Q:

Stack smashing detected and no source for getenv

I'm having the weirdest problem ever when programming in C. My function sometimes runs and other times it doesn't, and even though I tried searching for these errors (stack smashing detected and no source for getenv) I can't seem to find the answer to why it fails sometimes. I tried debugging it and it only has a problem in the last character (a "}"), so it runs all those other functions (and they have been tested a lot separately) but sometimes it just falls apart and doesn't run the last function (it works, that I can guarantee, because some other times it even runs inside this function). Also the few last times I ran the function it gave Segmentation Fault even though sometimes it ran all the way to the end. Is there any way I can debug this problem? Here goes my code:
void main(int argc, char * argv[]) {
FILE * fin=fopen(argv[1],"r");
char v[1024];
int col;
matrix m=emptyS();
while(fscanf(fin, "%s",v)!=EOF) {
    int i=0;
    int * w = (int*) malloc(sizeof (int));
    int str=strlen(v);
    int size=0;
    while(i<str) {
        char a[4];
        int y;
        for(y=0;y<4;y++)
            a[y]='\0';
        int x=0;
        while(v[i]!=',') {
            a[x]=v[i];
            i++;
            x++;
        }
        i++;
        size++;
        w=realloc(w,size*sizeof(int));
        w[size-1]=atoi(a);
    }
    m=add(m,w,size);
    col=size;
}
fclose(fin);
graphW wg=discWD(col-1);
int k=0;
while(k<(col-2)) {
    int j=k+1;
    while(j<(col-1)) {
        wg=add_Wedge(wg,k,j,mi(m,k,j,col));
        j++;
    }
    k++;
}
        int* ms=MST(wg);
graph gbayes=tree(wg,ms);
bayes b=newBN(gbayes,m);
FILE * fout=fopen(argv[2],"w");
serialize(b,fout);
fclose(fout);
}  

Thank you in advance!

A:

You don't check x for going out of bounds. Valgrind would have told you.
while(i<str) {
        char a[4] = { 0 };
        int x=0;
        while(v[i]!=',') {
            a[x]=v[i]; /* here you access a[x] without check for x */
            i++;
            x++; /* Here it may go >= 4*/
        }
        i++;
        size++;
        w=realloc(w,size*sizeof(int));
        w[size-1]=atoi(a);
    }

