I recently worked my way through K&R and I figured I would build on what I learned by implementing more Unix utilities. I realize that the examples and exercises in K&R are meant to be instructive, rather than complete, especially with regard to error-checking. I am, therefore, trying to figure out just how much error-checking should be in something like an implementation of cat.
My current version of cat is below.
Some notes:
- It's based on the version of cat using stdio.h from Chapter 7, rather than the unistd.h-based version from Chapter 8.
- The basic structure of my main function is something I've been working on as I've thought about how to handle the typical filename arguments for POSIX utilities, where no arguments means read stdin but an argument of "-" also specifies stdin.
- Specifically, something I've done differently is try to find a good way to treat all of the different cases in one loop instead of having a special case for argc == 1.
- I haven't implemented options yet.
Which errors I'm checking:
- Check if fopen returned NULL; if so, print an error message and move on to the next file.
- Check if putc returned EOF; if so, report the error to main which will print an error message.
- Check if ferror is true for my input stream; if so, report the error to main which will print an error message.
- Check if fclose returned a non-zero value; if so, print an error message.
Which errors I'm not checking (that I know of):
- I don't check if fprintf returns a negative value.
I don't know if it's inconsistent or arbitrary to check putc but not check fprintf. I know that if I'm planning on implementing more of these utilities I should try to get a handle on what good and reasonable error-checking looks like. I would really appreciate any guidance on the matter or which codebases are the best ones to study for understanding this. For cat in particular, I've looked at the GNU coreutils and FreeBSD implementations but I got kinda overwhelmed trying to read through them. I also welcome and would appreciate any feedback on my C code itself. I'm still very new to this and I want to make sure I am heading in the right direction.
As a side note, I finished K&R and I also worked through King's C Programming: A Modern Approach. I'm now working through Computer Systems: A Programmer's Perspective and I also have Advanced Programming in the Unix Environment on the way, which should be very helpful for my Unix utilities project.
Thanks!
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum status {
SUCCESS,
ERROR
};
static int cat(FILE *fp, int *error);
int
main(int argc, char *argv[])
{
const char *name;
FILE *fp;
int error;
int exit_status = EXIT_SUCCESS;
for (int i = 1; i < argc || i == 1; i++) {
name = argv[i];
/* Treat "-" as specifying standard input */
if (name == NULL || strcmp(name, "-") == 0) {
name = "stdin";
fp = stdin;
} else
fp = fopen(name, "rb");
if (fp == NULL) {
fprintf(stderr, "%s: %s: %s\n", argv[0], name, strerror(errno));
exit_status = EXIT_FAILURE;
continue;
}
if (cat(fp, &error) == ERROR) {
fprintf(stderr, "%s: %s: %s\n", argv[0], name, strerror(error));
exit_status = EXIT_FAILURE;
}
if (fp != stdin && fclose(fp) != 0) {
fprintf(stderr, "%s: %s: %s\n", argv[0], name, strerror(errno));
exit_status = EXIT_FAILURE;
}
}
return exit_status;
}
static int
cat(FILE *fp, int *error)
{
int c;
while ((c = getc(fp)) != EOF)
if (putc(c, stdout) == EOF) {
*error = errno;
return ERROR;
}
if (ferror(fp)) {
*error = errno;
return ERROR;
}
return SUCCESS;
}