Windows porting of tailf command from Linux

• Download tailf.zip-11.82KB

Introduction

The UNIX command tail -f or tailf are indispensible for monitoring log files in real-time. tail has a special command line option -f (follow) that allows a file to be monitored. Instead of displaying the last few lines and exiting, tail displays the lines and then monitors the file. As new lines are added to the file by another process, tail updates the display.

This is particularly useful for monitoring log files. The following commands will display the last 10 lines of messages and append new lines to the display as new lines are added to messages:

tail -f /var/adm/messages

or

tailf -n 10 /var/adm/messages

tailf is similar to tail -f, but it does nothing when the file is not growing.To interrupt tail or tailf while it is monitoring, break-in with Ctrl+C.

Because this very useful command is not available in windows world I decided to port the Linux code on windows. It's not a bid deal. I just replaced some headers, I had changed some data types and functions and works!

The source code explanation

First of all we have static int tailf(const char *filename, int lines). This function will display the last n lines from a file.

static int tailf(const char *filename, int lines) { char *buf, *p; int head = 0; int tail = 0; FILE *str; int i; if (!(str = fopen(filename, "r"))) { printf("Could not open file\n"); return -1; } buf = (char*)malloc(lines * BUFSIZ); p = buf; while (fgets(p, BUFSIZ, str)) { if (++tail >= lines) { tail = 0; head = 1; } p = buf + (tail * BUFSIZ); } if (head) { for (i = tail; i < lines; i++) fputs(buf + (i * BUFSIZ), stdout); for (i = 0; i < tail; i++) fputs(buf + (i * BUFSIZ), stdout); } else { for (i = head; i < tail; i++) fputs(buf + (i * BUFSIZ), stdout); } fflush(stdout); free(buf); fclose(str); return 1; }

The following function it's used to appends new lines to the display when new lines are added to the file.The algorithm is pretty simple: It checks if the file size is changed and display only the appended lines of data.

static int roll_file(const char *filename, off_t *size) { char buf[BUFSIZ]; int fd; struct stat st; if (!(fd = open(filename, O_RDONLY))) { printf("Could not open file\n"); return -1; } if (fstat(fd, &st) == -1) return -1; if (st.st_size == *size) { close(fd); return 0; } if (lseek(fd, *size, SEEK_SET) != (off_t)-1) { size_t rc; while ((rc = read(fd, buf, sizeof(buf))) > 0) { buf[rc] = NULL; printf("%s",buf); } fflush(stdout); } close(fd); *size = st.st_size; return 1; }

The following function it's used to loop and to continuously check if any lines are appended to the file.

static void watch_file(const char *filename, off_t *size) { do { roll_file(filename, size); Sleep(1500); } while(1); }

That's all. Of course the application it have a main function where it parse the command line to see which file is monitoring and how much last lines will display .

License

This article, along with any associated source code and files, is licensed under The Intelliproject Open License (IPOL)

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