[lmikle]

Ну а че, гуглом + немножко подумать религия не позволяет?

Код:

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

int main(void)
{
    FILE * fp_in;
    FILE * fp_out;
    char * line = NULL;
    size_t len = 0;
    size_t read;

    fp_in = fopen("/etc/in.txt", "r");
    fp_out = fopen("/etc/out.txt", "w+");
    if (fp_in == NULL || fp_out == NULL)
        exit(EXIT_FAILURE);

    int num = 1;
    while ((read = getline(&line, &len, fp_in)) != -1) {
        fprintf(fp_out,"%d %s\n", num, line);
        num++;
    }

    fclose(fp_in);
    fclose(fp_out);
    if (line)
        free(line);
    exit(EXIT_SUCCESS);
}

Не проверял.