Any chance you are running this on an OS with a "new line" character that isn't "\n"? Windows for example could be "\r\n". I tried on Linux and it worked, although I only grabbed the necessary parts, so if something else is breaking, I can't say. Or perhaps I misunderstood the issue altogether. Here is what I see:

The function:

# code block
int print_file(char* buffer) {
	printf("print_file\n");

	int line_num = 1;

	char* line = strtok(buffer, "\n");

	while(line != NULL) {
		printf("%4d\t%s\n", line_num, line);
		line = strtok(NULL, "\n");
		line_num++;
	}
	return 0;	
}

And the output:

# code block
test file: test.txt 
hello 123 123 123 
asdf asdf asdf 
0oijf0w0j s0w0wfjwef 
0wefjfjwefjkwejijiowefjo

output:
   1	hello 123 123 123 
   2	asdf asdf asdf 
   3	0oijf0w0j s0w0wfjwef 
   4	0wefjfjwefjkwejijiowefjo
count: 4 lines | 0 words | 82 characters

I'll attempt to give a more hand wavey approach, but one that is hopefully easier to understand.

First, implement:

\\ find first index of `c` within string `str`.
\\ returns true if found, sets `out` to result.
\\ otherwise returns false.
bool find(char* str, size_t len, char c, size_t* out)

Now this can be used in a loop, where you supply a slice starting at the last line (or the beginning of the slice, on the first iteration), where len is the length of the original string minus the amount of characters consumed (index of the beginning of the slice, relative to the start of the original string).

Note this requires calculating the string length beforehand, if this is a c-string (as in null terminated).

This should be relatively simpler to understand and implement than what the standard library supplies, sadly the C standard library is not easy to use or very reliable, so you'll mostly have to make do yourself.

I hope this helped, I'm currently away from a computer so i can't give a code example, good luck.

the strtok issue is, per the man page, that strtok returns only “non empty” tokens with your given delimiters.

Given only one delimiter, the strchr approach is likely what I'd use. understanding strchr (and the similar strstr) is a pretty useful thing to have anyway, so it’s a good time to learn it. strchr makes no modifications and isn’t aiming to make tokens for you. It is looking for the first instance of a given byte in a C string and, provided it found it, returning you a pointer to that byte. as such, if it returns you something other than NULL, you’ve got a pointer to a byte. assuming you’re fine with the function modifying the buffer, you can convert that byte to a nul byte (\0), then print buffer, then a new line. You then advance your buffer pointer to the byte past the now-nul byte. Continue until strchr returns NULL, which is your last line - assuming the buffer is nul-terminated

Edit: example, now that I've got a real keyboard:

void strchr_loop(char *buf) {
    char* p;
    while ((p = strchr(buf, '\n')) != NULL) {
        *p = '\0';
        printf("Line = %s\n", buf);
        buf = p + 1;
    }
    printf("Last line = %s\n", buf);
}

Before trying to learn and use the standard library, I'd advise you to write a few simple functions yourself:

1. void print_until(const char* str, char c) - print every character in str until you see c or null (Note: Don't print the c or null, which should ensure that a str that only contains a \0 doesn't crash.)
2. const char* print_until_2(const char* str, char c) - as before, but return the pointer to where you found c or null
3. void print_delimited_by(const char* str, char delimiter, char separator) - use print_until_2 in a while-loop, don't print the delimiter, but do print a separator.

For example, print_delimited_by("1;2;3",';',',') should print 1,2,3.

Finally, void num_delimited_by(const char* str, char del) - like print_delimited_by, but add a number before every output, and use newline as the separator.

These fundamental string exercises will give you a solid foundation for understanding oantby's explanations about the standard library. Note that no memory allocation is necessary.

It is not beginner friendly but it is optimized. It doesn't allocate memory or whatever. I don't expect you to understand all this but I did it for fun anyway.

int print_file(const char* buffer)
{
    // Validate the buffer
    if (!buffer || *buffer == '\0') return 1;

    // Prepare the first line prefix
    unsigned int line = 1;
    printf("%4d\t", line); // You could pre-format it if u want

    const char* cursor = buffer; // The pointer that points to the first char
    const char* linestart = cursor; // The start of the line
    char ch; // Character register

    while (true) {
        ch = *cursor++; // Read character THEN advance the cursor.

        // Check if the character is null or is newline or windows thing

        if (ch == '\0') {
            int linelength = cursor - linestart - 1; // Minus the null terminator
            printf("%.*s\n", linelength, linestart); // Print line using the length of string
            break;
        } else if (ch == '\n') {
            int linelength = cursor - linestart - 1; // Minus the newline
            printf("%.*s\n", linelength, linestart); // Print line using the length of string
            linestart = cursor;

            printf("%4d\t", ++line); // Print next line prefix
        } else if (ch == '\r') {
            continue; // Ignore the Windows thing
        }
    }
    
    return 0;
}