Why does my dynamic array of string pointers cause a segmentation fault?

Question

I would like to hold a dynamic list of objects. I am not allowed to use VLAs, so I wrote the following function: (Originally this was a list of structs I defined, but I reproduced the same problem with strings.)

#include <stdlib.h>
#include <string.h>

void appendSeq(char ** seq, char *** seqList, size_t curSeqListSize) {
    if (*seqList == NULL)
        *seqList = (char **) malloc(sizeof(char *));
    else
        *seqList = (char **) realloc(*seqList, sizeof(char *) * (curSeqListSize + 1));
    if (*seqList == NULL)
        exit(EXIT_FAILURE);
    *seqList[curSeqListSize] = *seq;
}

int main()
{
    char *curSeq = "";
    char ** seqList = NULL;

    for (int i = 0; i < 3; i++)
    {
        curSeq = "Sequence";
        char* curSeqCopy = strdup(curSeq);
        appendSeq(&curSeqCopy, &seqList, i);
    }
}

This fails with a segmentation error, and it seems that the realloc() function is not doing its job - Why?

when you step through what value of i is the last value before it segfaults, what line does it segfault, what are the values of the params when it segfaults. — Bwebb
– Bwebb, Commented Nov 13, 2018 at 21:41
*seqList[curSeqListSize] ==> (*seqList)[curSeqListSize]. And fyi, you never adjust curSeqListSize, which ideally would be passed by pointer as well and updated accordingly for the caller. — WhozCraig
– WhozCraig, Commented Nov 13, 2018 at 21:50
@WhozCraig That solves it, thanks! If you'd like to post an answer I'd be happy to accept it. — H.Rappeport
– H.Rappeport, Commented Nov 13, 2018 at 21:55

WhozCraig · Accepted Answer · 2018-11-13 22:04:25Z

1

Your a victim of failing to understand operator precedence. In C, [] has higher precedence than *, so this:

*seqList[curSeqListSize] = ...

says "find the curSeqListSize slotted char** in the sequence of char** pointed to by seqList, then dereference that. That's wrong. You need to shove higher precedence in to your seqList expression, and () will do that:

(*seqList)[curSeqListSize] = ...

says "Get the char** pointed to by seqList, then get the curSeqListSize slotted char* from that.

I'd change a number of other things, but that's where your wheels fell off the wagon.

answered Nov 13, 2018 at 22:04

WhozCraig

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