Bounded line input routine `get_line`

errval.c

#include <errno.h>
#include <stdio.h>
#include <string.h>

#include "errval.h"

static char const* errstrings[] = {
	[ErrOK]    = "no error",
	[ErrIO]    = "I/O error",
	[ErrMore]  = "more data available",
	[ErrParam] = "invalid parameter",
	//[ErrSys] = strerror(errno),
	[ErrMax]   = "unknown error",
};

char const*
errstr(errval err) {
	if (err == ErrSys) {
		return strerror(errno);
	} else if (err > ErrMax) {
		err = ErrMax;
	}
	return errstrings[err];
}

void
errprint(errval err, char const* msg) {
	if (msg) {
		fprintf(stderr, "%s: ", msg);
	}
	fprintf(stderr, "%s\n", errstr(err));
}

errval.h

#ifndef ERRVAL_H
#define ERRVAL_H 1

#ifdef __cplusplus
extern "C" {
#endif

/*
 * Error constants
 */
typedef enum {
	ErrOK,
	ErrIO,
	ErrMore,
	ErrParam,
	ErrSys,

	ErrMax,
} errval;

/*
 * Return a string description of an error
 *
 * If `err` is `ErrSys`, `strerror(errno)` is returned.
 */
char const* errstr(errval);

/*
 * Print a string description of an error to `stderr`
 *
 * If `msg` is not a null pointer, it is printed before the error
 * description, followed by a space and a colon like `perror`.
 */
void errprint(errval err, char const* msg);

#ifdef __cplusplus
}
#endif

#endif  /* !ERRVAL_H */

get_line.c

#include <stdio.h>

#include "errval.h"

/*
 * Read a line from standard input
 *
 * Upon return, successful or not, the `buffer` argument contains a
 * null-terminated string that is `size-1` characters or less in length,
 * and the `len` parameter, if not a null pointer, will contain the
 * length of that string.
 *
 * A newline character, if read, is discarded and causes the function to
 * return as if `EOF` was encountered.
 *
 * The return value is a value suitable for use with `errstr` or
 * `errprint`.
 *
 * Errors:
 *   ErrOK    - No error
 *   ErrIO    - I/O error (only used if ErrSys is not returned instead)
 *   ErrMore  - line was longer than `size-1` characters, excluding \n
 *   ErrParam - buffer is a null pointer, or size is less than 2
 *   ErrSys   - error is stored in errno
 */
errval
get_line(char* restrict buffer, size_t size, size_t* restrict len) {
	// size == 0 cannot store any characters.
	// size == 1 can only store a null terminator.
	if (!buffer || size < 2) {
	    return ErrParam;
	}

	int c;
	for (size_t i = 0; i < size; i++) {
		c = getchar();
		if (c == '\n' || c == EOF) {
			buffer[i] = 0;
			if (len) {
				*len = i;
			}
			if (ferror(stdin)) {
				if (errno) {
					return ErrSys;
				}
				return ErrIO;
			}
			return ErrOK;
		}
		buffer[i] = c;
	}

	// If we're outside the loop, then we didn't read a full
	// line.  This means we don't have room for a null terminator,
	// so we need to push the last char back onto stdin.
	ungetc(c, stdin);
	buffer[size-1] = 0;
	if (len) {
		*len = size-1;
	}
	return ErrMore;
}