Segmentation fault when accessing array with variable, but not with constant

Question

I'm sure I'm just overlooking something simple, but I haven't been able to figure it out. My code is

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <stdbool.h>
#include <stdlib.h> // For exit()
#include <unistd.h> // for getopt()


const int MAX_PID_LENGTH = 10; // #(PID allowed for user to input)

int main(int argc, char * argv[]) {
    int opt; // i.e. option to be parsed
    int pidCount = 0; // i.e. #(counted PID)
    char ** pidArray;
    int pidArrayLength = 10;
    while ((opt = getopt(argc, argv, "p:")) != -1) {

        switch (opt) {

        case 'p':
            if (!optarg) {
                printf("Invalid command line arguments.\n");
                exit(EXIT_FAILURE);
            }
            if (!pidArray) {
                pidArray = (char ** ) malloc(sizeof(char * ) * pidArrayLength);
                if (!pidArray) {
                    printf("Unable to allocate memory.\n");
                    exit(EXIT_FAILURE);
                }
            } else if (pidCount >= pidArrayLength) {
                pidArrayLength *= 2;
                pidArray = (char ** ) realloc(pidArray, sizeof(char * ) * pidArrayLength);
                if (!pidArray) {
                    printf("Unable to reallocate memory./n");
                    exit(EXIT_FAILURE);
                }
            }
            pidArray[pidCount] = (char * ) malloc(sizeof(char) * MAX_PID_LENGTH);
            strcpy(pidArray[pidCount], optarg);
            pidCount++;
        }
    }
}

When I run this code with the command line arguments "-p 1" (or anything else in place of "1"), I get a Segmentation Fault at the line "pidArray[pidCount] = (char *) malloc(sizeof(char) * MAX_PID_LENGTH);". Notably, if I change pidCount in this line to 0, I don't get the Segmentation Fault, even though pidCount has the value 0. Furthermore, when I ssh to one of my school's computer and run the code there, I don't get any such error.

Any insight into what's causing the bug and how to fix it would be greatly appreciated!

0___________ · Accepted Answer · 2020-09-18 15:52:59Z

1

You accessing the uninitialized variable pidArray in if statement. You need to initialize ot NULL or malloc it in the initialization
You do not need to cast result of malloc
Rather use objects in sizeof instead of types: pidArray = malloc(sizeof(*pidarray) * pidArrayLength);
sizeof(char) is by definition 1 and is completelly not needed

answered Sep 18, 2020 at 15:52

0___________

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2 Comments

user12116219 Over a year ago

Thanks, I'll make those changes, although I don't see how any of those would cause the bug. Could you please clarify?

0___________ Over a year ago

You are using not initialized variable. Automatic variables are not zeroed.

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