c - Reading from a file and setting an offset? -

i new c, , trying build c program scans through file until eof, picks out lines contain keyword , sets offset after last line searched. when scan executed again, scans file, time starting saved offset , continues downward until eof.

i trying wrap head around different functions of file i/o , i'm having trouble piecing procedure call fopen(), fseek(), fgets(), ftell(), etc want do. can point me in right direction or walk me through need done?

thank you!

i recomment using getline reading, , ftell , fseek getting/setting offset (and strstr searching individual lines) in case.

i'm not sure understand saving of offset about, might this:

int pick_lines(const char *filename, const char *keyword, long *offset) {     file *fp;     char *line = null;     size_t len = 0;      if (offset == null || (fp = fopen(filename, "r")) == null)         return 1;      if (*offset > 0 && fseek(fp, *offset, seek_set) != 0) {         fclose(fp);         return 1;     }      while (getline(&line, &len, fp) != -1) {         if (strstr(line, keyword) != null)             printf("%s", line); // or else chosen line     }      if ((*offset = ftell(fp)) < 0) {         free(line);         fclose(fp);         return 1;     }      free(line);     fclose(fp);     return 0; } 

here offset in/out parameter. it's dereferenced value used seek given offset (start *offset == 0) , reset new offset.

this function print every line contains keyword. if want return array of lines instead, little work needed.

an example of usage might be:

long offset = 0; pick_lines(filename, keyword, &offset); // append lines file pick_lines(filename, keyword, &offset); // ...