To malloc or not to malloc, that is the question!

Question

Do I need to malloc when creating a file to write to?

The file will be based on the contents of 2 others, so would I need to malloc space for the writeable file of sizeof( file a ) + sizeof( file b) + 1?

Sorry if this makes no sense; if it doesn't then I guess I need to go read some more :D

Essentially, I have 2 txt files and a string sequence - I am writing each line of each file side by side separated by the string sequence.

txt file a

hello stack over 
flow this 
is a test

txt file b

jump the
gun i am
a novice

seperator == xx

output ==

hello stack overxxjump the
flow thisxxgun i am
is a testxxa novice

Answer 1

If you're writing it in order, can't you just use fprintf() or fwrite() whenever you need to write something out, instead of writing the entire file at once?

EDIT: Based on your update, here's basically what you have to do (probably not valid C since I'm not a C programmer):

EDIT2: With some help from msw:

const int BUFSIZE = 200;
FILE *firstFile = fopen("file1.txt", "r");
FILE *secondFile = fopen("file2.txt", "r");
FILE *outputFile = fopen("output.txt", "w");
char* seperator = "xx";
char firstLine[BUFSIZE], secondLine[BUFSIZE];

// start a loop here
fgets(firstLine, 200, firstFile);
fgets(secondLine, 200, secondFile);
// Remove '\n's from each line

fprintf(outputFile, "%s%s%s", firstLine, seperator, secondLine);
// end a loop here

fclose(outputFile);
fclose(firstFile);
fclose(secondFile);

Answer 2

Files are on disk, malloc is for RAM.

You'd only malloc if you needed space in memory to store the data PRIOR to writing out to the file, otherwise, typically you'd use a stack allocated buffer of say 8k to write to the file in chunks.

So taking your question as-is, no you'd rarely malloc just to write to a file.

If your goal is to keep the file in memory in completion, then you'd malloc sizeof file.

Answer 3

You only need to malloc the entire size of a file if you need to hold the entire file in memory (and even then, you can probably use mmap or something instead). Allocate as much memory as you need for the data you intend to work with in memory: no more, no less.