Reading file Line By Line in C

Question

Preface:

This question is about reading a file, line by line, and inserting each line into a linked list.

I have already written the implementation for the linked list, and tested the insert() function manually. This works. I have also written the code for reading text from a file, and writing it out. Again, this also works.

OKAY: HERE'S MY QUESTION

How can I merge these concepts, and write a function that reads text from a file, line by line, and inserts each line as a node in the linked list?

When reading from a file, I do the following:

//Code for reading a file
int c;
while((c = getc(f))!= EOF) {
    putchar(c);  //Prints out the character
}
fclose(f);  //Close the file

The insert() function takes two parameters, one being the linked list head node, and the second one being the dataEntry ("string") to be held in that node.

void insert(node_lin *head, char *dataEntry) { ... }

Hence, since the function getc gets each character separately, and the putchar writes out each character to the screen, I imagine the code to do something along these lines:

Read each character until the end of file (EOF)
For each character, until reaching a new line ('\n'), append this to the previously read characters (building a "string)
If reaching the end of the line, insert this "string" into the linked list

Repeat until reaching EOF

 //Code for reading a file
 int c;
 while((c = getc(f))!= EOF) {
      //Build a string here consisting of characters from c, until reaching a new line.
      /*
      if(c == '\n') {  //Indicates a new line
          //Insert the line you have into the linked list: insert(myLinkedList, line);
      }
      */
 }
 fclose(f);  //Close the file

The thing is, I already have a working read_file function, as well as a working insert() function. The thing I need help with is separating the file into lines, and inserting these.

Thanks guys!

Or even better, use [getline(3)](http://man7.org/linux/man-pages/man3/getline.3.html) if your system has it. — Basile Starynkevitch, Sep 16 '14 at 10:13
You could read entire file in one blob, yhen replace newlines with 0, and build your list to point to the starts of lines in the blob. — hyde, Sep 16 '14 at 10:29

score 5 · Accepted Answer · answered Sep 16 '14 at 10:14

5

Replace your character-by-character reading by something more high-level.

The most typical choice would be fgets(), but that requires you to specify a static limit for the line's length.
If you have getline() you can use that, it will handle any line-length but it is POSIX, not standarc C.

Also, you should change your insert() function to accept const char * as the second argument (and remember to allocate memory inside and copy the text, of course).

answered Sep 16 '14 at 10:14

unwind

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`getline` is standard in Posix2008 (and Posix is a standard) – Basile Starynkevitch Sep 16 '14 at 10:14
@BasileStarynkevitch Correct of course, which is why I said "not as standard", since I consider "C" to be a higher standard than "POSIX". – unwind Sep 16 '14 at 10:15

score 3 · Answer 2 · answered Sep 16 '14 at 10:18

You can use fgets to read entire line from the file until new line character is encountered

fgets (buffer, 128, f);

When reading from a file, you can do the following:

//Code for reading a file
char buffer[128];                 // decide the buffer size as per your requirements.
while((fgets (buffer, 128, f))!= NULL) {
    printf (buffer);
}
fclose(f);  //Close the file

Reading file Line By Line in C

2 Answers2

Linked