C 使用文件作为参数

Question

我遇到的问题是，每当我运行代码时，我什至在将文件名作为参数传递给程序之前就得到了 myfile 的 Null 值，我不确定为什么任何帮助深表感谢。

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
    FILE* myFile; // file pointer
    myFile = fopen(argv[1] , "r"); //open file
    if(myFile==NULL)
    {
        printf("Can't Open FIle.\n"); // if file doesn't exist then exit
        exit(0);
    }

    int A[10000]={0};
    int B[10000]={0};
    double C[10000]={0};
    int N,M;
    int i=0;

    fscanf (myFile, "%d", &N); //input N from file
    printf("%d\n",N);
    if(N>100)
    {
        exit(0); // if N>100 then exit
    }
    while (!feof (myFile)) // loop until file pointer reaches to the end of file
    {
        fscanf (myFile, "%d", &A[i]); //input source
        fscanf (myFile, "%d", &B[i]); // input destination
        fscanf (myFile, "%lf", &C[i]); // input time
        i++;
    }
    fclose (myFile); //close file

    M=i; // number of lines = M

    for (i = 0; i < M; i++)
    {
        if(A[i]==0) //end of output
            break;
        else
        {
            printf("%d %d %lf:\n",A[i],B[i],C[i]); //print source, destination and time
            if(A[i]>=1&&A[i]<=N)
            {
                if(B[i]>=1&&B[i]<=N)
                {
                    if(A[i]==B[i])
                    {
                        printf("Error:Source city is the same as destination city.\n"); //same source and destination error:condition
                    }
                    else
                    {
                        if(C[i]<0)
                        {
                            printf("Error:Invalid Time.\n"); //invalid time
                        }
                        else
                        {
                            //
                        }
                    }
                }
                else
                {
                    printf("Error: Invalid destination city.\n"); //invalid destination condition
                }
            }
            else
            {
                printf("Error: Invalid source city.\n"); //invalid source condition
            }
        }
    }

    return 0;
}

Answer 1

如果您不将文件名作为参数传递给程序，argv[1] 的内容是未定义的。

由于当程序试图打开文件时 argv[1] 中可能存在任何内容，最有可能的是不存在具有该名称的文件，因此 fopen() returns NULL.

您不应该在不检查是否提供命令行参数的情况下尝试访问命令行参数，对于该检查 argc，如果您只期望 1 个参数，那么一个简单的检查将是

char filename[256]; /* the size should be reasonable
                     * can be PATH_MAX or MAX_PATH,
                     * depending on whether it's Windows
                     * or not
                     */
if (argc < 2)
 {
    size_t length;

    fprintf(stdout, "Error: %s program expects one argument\n", argv[0]);
    fprintf(stdout, "Please input the file name: ");
    if (fgets(filename, sizeof(filename), stdin) == NULL)
     {
        fprintf(stdout, "Error: unexpected error\n");
        return -1;
     }
    length = strlen(filename);
    if (length == 0)
     {
        fprintf(stdout, "Error: the provided file name is invalid.\n");
        return -1;
     }
    if (filename[length - 1] == '\n')
        filename[length - 1] = 0;
 }
else
    strcpy(filename, argv[1]);

C 使用文件作为参数

C using a file as an argument

c

input

command-line-arguments