How to handle memory allocation in C

Question

I am new in C and i am creating a simple program in which i am writing value to the multidimensional error .But I am getting error "Segmentation fault" or some time when i am running the code it is getting in Infinite loop.

int main(){
int i,j;
int num1=10; 
int num2=10; 
double data[num1][num2];


    for(i=0;i<=num1;i++){
        for(j=0;j<=num2;j++){
            if(i==0) {
                data[i][j]=121.21;
            }
            else {
                data[i][j]=0.0;
            }
        }
    }
}

When i am doing the hardcoding of values on below structure it is working fine : change double data[num1][num2]; to double data[10][10];

How can we fix this issue in C.

dbush · Accepted Answer · 2019-04-16 23:34:07Z

5

You're overrunning the bounds of the array in your loops:

for(i=0;i<=num1;i++){
    for(j=0;j<=num2;j++){

In the outer loop i ranges from 0 to num1, while in the inner loop j ranges from 0 to num2. However, the maximum valid index for each array dimension is num1-1 and num2-1 respectively. Reading / writing past the end of an array invokes undefined behavior, which is why it sometimes worked and sometimes didn't.

Change the loop conditions to <:

for(i=0;i<num1;i++){
    for(j=0;j<num2;j++){

edited Apr 16, 2019 at 23:34

answered Apr 16, 2019 at 18:14

dbush

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Comments

Justin · Accepted Answer · 2019-04-16 18:17:44Z

0

It seems you are going over the memory the program is allowing you to access. You are allocating memory for 100 values (10*10). However, when dealing with arrays in most programming languages, it is important that you know index 0 counts as one such that you can go from array[0] - array[9] and that is declaring an array with memory for 10 spots since we start at 0 in most programming languages.

A clean way to fix this code would be:

for (int i=0; i < num1; i++) {
    for (int j=0; j < num2; j++ {

This would avoid accessing memory location at index 10 which is not allocated.

answered Apr 16, 2019 at 18:17

Justin

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