c,arrays,pointers,segmentation-fault , C program: Segmentation Fault [closed]

## Question:

Tag: c,arrays,pointers,segmentation-fault

I am currently trying to solve a task, which is quite hard for me, a beginner to C, to handle and so i came to this point where I do not know what to do anymore.

My task is to implement polynomials with several functions.... The functions should be clear when you look at the code I think.

My exact problem is that i dont get a compiler error but a Segmentation Fault. I marked where my attempts to debug lead me to. But I have absolutely no clue on what I have to change. I hope someone can help me fix my code.

So here are the three code parts: Number one: poly.c

``````#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include "poly.h"

struct poly_t {
unsigned degree;
int *coeffs;
};

//constructor: heap
poly_t *poly_alloc(unsigned degree){

poly_t *heap_p;
heap_p = malloc(sizeof(*heap_p)+(degree+1)*sizeof(int)); //or malloc(sizeof(*heap_p)*(degree+1)) furthermore not sure if degree or degree +1

}

//free heap
void poly_free(poly_t *p){
int *coeffs = p->coeffs;
free(coeffs);
free(p);
}

void poly_set_coeff(poly_t *p, unsigned deg, int coeff){
p->degree = deg;
p->coeffs += deg;
p->coeffs[deg] = coeff;

//does not work Segmentation Fault not sure what to do
//p->coeffs += deg;
//*p->coeffs = coeff;
printf("%d",*p->coeffs);
}

//different variations
poly_t *poly_linear(poly_t *p, int a1, int a0){
p->degree=1;
*p->coeffs=a1;
p->coeffs++;
*p->coeffs=a0;
p->coeffs--;
}

poly_t *poly_quadratic(poly_t *p, int a2, int a1, int a0){
p->degree=2;
*p->coeffs=a2;
p->coeffs++;
*p->coeffs=a1;
p->coeffs++;
*p->coeffs=a0;
p->coeffs-=2;
}

//evaluate using horner
int poly_eval(poly_t const *p, int x){
int d = p->degree;
int next;
int *arr = p->coeffs;
int res = arr[d];
for(int i=0; i<=d; i++){
res = res*x+next;
}
return res;
}

//constructor : .txt
poly_t *poly_alloc_d(){
//needs to be finished
}
``````

Number Two: main.c

``````#include <stdlib.h>
#include <stdio.h>
#include "poly.h"

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

if(argc<3){
fprintf(stderr, "syntax: %s x coeffs...", argv[0]);
return 1;
}

poly_t *p = poly_alloc(argc-3);

for(int i = 2; i<argc; i++){
int coeff = atoi (argv[i]);
poly_set_coeff(p, i-2, coeff);
}
return 0;//for debugging

int x=atoi(argv[1]);
int y=poly_eval(p,x);
poly_free(p);
printf("%d\n", y);

return 0;

}
``````

And at last my header file: poly.h

``````#ifndef POLY_H
#define POLY_H

/* unvollständiger Verbund */
typedef struct poly_t poly_t;
poly_t *poly_alloc(unsigned degree);
void poly_free(poly_t *p);
void poly_set_coeff(poly_t *p, unsigned deg, int coeff);
int poly_eval(poly_t const *p, int x);

#endif /* POLY_H */
``````

I appreciate every help. I hope you can help me sort this out and please be patient with me a newbie to C... Thanks in advance

You have not allocated or freed memory correctly, and the function didn't even return the pointer! I think you were trying to allocate one block of memory for the struct and the array it contains, but the struct does not contain an array: only a pointer to an array. You have to allocate them separately:

``````typedef struct {
unsigned degree;
int *coeffs;
} poly_t;

//constructor: heap
poly_t *poly_alloc(unsigned degree){

poly_t *heap_p;
heap_p = malloc(sizeof(*heap_p));
if (heap_p == NULL)
exit (1);           // allocation error

heap_p->coeffs = malloc(degree * sizeof(int));
if (heap_p->coeffs == NULL)
exit (1);           // allocation error
return heap_p;
}

//free heap
void poly_free(poly_t *p){
free(p->coeffs);
free(p);
}
``````

There are other mistakes too, for example

``````p->coeffs += deg;
``````

You mustn't play with the allocated memory pointer, you already did it correctly like this

``````p->coeffs[deg] = coeff;
``````

although you can use an intermediate pointer if you want:

``````int *ptr = p->coeffs + deg;
*ptr = coeff;
``````

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