warning when compiling with make-CodePudding

I wrote a program that copies the standard entry into the stdout as well as into a file. The program works but I have a problem, I receive a warning when compiling with make:

warning: implicit declaration of function ‘isprint’ [-Wimplicit-function-declaration]
    if (isprint(optopt))
        ^~~

code:

Although it's not a big deal, I'd like it to stop displaying this warning. What would be the problem? I would also like a review of the code, what could I improve? The program behaves like the tee -a file command.

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

static int append_mode = 0;

int parse_args(int argc, char *argv[])
{
    char c;
    while ((c = getopt(argc, argv, "a")) != -1) {
        switch (c) {
        case 'a':
            append_mode = 1;
            break;
        case '?':
            if (isprint(optopt))
                fprintf(stderr, "Unkonw option `-%c'.\n", optopt);
            else
                fprintf(stderr,
                    "Unknown option character `\\x%x'.\n", optopt);
            return 1;
        default:
            abort();
            break;
        }
    }
    return 0;
}

int main(int argc, char *argv[])
{
    char buf[100];
    size_t len;
    char *file_mode;
    int i;
    FILE *files[20];
    int num_files;
    
    if (parse_args(argc, argv)) {
        return 1;
    }
    
    file_mode = (append_mode ? "a" : "w");
    
    num_files = argc - optind;
    if (num_files > 0) {
        if (files == NULL) {
            fprintf(stderr, "Unable to allocate file buffer space\n");
            return 1;
        }

        /* go through file arguments and either open for writing
           or append based on the -a flag */
        for (i = optind; i < argc; i  ) {
            FILE *pFile = fopen(argv[i], file_mode);
            if (pFile == NULL)
            {
                fprintf(stderr, "Unable to open file %s for mode %s",
                    argv[i], file_mode);
                goto main_cleanup;
            }
            files[i - optind] = pFile; /* mind the offset */
        }
    }
    
    FILE *not_stdin = fopen("tee.c", "r");
    while ((len = fread(&buf[0], 1, sizeof(buf), not_stdin)) > 0) {
        fwrite(&buf[0], 1, len, stdout);
        for (i = 0; i < num_files; i  ) {
            fwrite(&buf[0], 1, len, files[i]);
        }
    }

    main_cleanup:
    if (num_files > 0) {
        for (i = 0; i < num_files; i  ) {
            fclose(files[i]);
        }
    }

    return 0;
}

CodePudding user response：

Mostly this warning appears when you are trying to use a function without including its required header.

To use isprint() add #include <ctype.h> to your included headers.

CodePudding user response：

For use isprint you have to enter, before the main, the library <ctype.h>.

#include <ctype.h>