Reading a text file backwards in C

Question

What's the best way to read a file backwards in C? I know at first you may be thinking that this is no use whatsoever, but most logs etc. append the most recent data at the end of the file. I want to read in text from the file backwards, buffering it into lines - that is

abc
def
ghi

should read ghi, def, abc in lines.

So far I have tried:

    #include <stdio.h>
    #include <stdlib.h>

    void read_file(FILE *fileptr)
    {
        char currentchar = '\0';
        int size = 0;

        while( currentchar != '\n' )
        {
            currentchar = fgetc(fileptr); printf("%c\n", currentchar);
            fseek(fileptr, -2, SEEK_CUR);
            if( currentchar == '\n') { fseek(fileptr, -2, SEEK_CUR); break; }
            else size++;

        }
        char buffer[size]; fread(buffer, 1, size, fileptr);
        printf("Length: %d chars\n", size);
        printf("Buffer: %s\n", buffer);


    }


    int main(int argc, char *argv[])
    {
        if( argc < 2) { printf("Usage: backwards [filename]\n"); return 1; }

        FILE *fileptr = fopen(argv[1], "rb");
        if( fileptr == NULL ) { perror("Error:"); return 1; }

        fseek(fileptr, -1, SEEK_END); /* Seek to END of the file just before EOF */
        read_file(fileptr);


        return 0;


    }

In an attempt to simply read one line and buffer it. Sorry that my code is terrible, I am getting so very confused. I know that you would normally allocate memory for the whole file and then read in the data, but for large files that constantly change I thought it would be better to read directly (especially if I want to search for text in a file).

Thanks in advance

* Sorry forgot to mention this will be used on Linux, so newlines are just NL without CR. *

Answer 1

You could just pipe the input through the program tac, which is like cat but backwards!

http://linux.die.net/man/1/tac