mKay February 2016

Pointer to pointer in C

I hope you can help me.

I have a function in c, which takes a file, reads line for line and stores every line as a string. It works in this function

int createDownloadList(FILE **dllistref, dltask* taskList) {
    ...
    taskList = (dltask*) malloc(tasksize*allocsize);
    int num = 0;
    while(getline(&line, &linesize, *dllistref) > 0) {
        ...
        taskList[num] = task;
        num++;
        if(num%8 == 0) {
            taskList = realloc(taskList, (num+allocsize)*tasksize);
        }
    }
    return num;
}

But I want to access the pointer to the taskList outside of the function. I tried it with this change

int createDownloadList(FILE **dllistref, dltask** taskList) {
    size_t linesize = 256;
    char* line = (char*) malloc(linesize);
    size_t tasksize = sizeof(dltask);
    int allocsize = 8;
    *taskList = (dltask*) malloc(tasksize*allocsize);
    int num = 0;

    while(getline(&line, &linesize, *dllistref) > 0) {
        ...
        *taskList[num] = task;
        num++;
        if(num%8 == 0) {
            *taskList = realloc(taskList, (num+allocsize)*tasksize);
        }
    }
    return num;
}

But I get always a segmentation fault after the third task and don't know why. I hope someone can help me, I am clueless, why it won't work. Oh, and that's how I call the second function in the main method:

dltask* taskList = NULL;
numOfTasks = createDownloadList(&fileref_dllist, &taskList)

I only added the "&" in the call, otherwise it's the same call for the first function.

Answers


LPs February 2016

Line

*taskList = realloc(taskList, (num+allocsize)*tasksize);

have to be

*taskList = realloc(*taskList, (num+allocsize)*tasksize);

EDIT

The second error, found out by @user3121023, is:

*taskList[num] = task;

that should be

(*taskList)[num] = task;

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Asked in February 2016
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