Segmentation fault, fgets in while loop

Question

I try to read in a file multiple times instead of just once. While trying that I got alot of segementation faults. The part of the program with the while loop looks like this:

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

/* General use buffer */
#define STRLEN 8196
char    string[STRLEN];


int lines = 1024;
char    **line;
int linemax;

int longest=0;


int main(){
int len,i;
int zwei = 1;
FILE * fp;
char    *s;
int debug =  0;
line=(char **)malloc(sizeof(char *) * 1024);
do{
    if ( (fp = fopen("rhel7_160731_0606.nmon", "r")) == NULL) {
        perror("failed to open file");      

                perror("fopen");
        exit(75);
    }
printf("where is the problem1,3\n");
    for (i = 0; fgets(string, STRLEN, fp) != NULL; i++) {
        if (i >= lines) {
            lines += 1024;
            line = (char **)realloc((void *)line, sizeof(char *) * lines);
        }

        if (string[strlen(string)-1] == '\n')
            string[strlen(string)-1] = 0;
        if (string[strlen(string)-1] == '\r')
            string[strlen(string)-1] = 0;
        if (string[strlen(string)-1] == ' ')
            string[strlen(string)-1] = 0;
        if (string[strlen(string)-1] == ',')
            string[strlen(string)-1] = 0;
        len = strlen(string) + 1;

        if (len > longest)
            longest = len;
        s = malloc(len);
        strcpy(s, string);
        line[i] = (char *)s;
    }
    linemax = i;
    lines = i;

    if (debug)
        for (i = 0; i < linemax; i++)
            printf("line %d lastline %s\n", i, line[i-1]);

zwei++;

}while(zwei<4);

return 0;
}

It hangs nothing or ends with a segmentation fault.

Program received signal SIGSEGV, Segmentation fault. 0x00000000004009df in main () at test.c:53 53 line[i] = (char *)s; — bastel
– bastel, Commented Sep 16, 2016 at 17:44
So now this is what happens: #0 __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95 #1 0x00007ffff7a8924b in _IO_fgets (buf=0xeb2d50 "\002", n=8196, fp=0xea1c40) at iofgets.c:50 #2 0x000000000040a7f9 in main (argc=8, argv=0x7fffffffdf68) It runs twice and then something goes wrong. Is it a Buffer overflow? — bastel
– bastel, Commented Sep 16, 2016 at 17:56
} while(zwei>4); is never true, so it cannot loop twice, it's not the cause but I wonder how much else is careless. — Weather Vane
– Weather Vane, Commented Sep 16, 2016 at 17:58

RedRidingHood · Accepted Answer · 2016-09-16 18:01:21Z

2

You seem to have forgotten to allocate memory for line. It fails here: line[i] = (char *)s. I think you need to set lines to zero, since you reallocate line only if your iterator i gets grower than lines.

Also, fix this: while(zwei > 4) to while(zwei < 4). And, you need to free the memory you allocate - because you store all the pointers in line, it is not going to be complicated - just one loop.

edited Sep 16, 2016 at 18:01

answered Sep 16, 2016 at 18:00

RedRidingHood

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8 Comments

Weather Vane Over a year ago

It's a global variable, therefore initialised to 0 or NULL which realloc can deal with. But that is a good catch.

RedRidingHood Over a year ago

@WeatherVane I mean, he tries to write data in line[i], but line, as you correctly say, sets to NULL by default.

bastel Over a year ago

how do I free the memory?

RedRidingHood Over a year ago

@bastel Using free( line[i] ) in a single, say, for loop. I have dealt with it putting the following loop right before while statement: for (i = 0; i < lines; i++){free( (void*) line[i] );} But if you want to store all the lines until the end, you may need something more elaborate.

RedRidingHood Over a year ago

@bastel I don't think you need to malloc it first, just set lines to 0, and, as WeatherVane noticed, realloc can deal with NULL and at the first time it will work as malloc

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