C基本数据head命令 [英] C Basic Head Command

问题描述

我试图重新头部，并从Linux命令尾为我的编程类。
我们刚刚开始使用C，所以我是新来分配内存和指针的想法。
我不知道为什么这是行不通的。

I'm trying to recreate the head, and tail commands from linux for my programming class. We just started using C so I'm new to the idea of allocating memory and pointers. I'm wondering why this doesn't work.

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

int main(int argc,char **argv){

    /* Checks if correct amount of arguements */

    if(argc != 2 || argc != 4){
        printf("Usage: %s head <file> \n Or: head <file> -n <number of characters>", argv[0]);
        exit(-1);
    }

    if(strcmp(argv[1], "-n" != 0)){
        char fileName[strlen(argv[1])] = argv[1];
    }
}

//Compile error on char fileName[strlen(argv[1])] = argv[1];

任何额外的见解也将是有益的。

Any additional insight would also be helpful.

推荐答案

首先第一件事情，您的使用不符合您的参数检查。根据使用情况，您必须使用之一：

First things first, your usage doesn't match your argument checking. According to the usage, you must use one of:

head <filename>
head <filename> -n <count>

在换句话说，的argv [1] 是总是的文件名，的argv [2] 是需要被设置为 -n 如果有两个以上的参数之一。

In other words, argv[1] is always the filename, argv[2] is the one that needs to be set to -n if there are more than two arguments.

其次，除非你想用沃拉斯（可变长度数组），你应该只设置一个指向文件名参数的东西，如：

Secondly, unless you want to use VLAs (variable length arrays), you should probably just set up a pointer to the filename argument with something like:

char *fileName = argv[1];

您不需要改变它在所有（你只可以把它传递给的fopen ，presumably），所以它试图使废另一个副本。

You don't need to change it at all (you'll just be passing it to fopen, presumably), so it's a waste trying to make another copy.

另外，你的如果说法是错误的，因为一个或，它应该是一个和。它保证的argc 将要么不为2或不为4，因为它不能在同一时间既

In addition, your if statement is wrong as an or, it should be an and. It's guaranteed that argc will either not be 2 or not be 4, since it can't be both at the same time.

我就开始是这样的：

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

static int usage (void) {
    printf ("Usage: head <file>\n");
    printf ("   or: head <file> -n <number of characters>\n");
    return -1;
}

int main (int argc,char *argv[]) {
    char *fileName;
    int lineCount;

    // Checks if correct arguments

    if ((argc != 2) && (argc != 4)) return usage();

    if ((argc == 4) && (strcmp(argv[2], "-n" != 0)) return usage();

    // Get file spec and line count

    fileName = argv[1];

    lineCount = (argc == 2) ? 10 : atoi (argv[3]); // or strtol for purists
    if (linecount < 0) lineCount = 0;

    // Now go ahead and implement the logic for head.

}

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