Saving lines from a text file to a dynamic array of strings - c

Question

This is my first year with C, so I am a bit lost.
I have the function:
void read(char** lines){
FILE *fpointer = fopen("input1.txt","r");
char *p_input = (char*) malloc(sizeof(char)*200);
int i,len;
i=0;
lines = malloc(sizeof(char*));
while( fgets(p_input,200,fpointer) ){
len = strlen(p_input);
char temp[len];
strcpy(temp,p_input);
lines[i] = temp;
i++;
}
}
and in main:
int main(){
char **lines;
read(lines);
return 0;}
And when I try printing something from the array, I face errors and the code stops, something like:
printf("%s\n",lines[0]);
Can you please tell me what is wrong.

Answer 1

lines = malloc(sizeof(char*));
...
lines[i] = temp;
This is wrong, you don't have enough space for an array of pointer to chars (you need to know the number of lines to reserve)
Change to something like
char **read(void) {
size_t n = file_lines;
char **lines = malloc(sizeof(char*) * n);
...
return lines;
}
int main(void) {
char **lines;
lines = read();
return 0;
}
If you dont know the number of lines before-hand you can use realloc on each iteration of the while loop.
char **read(void) {
...
char **lines = NULL;
char **tmp;
...
while (fgets(p_input,200,fpointer)) {
...
tmp = realloc(lines, sizeof(char *) * (i + 1));
if (tmp != NULL) {
lines = tmp;
} else {
return NULL;
}
lines[i] = temp;
i++;
}
return lines;
}
int main(void) {
char **lines;
lines = read();
if (lines == NULL) {
perror("read");
exit(EXIT_FAILURE);
}
return 0;
}

Answer 2

In the realloc() example, if you use that together with the original code:
while( fgets(p_input,200,fpointer) ){
len = strlen(p_input);
char temp[len];
...
That's a mistake because temp[len] is declared inside the loop which means it'll be destroyed upon exiting the while loop. So your entries inside your realloc() array will point to nothing.
You would want to use malloc() inside the while loop to generate a separate space for each entry rather than declare a static array like the above.
And to tidy up at the end remember to free() the space at the end before your program exits completely.