listdir.c

#include <dirent.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdbool.h>

static
bool startswith(const char* str, const char* prefix) {
    if ((str == NULL) || (prefix == NULL)) {
        return false;
    }
    size_t str_len = strlen(str);
    size_t prefix_len = strlen(prefix);
    
    if (prefix_len > str_len) {
        return false;
    }

    return strncmp(prefix, str, prefix_len) == 0;
}

typedef void(*DirVisitor)(void* ctx, const struct dirent*);

static
bool listdir(const char* dirname, DirVisitor visitor, void* ctx) {
    DIR* dir = opendir(dirname);
    if (dir == NULL) {
        fprintf(stderr, "Couldn't open directory '%s': %s\n", dirname, strerror(errno));
        return false;
    }

    struct dirent* dp;
    while ((dp = readdir(dir)) != NULL) {
        if (!startswith(dp->d_name, "."))
            visitor(ctx, dp);
    }
    closedir(dir);
    return true;
}

void dir_printer(void* ctx, const struct dirent* dp) {
    const char* parent_dir = (const char*) ctx;
    fprintf(stdout, "%s/%s\n", parent_dir, dp->d_name);
}

char* dir_path(const char* dir) {
    const size_t maxlen = 4096;
    char* result;
    if ((result = realpath(dir, NULL)) == NULL) {
        fprintf(stderr, "Couldn't get path for dir '%s': %s\n", dir, strerror(errno));
        return NULL;
    }
    return result;
}

int main(int argc, char* argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <dirname>\n", argv[0]);
        return EXIT_FAILURE;
    }
    char* path = dir_path(argv[1]);
    int result = EXIT_SUCCESS;
    do {
        if (!listdir(path, dir_printer, (void*)path)) {
            fprintf(stderr, "Failed\n");
            result = EXIT_FAILURE;
            break;
        }
    } while(false);
    
    free(path);
    return result;
}