CODEWITHSUNDEEP
cmalloc
Abraham Adam
Abraham Adam

Reputation: 633

malloc causing a SIGSEGV:Segmentation fault

typedef struct Matrix
{
    double * matrix;
    int sizex;
    int sizey;
}Matrix;

int nn = 257;
Matrix * g = (Matrix *)malloc(sizeof(Matrix *));
g->matrix = malloc(sizeof(double) * nn * nn);
g->sizex = nn;
g->sizey = nn;

This code give an error when it gets to g->matrix = malloc(sizeof(double) * nn * nn); anyone see a problem with it ?

edit: found problem to be accessing unallocated memory in a place before the allocation shown, it was causing a SIGSEGV:Segmentation fault.

Upvotes: 3

Views: 2043

Answers (7)

gdjs
gdjs

Reputation: 1

I'll just add the direction of not casting malloc's return. It is unnecessary, pollutant, and might lead to unwanted behavior (as described here: http://c-faq.com/malloc/mallocnocast.html

Upvotes: 0

Tony Delroy
Tony Delroy

Reputation: 106236

Matrix * g = (Matrix *)malloc(sizeof(Matrix *));

This reserves enough heap space for a pointer to a Matrix, but you want enough heap space for the Matrix itself. Try:

Matrix* g = (Matrix*)malloc(sizeof(Matrix));

For a complete, working program:

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

typedef struct Matrix {
    double * matrix;
    int sizex;
    int sizey;
} Matrix;

int main()
{
    int nn = 257;
    Matrix * g = (Matrix *)malloc(sizeof(Matrix));
    if (g == NULL)
    {
        printf("g = malloc() failed\n");
        return 1;
    }
    g->matrix = malloc(sizeof(double) * nn * nn);
    g->sizex = nn;
    g->sizey = nn; 
    printf("g %p, g->matrix %p, g->sizex %d, g->sizey %d\n",
            g, g->matrix, g->sizex, g->sizey);
    return 0;
}

Output on my Linux box:

g 0x8822008, g->matrix 0xf6ea6008, g->sizex 257, g->sizey 257

Upvotes: 0

codaddict
codaddict

Reputation: 455380

You need to pass malloc the sizeof the Matrix not sizeof pointer to the Matrix.

Change

Matrix * g = (Matrix *)malloc(sizeof(Matrix *));
                                           ^^

to

Matrix * g = (Matrix *)malloc(sizeof(Matrix));

Also you must always check the return value of malloc and make sure allocation succeed before you go and use the allocated memory.

Upvotes: 6

Neilvert Noval
Neilvert Noval

Reputation: 1695

Parameter of your sizeof should not be a pointer.

Upvotes: 0

wkl
wkl

Reputation: 80031

You do not allocate memory for a Matrix object, you are allocating for a pointer.

Change your first malloc call to do this:

Matrix * g = malloc(sizeof(*g));

I prefer this style as you do not need explicit pointer cast from void * in C, and you are allowed to do sizeof on the variable's underlying type. This could save you headaches just in case you change the type of g (or for future code).

Similarly for style:

g->matrix = malloc(sizeof(double) * nn * nn);

should be:

g->matrix = malloc(sizeof(*(g->matrix)) * nn * nn);

Upvotes: 0

developmentalinsanity
developmentalinsanity

Reputation: 6249

Matrix * g = (Matrix *)malloc(sizeof(Matrix *));

should be

Matrix * g = (Matrix *)malloc(sizeof(Matrix));

You're only allocating the size of a pointer to Matrix, rather than a Matrix itself.

Upvotes: 0

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215547

I'm guessing you're using some ancient 16-bit compiler, probably Turbo C. Junk it and get gcc, either djgpp if you want to build DOS programs or mingw or cygwin if you want to build Windows programs.

Assuming I'm right, 257*257 overflows the maximum addressable size, 65536, not to mention what happens when you multiply it by 8.

Edit: OP changed the question after I wrote this so it may be completely off. If so I'll delete it.

Upvotes: 0

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