CODEWITHSUNDEEP
c
dmolz
dmolz

Reputation: 13

Segmentation Fault (SIGSEGV) when using malloc on null pointer

This is what my main functions looks like. This cannot be changed as it is being used in the file my professor is using to grade the assignment.

char** lines = NULL;
int line_count = 5;
add_lines(&lines, line_count);

In this function, I need to dynamically allocate the lines array and store multiple strings inside. The memory is then being freed in main. This is how I am currently doing it, but I keep getting a segfault. (This function also must take a char***).

void add_lines(char*** lines, int line_count) {
    *lines = (char**)malloc(line_count*sizeof(char*));
    for (int i = 0; i < line_count; i++)
        *lines[i] = (char*)malloc(64*sizeof(char));
}

I am assuming this error has to do with dereferencing and trying to dynamically allocate a NULL pointer, but I'm not sure how I'd go about fixing it.

Upvotes: 0

Views: 82

Answers (1)

dbush
dbush

Reputation: 225757

This line isn't doing what you think:

 *lines[i] = (char*)malloc(64*sizeof(char));

The array index operator [] has higher precedence than the unary dereference operator *. So it attempts to access lines as an array (which it isn't), then dereference the array member. This ends up working when i is 0 because it will point to the first allocated array element, but when i is larger you access memory past lines in the calling function.

You need to use parenthesis to make sure you dereference lines first, then index the array elements.

 (*lines)[i] = malloc(64*sizeof(char));

Also, don't cast the return value of malloc. It's unnecessary and can mask subtle bugs.

Upvotes: 2

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