C Segmentation Fault from function that returns the maximum of an array

user_13716679 0
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I just have a function that finds out the maximum value of an array of integers, but I get a segmentation fault, which I can't find because the compiler doesn't show me the line of the error.

This is my C code:

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

//Funktion ermittelt den größten Wert eines Arrays
int groesstesElement(int **arrayPointer){
    int max = 0;
    for (int i = 0; i < 3; i++) {
        if (*arrayPointer[i]>max) {
            max = *arrayPointer[i];
        }
    }
    return max;
}


int main (int argc, char **argv) {
    int array[4]={1,2,3,4};
    int *ptr = array;
    int z = groesstesElement(&ptr);
    printf("%d\n", z);

    return EXIT_SUCCESS;
}

I use macOS and VSC.

max segmentation-fault c arrays
2 Answers
user_221470

Due to the operators precedence, with

*arrayPointer[i]

you are telling your program:

Take the i-th element of the array arrayPointer and dereference it.

But arrayPointer is a pointer to int *, so all you get is the address of ptr (the int ** pointer defined from main) with an offset. When you finally dereference it you are likely accessing an invalid address, causing segmentation fault.

As already suggested by the main answer, the fix is done using parenthesis in order to apply the operators in the order you want:

(*arrayPointer)[i]

Secondary issues in the code

Even though with the correction above the program won't crash anymore, it won't work. Searching through the array with the loop

for (i = 0; i < 3; i++)

you won't include the last item in the search (index 3), skipping precisely the index that, in your example, contains the maximum value. You probably meant for (i = 0; i < 4; i++) or for (i = 0; i <= 3; i++).

Anyway, using magic numbers in the code (in this case the dimension of the array) is considered bad practice. A better solution would have been using a #define, but an even better solution would have been passing to the function the size of the array:

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

#define ARR_SIZE 4

//Funktion ermittelt den größten Wert eines Arrays
int groesstesElement(int *arrayPointer, unsigned int arrSize){
    int max = 0, i;
    for (i = 0; i < arrSize; i++) {
        if (arrayPointer[i]>max) {
            max = arrayPointer[i];
        }
    }
    return max;
}


int main (int argc, char **argv) {
    int array[ARR_SIZE]={1,2,3,4};
    int z = groesstesElement(array, ARR_SIZE);
    printf("%d\n", z);

    return EXIT_SUCCESS;
}

Please note how there's no need to use a double pointer anymore.

user_113367620
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