Update array of strings in function - arrays

Question

I have a working example of copy lines from a file into an array of strings. I want to move the code to copy the lines into a function to which I simply pass a pointer to the array of strings, where the lines will be stored, and a pointer to the file. However, I have tried to move the code into a function and keep getting seg faults. I have tried debugging using GDB and it seems like the problem is with the memory allocation to rows. But I can't work out what the problem is. realloc seems to be working correctly since I find the size of row increases on the 3rd iteration (using malloc_usable_size(*rows)), but then seg faults. I'm compiling with gcc -Wall -Wextra -pedantic -std=c99 -g c_programs/read_file_function.c on Linux.
Working example
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char **argv)
{
if (argc != 2)
{
fprintf(stderr, "Please supply a file path:\n%s <file path>\n", argv[0]);
return EXIT_FAILURE;
}
FILE *fp = fopen(argv[1], "r");
if (!fp)
{
perror("ERROR");
return EXIT_FAILURE;
}
char **rows = (char **)malloc(sizeof(char *));
char *lineBuf = NULL;
size_t n = 0;
size_t nLines = 0;
ssize_t lineLength = 0;
size_t i = 0;
while ((lineLength = getline(&lineBuf, &n, fp)) != -1)
{
lineBuf[strcspn(lineBuf, "\n")] = 0;
lineBuf[strcspn(lineBuf, "\r")] = 0;
rows[i] = (char *)malloc(lineLength + 1);
strcpy(rows[i], lineBuf);
i++;
nLines = i;
rows = (char **)realloc(rows, (nLines + 1) * sizeof(char *));
}
printf("nLines: %lu\n", nLines);
printf("row 1: %s\n", rows[0]);
printf("row 2: %s\n", rows[1]);
printf("row 2: %s\n", rows[10]);
return 0;
}
Non working function version
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
size_t readFile(FILE **fp, char ***rows)
{
char *lineBuf = NULL;
size_t n = 0;
size_t nLines = 0;
ssize_t lineLength = 0;
size_t i = 0;
while ((lineLength = getline(&lineBuf, &n, *fp)) != -1)
{
lineBuf[strcspn(lineBuf, "\n")] = 0;
lineBuf[strcspn(lineBuf, "\r")] = 0;
*rows[i] = (char *)malloc(lineLength + 1);
strcpy(*rows[i], lineBuf);
i++;
nLines = i;
*rows = (char **)realloc(*rows, (nLines + 1) * sizeof(char *));
}
return nLines;
}
int main(int argc, char **argv)
{
if (argc != 2)
{
fprintf(stderr, "Please supply a file path:\n%s <file path>\n", argv[0]);
return EXIT_FAILURE;
}
FILE *fp = fopen(argv[1], "r");
if (!fp)
{
perror("ERROR");
return EXIT_FAILURE;
}
char **rows = (char **)malloc(sizeof(char *));
size_t nLines = readFile(&fp, &rows);
printf("nLines: %lu", nLines);
printf("row 1: %s", rows[0]);
printf("row 2: %s", rows[1]);
return 0;
}

Answer 1

*rows[i] is doing *(rows[i]) - accessing ith element in the array of rows, and then dereferencing it. You want to do (*rows)[i] - dereference rows and then access ith element.
I advise to:
readFile(..., char ***rows0) {
char **rows = NULL; // temporary internal variable
...
// use rows normally
rows = stuff();
...
// when finished, assign once
*rows0 = rows;
return nLines;
}
But do not be a 3-star programmer. At best, use a structure, -> is easy to use. Like:
struct string {
char *str;
};
struct lines {
struct string *strs;
size_t cnt;
};
// #return 0 on success, otherwise error
int readFile(...., struct lines *p) {
// initialization
p->cnt = 0;
p->strs = NULL;
...
void *pnt = realloc(p->strs, (p->cnt + 1) * ....);
if (!pnt) { /* handle error */ return -1; }
p->strs = pnt;
p->strs[p->cnt]->str = malloc(lineLenght + 1);
if (!p->strs[p->cnt]->str) { /* handle error */ return -2; }
strcpy(p->strs[p->cnt]->str, lineBuf);
p->cnt++;
...
return 0; /* success */
}
int main(int argc, char **argv) {
struct lines p = {0};
if (readFile(..., &p)) {
/* handle error */
}
printf("nLines: %zu\n", p.cnt);
Do not pre-allocate memory. Initialize memory with NULL and call realloc before using memory. realloc(NULL is the same as malloc().
Check for allocation errors.