Why my double pointer is null? [duplicate] - c

Question

This question already has an answer here:
Dynamic memory access only works inside function
(1 answer)
Closed 5 years ago.
this is my code:
void n(FILE *f, int *pocet, char **spz) {
int i = 0;
int pocett = 0;
char* r = (char*)malloc(50 * sizeof(char));
if (spz != NULL) {
free(spz);
}
spz = (char**)malloc(sizeof(char*));
if (f == NULL) {
printf_s("File is null");
return;
}
while (fgets(r, 50, f)) {
switch (i % 6) {
case 1: {
if (i == 1) {
spz[0] = (char*)malloc(50 * sizeof(char));
spz[0] = r;
}
else {
spz = (char**) realloc(spz,((i - 1) / 6 + 1) * sizeof(char*));
spz[((i - 1) / 6)] = (char*)malloc(50 * sizeof(char));
strcpy_s(spz[(i-1)/6], 50 ,r);
}
break;
}
case 5: {
pocett++;
break;
}
}
i++;
}
*pocet = ++pocett;
}
and I call my function as this:
int main() {
FILE *f = NULL;
int c;
int pocet = 0;
int* p = &pocet;
char** spz = NULL;
while ((c = getchar()) != 'k') {
getchar();
switch (c) {
case 'v': {
v(&f);
break;
}
case 'o': {
//printf_s("AHOJ\n");
break;
}
case 'n': {
n(f, p, spz);
break;
}
case 's': {
s(spz, *p);
break;
}
case 'p': {
printf_s("AHOJ\n");
break;
}
case 'z': {
printf_s("AHOJ\n");
break;
}
default: {
printf_s("Skus znova\n");
break;
}
}
}
return 0;
}
EDIT: I have edited my code to add main method.
the problem is, when I pass the spz to another method after, it is null, that means I have to use it in method n() as triple pointer, but it is not working when I add asterisk before each spz in method n. Do you have any ideas how to fix this ?

Answer 1

When you call a function you pass a copy of the spz variable from the main() function to an spz variable in the n() function. Then, assignment in n() affects only its local spz.
If you want to get a pointer value back, you need to pass a pointer to the variable and the routine must dereference the pointer to reach the original variable:
void n( char ***spz_ptr) // a pointer to (char **) variable
{
*spz_ptr // dereferenced pointer. i.e. the main's spz
= malloc( ... );
}
void main()
{
char **spz;
n( & spz ); // pass a pointer to spz
if( spz == NULL) { // test the value assigned by n()
.... // handle the error
}
else {
.... // proceed with actual work
}
}