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获得了 myfile 的 Null 值，我不确定为什么非常感谢任何帮助。

#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。

由于当程序尝试打开文件时，argv[1] 中可能存在任何内容，最可能的情况是没有具有该名称的文件，因此fopen() 返回NULL。

你不应该尝试访问命令行参数而不检查它们是否被提供，检查argc，如果你只期望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]);