使用命令行参数时出现分段错误

我是编程新手。以下是我编写的一个程序,用于计算6面随机掷骰子的概率分布。它可以完美地工作,但是,如果我使用命令行参数作为抛出次数,它将开始抛出分段错误错误。有人可以帮助我了解我在做什么错吗?

// distribution of rand numbers

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

const unsigned short NUM_FACES = 6;

int main(int argc, char * argv[])
{
    if(argc != 2 || isdigit(argv[1]))
    {
        printf("Invalid arguments!!\n");
        printf("Usage: %s numThrows\n", argv[0]);                      //correct usage of arguments
        exit(EXIT_FAILURE);
    }

    system("clear");

    srand(time(0));                                                    //updating seed
    long upperLim = atol(argv[1]);
    long dist[7] = {0};
    double probab = 0.0;
    unsigned int i;
    for(i = 0; i < upperLim; i++)
        ++dist[rand()%6 + 1];                                         //generating random numbers (1-6)

    for(i = 0; i < NUM_FACES; i++)
    {
        probab = 100.0*dist[i]/upperLim;                              //calculating probability of each throws
        printf("DICE THROW %d ->     Number of throws: %ld     Distribution: %.2lf%c\n", i+1, dist[i], probab, '%');
    }

    getchar();
    return 0;
}
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tfugit
tfugit
isdigit(argv[1])

isdigit wants an int, not a char *

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