This simple program, that I call trim(1), trims trailing lines and, if the flag -l is passed as an argument, it also removes empty lines and lines containing only blanks.

I did this as an exercise for learning C and good practices of C programming in UNIX.

It reads from standard input if no argument is passed or read from the files passed as arguments.

I am a bit suspicious of that realloc idiom for reallocing the buffer if it gets full. Is it right?

#include <err.h>
#include <errno.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define MID 0   /* in middle of line */
#define BEG 1   /* at beginning of line */

static int blankline = 0;
static int exitval = EXIT_SUCCESS;

static void trim(FILE *fp);
static void usage(void);

/* remove trailing blanks and delete blank lines */
main(int argc, char *argv[])
    int ch;
    FILE *fp;

    while ((ch = getopt(argc, argv, "l")) != -1) {
        switch (ch) {
        case 'l':
            blankline = 1;
    argc -= optind;
    argv += optind;

    if (argc == 0) {
    } else {
        while (*argv) {
            if ((fp = fopen(*argv, "r")) == NULL) {
                warn("%s", *argv);
                exitval = EXIT_FAILURE;
            } else {

/* trim trailing blanks from fp; if blankline is set also remove blank lines */
static void
trim(FILE *fp)
    char *buf = NULL;
    size_t i, size, newsize;
    int status;
    int c;

    size = BUFSIZ;

    if (buf == NULL)
        if ((buf = malloc(size)) == NULL)
            err(EXIT_FAILURE, "malloc");

    i = 0;
    status = BEG;
    while ((c = getc(fp)) != EOF) {

        if (isblank(c)) {               /* collect blanks in buf */
            if (i >= size) {            /* realloc */
                newsize = size + BUFSIZ;
                if (newsize <= size)    /* check for overflow */
                    errc(EXIT_FAILURE, EOVERFLOW, "realloc");
                size = newsize;
                if ((buf = realloc(buf, size)) == NULL)
                    err(EXIT_FAILURE, "realloc");
            buf[i++] = c;
        } else if (c == '\n') {
            if (status == MID || (!blankline))
            status = BEG;
            i = 0;
        } else {
            fwrite(buf, 1, i, stdout);
            status = MID;
            i = 0;

    if (ferror(fp)) {
        exitval = EXIT_FAILURE;

/* show usage */
static void
    (void)fprintf(stderr, "usage: trim [-l] [file...]\n");

1 Answer 1


Well designed code.

I am a bit suspicious of that realloc idiom for reallocing the buffer if it gets full. Is it right?

I see no errors.

Nit: I'd rather see newsize declared locally. It is only used there.

// newsize = size + BUFSIZ;
size_t newsize = size + BUFSIZ;


Unneeded code

The initial allocation is not needed. realloc() in the loop can handle starting from 0

//size = BUFSIZ;
//if (buf == NULL)
//    if ((buf = malloc(size)) == NULL)
//      err(EXIT_FAILURE, "malloc");
size = 0;

Perhaps there is a concern about ...

fwrite(NULL, 1, 0, stdout); 

... yet given the spec "If size or nmemb is zero, fwrite returns zero and the state of the stream remains unchanged.", I see no issue.

Either way I'd consider the micro-optimization below, just for the sake of avoiding the fwite() function call as i==0 will be quite common. (Usually I do not encourage any micro-op - I'll make an exception here)

// fwrite(buf, 1, i, stdout);
if (i > 0) fwrite(buf, 1, i, stdout);

Checking output

Code does not check the return values of fwrite(), putchar(c). Robust code would and report the problem - even if it is rare.

Missing free()

Free the allocation, else trim() leaks memory.



Good that code checks this.

main() always reports success

main() employs the implied return 0;. Yet failure is possible.

Given code has exitval = EXIT_FAILURE;, I'd expect main() to return exitval; in the end.


Rather than declare then assign lines later, consider declare with initialization.

// int status;
// ...
// status = BEG;
int status = BEG;

// size_t size;
// ...
// size = BUFSIZ;
size_t size = BUFSIZ;

Same for i.

  • \$\begingroup\$ "Given code has exitval = EXIT_FAILURE;, I'd expect main() to return exitval; in the end." LOL, it was supposed to return exitval, I really forgot it. Thanks for seeing it! \$\endgroup\$
    – phillbush
    May 6, 2020 at 14:04
  • \$\begingroup\$ "Code does not check the return values of fwrite(), putchar(c)". Should I use an ferror(stdout) in the end of the function instead of checking the return values of fwrite(3) and putchar(3)? \$\endgroup\$
    – phillbush
    May 6, 2020 at 14:08
  • \$\begingroup\$ @barthooper Perhaps. A more direct approach would take action as soon as the error occurs. IMO a write error is most likely on the first write - otherwise it is rare. Your call. \$\endgroup\$ May 6, 2020 at 14:18

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