mention about the leak – Michi May 13 '16 at 06:26 — Michi, May 13 '16 at 06:26

score 2 · Answer 2 · answered May 13 '16 at 06:20

2

There is a limit of array allocation, the size u are trying causes stack overflow of the function, because your array can't fit into the memory allocated to the function stack.

There is a limit of 8MB on the maximum size of objects, due to internal compiler implementation limits. You should use malloc() to create large arrays instead.

answered May 13 '16 at 06:20

msc

33,420
29
119
214

does the max size of objects depend on the compiler? or is it always 8 MB? – tofutim May 13 '16 at 06:33
apparently you can set the stack size programmatically - I had no idea http://stackoverflow.com/questions/2275550/change-stack-size-for-a-c-application-in-linux-during-compilation-with-gnu-com/2284691#2284691 – tofutim May 13 '16 at 06:35
The stack size as far as I know is platform independent and of course there is a default stack size. Could you show me some reference where says that the default one is 8MB? – Michi May 13 '16 at 06:41

score 0 · Answer 3 · answered May 13 '16 at 06:09

I think there is no need of creating str variable, you could have used the a itself. Moreover as mentioned in the comments too, try using dynamic memory, as most programmers don't use stack for huge allocations. May be heap may have greater size than stack.

Larry.He · Answer 4 · 2016-05-13T06:44:56.097

0

In Linux Environment, ulimit -s can return the stack size.

root@ubuntu:# ulimit -s
8192

It means the max stack space that the system support is 8192 KB, that is 8MB. test program as follows, try to modify array size from 8*1024 to 7*1024.

#include<stdio.h>

void test()
{
}

int main()
{
    char a[7*1024][1024];
    test();
    return 0;
}

edited May 13 '16 at 06:44

answered May 13 '16 at 06:36

Larry.He

604
5
16

score 0 · Answer 5 · answered May 13 '16 at 07:52

You can try this.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void main(void) {

    FILE *fp = NULL;
    fp = fopen("read.txt", "rb");
    if(fp == NULL)
        printf("failure\n");
    else
        printf("success\n");

    char buffer[4096];
    while (fgets(buffer, sizeof(buffer), fp) != 0)
        fputs(buffer, stderr);

}

segmentation fault with array of string from file

5 Answers5