为什么这个程序不起作用？

Question

我在做C程序设计语言的时候，想做一个小程序来复习一下以前的这些知识点。这个程序似乎有一些问题。

该程序应该从输入中收集信息并以某种格式将其打印在文件 "reg.txt" 中。

然而，在输入第一行并按回车后，程序退出了，但我无法弄清楚它有什么问题。

#include <stdio.h>

int main()
{
    FILE *fp;
    struct profile
    {
        char *name;
        char *surname;
        int year;
        int month;
        int day;
    } people[10];
    int temp;
    int i = 0;
    char *line;


    fp = fopen("reg.txt", "a");
    while (fgets(line, 256, stdin)
    {
        sscanf(line, "%s %s %d/%d/%d", people[i].name, people[i].surname, &(people[i].year), &(people[i].month), &(people[i].day));
        ++i;
    }
    temp = i-1;

    for (i = 0; i <= temp; ++i)
        fprintf(fp, "NAME: %s %s\nBIRTHDAY: %d/%d/%d\n", people[i].name, people[i].surname, people[i].year, people[i].month, people[i].day);

    fclose(fp);
    return 0;
}

我采纳了 Ed Heal 的建议，我的目标是检查 'sscanf' 的 return 值。奇怪的是程序并没有真正到达'printf'部分。我想循环可能有一些问题吗？

#include <stdio.h>

int main()
{
    FILE *fp;
    void filecopy(FILE *, FILE *);
    struct profile
    {
        char *name;
        char *surname;
        int year;
        int month;
        int day;
    } people[10];
    int temp;
    int i = 0;
    char *line;
    int j;


    fp = fopen("reg.txt", "a");
    while (fgets(line, 256, stdin) != NULL)
    {
        j = sscanf(line, "%s %s %d/%d/%d", people[i].name, people[i].surname, &(people[i].year), &(people[i].month), &(people[i].day));
        ++i;
    }
    temp = i-1;

    //for (i = 0; i <= temp; ++i)
    //  fprintf(fp, "NAME: %s %s\nBIRTHDAY: %d/%d/%d\n", people[i].name, people[i].surname, people[i].year, people[i].month, people[i].day);

    printf("%d",j);
    fclose(fp);
    return 0;
}

Answer 1

在使用 line 和 gets

之前需要分配内存

#include <stdio.h>

int main()
{
    FILE *fp;
    struct profile
    {
        char *name;
        char *surname;
        int year;
        int month;
        int day;
    };
    struct profile people[10];
    int temp;
    int i = 0;
    char line[256]; //you can also do dynamic allocation by using malloc

    fp = fopen("reg.txt", "a");
   // while (gets(line) != NULL) //use fgets instead gets
    while( fgets(line, 256, stdin) ) //specify required buffer len
    {
        sscanf(line, "%s %s %d/%d/%d", people[i].name, people[i].surname, &(people[i].year), &(people[i].month), &(people[i].day));
        ++i;
    }
    temp = i-1;

    for (i = 0; i <= temp; ++i)
        fprintf(fp, "NAME: %s %s\nBIRTHDAY: %d/%d/%d\n", people[i].name, people[i].surname, people[i].year, people[i].month, people[i].day);

    fclose(fp);
    return 0;
}

Answer 2

即使在确保 line 已分配内存之后，当它尝试写入 people[0].name 和 people[0].surname 时，您应该会在 sscanf 上遇到段错误，因为那些指针将是未定义的。

您需要为这些字符串分配内存，静态或动态（使用 malloc）。

打印日志语句也有助于收集一些见解。

#include <stdio.h>

int main()
{
    FILE *fp;
    typedef struct
    {
        char name[64];
        char surname[64];
        int year;
        int month;
        int day;
    } profile;
    profile people[10];

    int temp;
    int i = 0;
    char line[512];

    printf("Starting profile line parser...\n");
    printf("Please enter up to 10 people in the format: (name surname year/month/day)\n");
    printf("Enter EOF to exit. (Linux: CTRL+D     Windows CTRL+Z)\n");
    fp = fopen("reg.txt", "a");
    while (gets(line) != NULL)
    {
        sscanf(line, "%63s %63s %d/%d/%d", people[i].name, people[i].surname, &(people[i].year), &(people[i].month), &(people[i].day));
        ++i;
    }
    printf("Processed %d lines.\n", i);
    temp = i-1;
    for (i = 0; i <= temp; ++i)
    {
        fprintf(fp, "NAME: %s %s\nBIRTHDAY: %d/%d/%d\n", people[i].name, people[i].surname, people[i].year, people[i].month, people[i].day);
    }

    fclose(fp);
    printf("Done with profile line parser...\n");
    return 0;
}

编辑：因为 gets 已被弃用，这里有一个 fgets 替代方案。进一步阅读获取：Why is the gets function so dangerous that it should not be used?

编辑：还添加了 chux 的缓冲区溢出保护。进一步阅读有关使用 scanf 防止缓冲区溢出的信息：Read no more than size of string with scanf()

#include <stdio.h>

int main()
{
    FILE *fp;
    typedef struct
    {
        char name[64];
        char surname[64];
        int year;
        int month;
        int day;
    } profile;
    profile people[10];

    int temp;
    int i = 0;
    char line[512];

    printf("Starting profile line parser...\n");
    printf("Please enter up to 10 people in the format: (name surname year/month/day)\n");
    printf("Enter EOF to exit. (Linux: CTRL+D     Windows CTRL+Z)\n");
    fp = fopen("reg.txt", "a");
    while (fgets(line, 512, stdin) != NULL)
    {
        sscanf(line, "%63s %63s %d/%d/%d", people[i].name, people[i].surname, &(people[i].year), &(people[i].month), &(people[i].day));
        ++i;
    }
    printf("Processed %d lines.\n", i);
    temp = i-1;
    for (i = 0; i <= temp; ++i)
    {
        fprintf(fp, "NAME: %s %s\nBIRTHDAY: %d/%d/%d\n", people[i].name, people[i].surname, people[i].year, people[i].month, people[i].day);
    }
    printf("Done with profile line parser...\n");
    fclose(fp);
    return 0;
}

为什么这个程序不起作用？

Why doesn't this program work?

c

gets

scanf

fgets