How to fix segmentation fault 11 error?

Question

I want the main returns the position of the occurrences of "mdl" in "dati". I set up the "schema" function to find the starting point of each occurrence, but when i run the program from the command line, it returns:

Segmentation fault: 11

I don't know how to fix the problem. Here's the code:

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

int schema(int testo[], int nT, int modello[], int nM, int primo) {

    int i, j, k;
    static int r[12];

    j=0;
    for(i=primo; i<nT; i++) {
        if(testo[i] == modello[0] && testo[i+1] == modello[1] && testo[i+2] == modello[2] && testo[i+3] == modello[3] && testo[i+4] == modello[4] && testo[i+5] == modello[5] && testo[i+6] == modello[6] && testo[i+7] == modello[7]) {
        r[j] = i+1;
        j++;
        }
    }

    return *r;
}



int main(int argc, char** argv) {

    FILE *in;
    FILE *out;

    int i, m;
    const int n = 100;
    int dati[n];
    int *soluzione;
    int start;

    if ((in=fopen("dati.txt", "r"))==NULL){
        return -1;
    }

    for(i=0; i<n; i++) {
        if (fscanf(in, "%d", &dati[i]) < 0){
            fclose(in);
            return i;
        }
    }

    int mdl[] = {0,0,0,1,1,1,0,1};
    m = sizeof(mdl)/sizeof(mdl[0]);

    *soluzione = schema(dati, n, mdl, m, start);

    for(i=0; i<12; i++) {
        printf("- risultato[%d] = %d\n", i, soluzione[i]);
    }

    //out = fopen("risultati.txt", "w");
    //...

    fclose(in);

    return 1;
}

I have to use the function to find the occurrences, I cannot use other ways.

Protip number one: enable warnings for your compiler, and pay attention to them (preferably by also setting them to be treated as errors). — n. m. could be an AI, Jan 19 '17 at 16:42
`*soluzione = schema(dati, n, mdl, m, start);` : `soluzione` and `start` are uninitialized. — BLUEPIXY, Jan 19 '17 at 16:44
@n.m. I have warnings enabled, and when i compiled the program there weren't errors or warnings... — FranzGoogle, Jan 19 '17 at 17:16
What compiler are you using? Consider getting a better one. An uninitialised variable like soluzione is an easy game for any modern compiler. — n. m. could be an AI, Jan 19 '17 at 17:28
[this](http://coliru.stacked-crooked.com/a/9707908cd47251fd) is what gcc outputs. — n. m. could be an AI, Jan 19 '17 at 17:44

dbush · Accepted Answer · 2017-01-19T17:40:22.443

3

You're dereferencing the pointer soluzione, but it was never initialized with a value:

int *soluzione;
...
*soluzione = schema(dati, n, mdl, m, start);

Reading an uninitialized value, as well as subsequently dereferencing that uninitialized value, invokes undefined behavior. In this case, it manifests in a segmentation fault.

You don't need a pointer in this case. Just declare the variable as an int.

int soluzione;
...
soluzione = schema(dati, n, mdl, m, start);

You also don't initialize start. As a result, you index into testo at an unknown location which could be outside the bounds of the array. This also invokes undefined behavior.

EDIT:

It looks like you're actually returning the wrong datatype from schema. If you want to return a pointer to the local array r (which in this case is fine since it's declared as static, the function needs to return an int * and you should return r.

Then in main you would keep soluzione as a pointer but assign to it directly.

int *schema(int testo[], int nT, int modello[], int nM, int primo) {
    ...
    return r;
}

int main(int argc, char** argv) {
    ...
    int *soluzione;
    ...
    soluzione = schema(dati, n, mdl, m, start);

edited Jan 19 '17 at 17:40

answered Jan 19 '17 at 16:42

dbush

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Note that `start` is not initialized either. – Vladislav Martin Jan 19 '17 at 16:45
@VladislavMartin Good catch. Edited to include. – dbush Jan 19 '17 at 16:48
@dbush Thank you. I understood and I fixed the code. But now I've got a new problem: it produces an error: `error: subscripted value is not an array, pointer, or vector printf("- risultato[%d] = %d\n", i, soluzione[i]);` – FranzGoogle Jan 19 '17 at 17:25
@FranzGoogle See my edit. – dbush Jan 19 '17 at 17:40
@dbush Thank you very much! Now it works perfectly! I have only one more question: if I want to keep `schema` (and not `*schema`), what I have to do with `r` and `soluzione`? – FranzGoogle Jan 19 '17 at 17:54
@FranzGoogle You need some way of getting the address of `r` back to the caller. So either the function returns an `int *` or you need to add a parameter of type `int **` and pass in the address of `soluzione`. – dbush Jan 19 '17 at 17:56
@dbush OK, I understood it. Thank you again! – FranzGoogle Jan 19 '17 at 18:00

score 1 · Answer 2 · answered Jan 19 '17 at 16:42

I suppose the error lies in the following code segment:

for(i=primo; i<nT; i++) {
    if(testo[i] == modello[0] && testo[i+1] == modello[1] && testo[i+2] == modello[2] && testo[i+3] == modello[3] && testo[i+4] == modello[4] && testo[i+5] == modello[5] && testo[i+6] == modello[6] && testo[i+7] == modello[7]) {

Note that you pass dati, which is an integer array of size n, as testo and you pass n as the value for nT. Hence, testo is of size nT. But in you loop, where i potentially runs until nt-1, you access testo[i+7], which exceeds the boundaries of testo, right?

I think you're right, thank you. But in an easier version of the same program it didn't make problems, I don't know why... I'm not an expert, I'm learning it. — FranzGoogle, Jan 19 '17 at 17:29

How to fix segmentation fault 11 error?

2 Answers2

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