leftpad

Pad a string to a desired length. Library & CLI.

Key features:

• Highly portable.
• No dependencies: CLI needs only libc, library is freestanding!
• Detailed man(1) documentation for both the CLI and library.

Testimonials

Couldn't you just printf with a variable format string?

~ kartoffelwaffel on Reddit

Thanks, I hate it

~ not_a_novel_account on Reddit

leftpad string length [padding]

leftpad outputs padding (or spaces if omitted) and string, where padding is repeated or cut off as needed to make for a minimum final string of length characters.

For example, to pad the string "5581" to 16 characters:

$ leftpad 5581 16 '*'
************5581

Using leftpad to repeat a string:

$ leftpad '' 40 _-
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-

Library

#include <stddef.h>     /* for size_t */
#include <leftpad.h>

size_t
leftpad(
    const char *str,
    const char *padding,
    size_t min_len,
    char *dest,
    size_t dest_sz);

leftpad() writes padding (or spaces if NULL or empty) and str to dest, where padding is repeated or cut off as needed to make for a minimum final string length of min_len.

If dest is NULL or dest_sz 0, nothing is written. Otherwise, up to dest_sz characters are written, including the \0 terminator, possibly truncating the output.

Rreturns the length of the output string. If less than dest_sz, the output has not been truncated.

For example, to pad the string "5581" to 16 characters, assuming buf is a character array of sufficient size:

leftpad("5581", "*", 16, buf, sizeof(buf));
puts(buf); /* outputs: ************5581 */

Using leftpad() to repeat a string:

leftpad(NULL, "_-", 40, buf, sizeof(buf));
puts(buf); /* outputs: _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_- */

Calling leftpad() without a buffer to find out the required buffer size:

size_t len = leftpad(some_str, " ", 10);
char *buf = malloc(len+1); /* +1 for terminating NUL */
leftpad(some_str, " ", 10, buf, len+1);

Building

The CLI requires only the C standard library. The library has no dependencies.

A Makefile is provided for Unix systems such as Linux, BSD, and macOS. To build, test, install, and uninstall:

make
make check
make install   [DESTDIR=] [PREFIX=/usr/local] [MANPREFIX=PREFIX/man]
make uninstall [DESTDIR=] [PREFIX=/usr/local] [MANPREFIX=PREFIX/man]

Note: to run leftpad without installing, it may be needed to set LD_LIBRARY_PATH as follows:

$ LD_LIBRARY_PATH=. leftpad

The following files are installed:

bin/leftpad
lib/libleftpad.so
lib/libleftpad.a
include/leftpad.h
man/man1/leftpad.1
man/man3/leftpad.3

Author

Sijmen J. Mulder ([email protected])