Program

[pacman169]

Hello,

I need to understand these lines of code, I'm studying the unix program grep. It is not overly long, only 100 lines or so :o
Comments, what the lines are supposed to do are welcome. Thanks.

#define  _POSIX_C_SOURCE 200809L
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <regex.h>
 
static void do_grep(FILE *const in, const char *const filename, regex_t *const pattern, const int use_ansi)
{
    char         *line = NULL;
    size_t        size = 0;
    ssize_t       len;
    unsigned long linenum = 0UL;
    regmatch_t    match[2];
 
    while (1) {
        len = getline(&line, &size, in);
        if (len < (ssize_t)0)
            return;
 
        if (len > (ssize_t)0 && line[len - 1] == '\n')
            line[--len] = '\0';
 
        linenum++;
        if (regexec(pattern, line, 1, match, 0) == REG_NOMATCH)
            continue;
 
        printf("%s: %lu: ", filename, linenum);
        if (use_ansi) {
            if (match[0].rm_so > 0)
                fwrite(line, match[0].rm_so, 1, stdout);
            if (match[0].rm_eo > match[0].rm_so) {
                printf("\033[1;31m");
                fwrite(line + match[0].rm_so, match[0].rm_eo - match[0].rm_so, 1, stdout);
            }
            printf("\033[0m");
            if (match[0].rm_eo < len)
                fwrite(line + match[0].rm_eo, len - match[0].rm_eo, 1, stdout);
        } else
            fwrite(line, len, 1, stdout);
 
        putc('\n', stdout);
    }
}
 
int main(int argc, char *argv[])
{
    regex_t pattern;
    int     result, arg;
    FILE   *in;
    int     use_ansi;
 
    use_ansi = isatty(STDOUT_FILENO);
    if (argc < 2 || !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help")) {
        fprintf(stderr, "\n");
        fprintf(stderr, "Usage: %s [ -h | --help ]\n", argv[0]);
        fprintf(stderr, "       %s PATTERN [ FILES ... ]\n", argv[0]);    
        fprintf(stderr, "\n");
        return EXIT_FAILURE;
    }
 
    result = regcomp(&pattern, argv[1], REG_NEWLINE);
    if (result) {
        const size_t length = regerror(result, &pattern, NULL, 0);
        char        *errmsg;
        errmsg = malloc(length + 1);
        if (!errmsg) {
            fprintf(stderr, "%s: Invalid pattern.\n", argv[1]);
            return EXIT_FAILURE;
        }
        (void)regerror(result, &pattern, errmsg, length + 1);
        fputs(errmsg, stderr);
        fputc('\n', stderr);
        return EXIT_FAILURE;
    }
 
    if (argc > 2) {
        for (arg = 2; arg < argc; arg++) {
            in = fopen(argv[arg], "rt");
            if (!in) {
                fprintf(stderr, "%s: %m.\n", argv[arg]);
                return EXIT_FAILURE;
            }
            do_grep(in, argv[arg], &pattern, use_ansi);
            if (ferror(in) || fclose(in)) {
                fprintf(stderr, "%s: Read error.\n", argv[arg]);
                return EXIT_FAILURE;
            }
        }
    } else {
        do_grep(stdin, "(standard input)", &pattern, use_ansi);
        if (ferror(stdin)) {
            fprintf(stderr, "Error reading from standard input.\n", argv[arg]);
            return EXIT_FAILURE;
        }
    }
    return EXIT_SUCCESS;
}

[MikeyBoy]

Which bits, specifically, are you having trouble understanding?

[pacman169]

> Which bits, specifically, are you having trouble understanding?

I don't understand the code because I'm not the author.
I found some explanations about the functions used here:

http://www.gnu.org/software/libc/manual/html_node/Matching-POSIX-Regexps.html

http://www.gnu.org/software/libc/manual/html_node/POSIX-Regexp-Compilation.html

My point was that I would be looking for line to line explanation of the above code.

[AbstractionAnon]

Sorry, but it's unlikely anybody here is going to go through 100 lines of code and give you a line by line explanation. The code is not particularly complex. If you have questions about specific bits, then post your questions about those bits, and we will try and explain.