Separate the list implementation from the main translation unit:

Currently, the list can not be used in any other program. Move it to a separate source file and include its corresponding header in the main program.

Define main() at the end of the translation unit:

If we define main() at the end, we wouldn't require all the forward declarations of the functions.

Extra arguments to main():

fileOpener() checks whether at least 2 arguments were provided to the program, but what when more than 2 are provided? Currently, one can pass around INT_MAX arguments to the program.

Prefer standard exit status values to non-standard ones:

Use EXIT_FAILURE and EXIT_SUCCESS from stdlib.h to signify failure and success respectively.

Eliminate redundant function calls:

fseek(file, 0, SEEK_SET);

does nothing at the beginning of calculate(), as the file position indicator is already at the beginning of the file.

As of the second call, it can be replaced with rewind(file).

Read the file in large chunks:

Currently you're reading it byte by byte, and whilst that has the benefit of not having to allocate anything, reading it in bigger chunks (4-64 Kib) should prove to be faster.

Use sizeof *ptr instead of sizeof (type) in calls to malloc() and family:

#if 0
Node *node = calloc(1, sizeof(Node));
#else
Node *node = calloc(1, sizeof *node);
#endif

It will reduce maintenance headaches if you ever change the type of node.

Separate the list implementation from the main translation unit:

Currently, the list can not be used in any other program. Move it to a separate source file and include its corresponding header in the main program.

Define main() at the end of the translation unit:

If we define main() at the end, we wouldn't require all the forward declarations of the functions.

Extra arguments to main():

fileOpener() checks whether at least 2 arguments were provided to the program, but what when more than 2 are provided? Currently, one can pass around INT_MAX arguments to the program.

Prefer standard exit status values to non-standard ones:

Use EXIT_FAILURE and EXIT_SUCCESS from stdlib.h to signify failure and success respectively.

Eliminate redundant function calls:

fseek(file, 0, SEEK_SET);

does nothing at the beginning of calculate(), as the file position indicator is already at the beginning of the file.

As of the second call, it can be replaced with rewind(file).

Read the file in large chunks:

Currently you're reading it byte by byte, and whilst that has the benefit of not having to allocate anything, reading it in bigger chunks (4-64 Kib) should prove to be faster.

Use sizeof *ptr instead of sizeof (type) in calls to malloc() and family:

#if 0
Node *node = calloc(1, sizeof(Node));
#else
Node *node = calloc(1, sizeof *node);
#endif

It will reduce maintenance headaches if you ever change the type of node.

strlen() returns size_t, not int:

int length = strlen(c);

The compiler would have complained about this if you had enabled warnings. Specify -Wextra -Werror -Wpedantic to gcc.

Use const for data that is not modified:

In:

Package parseString2List(char *c) {

c remains unmodified. Therefore, it should be declared as:

Package parseString2List(const char *c) {