Why segmentation fault after altering pointer to struct? - c

Question

I currently have functioning code, that when I attempted to make a function out of the conversion of a file to array, I get a segmentation fault. I know that the objects inside of fileToArray are correct (as far as the myData objects are concerned) because when inside of the function, the myData.length, and myData.array all return correctly. However, after the pointer is referenced in the main, I get a seg fault. I'm new to c, but all of this is working without that specific pointer to the struct.
So, if I call this program with an argument with a file with multiple rows of text, the set fault happens.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <string.h>
#include <unistd.h>
typedef struct {
int length;
char** array;
} FileStruct;
void fileToArray(FileStruct* fileDataPtr, int argc, char *argv[]){
int fd, i, n, count;
struct stat statbuf;
char *buf, *inbuf, *str, *saveptr;
char **array;
if ((fd = open(argv[1], O_RDONLY)) == -1) {
printf("Error opening file %s\n", argv[1]);
exit (-1);
}
if (lstat(argv[1], &statbuf) < 0) {
printf("Unable to lstat file %s\n", argv[1]);
exit (-1);
}
off_t filesize = statbuf.st_size;
buf = malloc(sizeof(char)*filesize);
array = malloc(sizeof(char *)*filesize);
count = 0;
if ((n = read(fd, buf, filesize)) > 0){
inbuf = buf;
for (i = 1; ; inbuf = NULL, i++) {
str = strtok_r(inbuf, "\n", &saveptr);
if (str == NULL)
break;
array[count] = malloc(sizeof(char)*(strlen(str)+1));
strcpy(array[count++], str);
}
} else {
printf("Error reading input file\n");
exit (-1);
}
close(fd);
// I know array works because it prints correctly here.
for (i = 0; i < count; i++) {
printf("%s\n", array[i]);
free(array[i]);
}
fileDataPtr->length = count;
fileDataPtr->array = array;
free(array);
free(buf);
}
int main(int argc, char *argv[]) {
int i;
FileStruct myData;
FileStruct* fileDataPtr = &myData;
fileToArray(fileDataPtr, argc, argv);
printf("length: %i", myData.length);
// I know this doesn't work because anything related to myData causes Seg fault.
// for (i = 0; i < 1; i++) {
// printf("%s\n", myData.array[i]);
// free(myData.array[i]);
// }
return 0;
}

Answer 1

Near the end of fileToArray, you assign array to fileDataPtr->array, then on the next line you free array. This will leave fileDataPtr->array pointing to freed memory (a dangling pointer). When you dereference it later, you're into Undefined Behavior and anything can happen.
Since the assignment transfers ownership of the allocated memory to fileDataPtr, you do not need to free array before returning from fileToArray.
Remove the free(array); line.