linear normalization segfault and memory errors - c

Question

I have a segfault in this function...the error message it gives me is this...
*** glibc detected *** /double free or corruption (fasttop): 0x0000000000603250 ***
*** glibc detected *** / memory corruption: 0x00007ffff7dd3710 ***
This is all my code:
struct Image * loadImage(const char* filename)
{
//VARIABLES
struct ImageHeader * hdr2; // on heap, malloc, free
FILE * fptr = fopen(filename, "r");
if (fptr == NULL)
{
return NULL;
}
int retval;
hdr2 = malloc(sizeof(struct ImageHeader));
if (hdr2 == NULL)
{
free(hdr2);
fclose(fptr);
return NULL;
}
retval = fread(hdr2, sizeof(struct ImageHeader), 1, fptr);
if (retval != 1)
{
free(hdr2);
fclose(fptr);
return NULL;
// error
}
if (hdr2 -> magic_bits != ECE264_IMAGE_MAGIC_BITS)
{
free(hdr2);
fclose(fptr);
return NULL;
// error
}
if (hdr2->width == 0||hdr2->height ==0)
{
free(hdr2);
fclose(fptr);
return NULL;
// error
}
struct Image * img = NULL;
img = malloc(sizeof(struct Image));
if(img == NULL)
{
fclose(fptr); // free(img);
return NULL;
//do something
}
img -> width = hdr2 -> width;
img ->height=hdr2->height;
img -> comment = malloc(sizeof(char) * (hdr2->comment_len));
img -> data = malloc(sizeof(uint8_t) * hdr2->width * hdr2->height);
retval = fread(img->comment, sizeof(char), hdr2->comment_len, fptr);
if(img -> comment == NULL)
{
free(hdr2);
free(img->data);
free(img->comment);
free(img);
fclose(fptr);
return NULL;
//do something, don't forget to free whatever you have allocated
}
//lookg at the img-> comment (should free)
if (retval != hdr2->comment_len)
{
free(hdr2);
free(img->data);
free(img->comment);
free(img);
fclose(fptr);
return NULL;
// error
}
/*
if(img->comment[hdr2->comment_len-1]=='\0')/////////////////////////THIS IS THE PROBLEM AREA
{
free(hdr2);
free(img->data);
free(img->comment);
free(img);
fclose(fptr);
return NULL;
}*/
if(img->data==NULL)
{
free(hdr2);
free(img->comment);
free(img);
fclose(fptr);
return NULL;
}
retval = fread(img->data, sizeof(uint8_t),hdr2->width * hdr2->height, fptr);
if (retval != (hdr2->width * hdr2->height))
{
free(hdr2);
free(img->data);
free(img->comment);
free(img);
fclose(fptr);
return NULL;
}
uint8_t j = 0;
if(fread(&j, sizeof(uint8_t),2,fptr)==2)
{
free(hdr2);
free(img->data);
free(img->comment);
fclose(fptr);
}
retval = fread(img ->data, sizeof(uint8_t), hdr2->width * hdr2->height +1, fptr);
if(retval==hdr2->width*hdr2->height+1)
{
free(hdr2);
free(img);
//error
fclose(fptr);
return NULL;
}
fclose(fptr);
return (img);
}
void freeImage(struct Image * image)
{
if(image!=NULL)
{
free(image->data);
free(image->comment);
}
free(image);
}
void linearNormalization(struct Image * image)
{
int index1 = 0;
int index2 = 0;
//int totaldata = image->height * image->width;
int max = 0;
int min = 255;
// if(image -> data[i] > max)
// set max equal to this image
for(index1=0; index1<(image->height * image->width); index1++)
{
if(image->data[index1] > max)
{
max= image->data[index1];
}
if(image->data[index1]<min)
{
min = image->data[index1];
}
}
for(index2 = 0; index2<(image->height * image->width);index2++)
{
image->data[index2] = (image->data[index2]- min) * 255.0 / (max - min);
}
}
I marked the area that is giving me issues...when I leave it in my function does not output the correct data, but does not get a segfault. Can someone tell me what this error code means with respect to that line?

Answer 1

The error message means you are using free on a pointer that you have already freed. The problem is that this kind of error is a bit difficult to find since using free only tells the SO that the memory is free to be used if it needs, but it does not necessarily prevents you from accessing that memory. For example:
int *a = malloc(sizeof(int));
*a = 5;
free(a);
printf("%d", *a);
Will not generate an error, and if you try to free a again, then you will get an error of double free.
From the man pages, free has undefined behaviour if you try to double free something, but if you try free(NULL) there will be no errors. So instead of using a bunch of if's to control what pointers you have to free, just use:
free(ptr);
ptr = NULL;
This way, if you try to free again you won't get and error.

Answer 2

Here's a couple of errors:
uint8_t j = 0;
if(fread(&j, sizeof(uint8_t),2,fptr)==2)
There is surely not room for two uint8_t's in j
retval = fread(img ->data, sizeof(uint8_t), hdr2->width * hdr2->height +1, fptr);
You have a +1 there, but you have not allocated room for that byte when you allocated img->data
For additional help, run your program in valgrind , it is of great help when tracking down bugs such as these.

Answer 3

Both what nos cenouro have pointed out are valid things. Instead of picking all your memory errors apart piece by piece though. Try this thing called Valgrind. It runs your whole program and checks for memory errors everywhere. It's really a life saver.