c - 如何为未知长度的输入字符串分配内存？

Question

这是结构:

typedef struct _friend {
        char *firstname;
        char *lastname;
        char birthdate[9];
} friend;

我对如何让用户输入一个字符串并将其作为 firstname(或 lastname)放在 friend 结构中感到困惑).另外，如果我使用 fgets 时用户输入了超过 256 个字符怎么办？这是我目前所拥有的...

friend *f = (friend *)malloc(sizeof(friend));  //initialize f pointer to friend
char *str;

fgets(str,256,stdin);
f->firstname = (char*)malloc(sizeof(char)*(strlen(str)+1));
strcpy(f->firstname,str);

Answer 1

好吧，由于 stdin 是一个缓冲输入，您可以使用 fgetc 逐个字符地读取输入，直到遇到换行符或 EOF。也许您正在寻找这样的东西:

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

struct friend {
        char    *firstname;
        char    *lastname;
        char     birthdate[9];
};

static char *read_line(FILE *f)
{
    char    *r = NULL;
    char    *p = r;
    char    *e = p;
    int  c;

    while ((c = fgetc(f)) != EOF) {
        if (p >= e) {
            size_t   l = e > r ? (size_t)(e - r) : 32u;
            char    *x = realloc(r, l);
            if (!x) {
                free(r);
                r = NULL;
                goto out;
            }
            p = x + (p - r);
            e = x + l;
            r = x;
        }
        if (c != '\n') {
            *p++ = (char)c;
        } else {
            *p++ = '\0';
            goto out;
        }
    }
    if (ferror(f) != 0) {
        free(r);
        r = NULL;
    }
out:
    return r;
}

int main(void)
{
    struct friend f;

    memset(&f, 0, sizeof(struct friend));

    printf("Please enter your first name: ");
    fflush(stdout);
    f.firstname = read_line(stdin);
    if (!f.firstname)
        goto on_error;
    printf("Please enter your last name: ");
    fflush(stdout);
    f.lastname = read_line(stdin);
    if (!f.lastname)
        goto on_error;

    printf("You first name is: %s\n", f.firstname);
    printf("Your last name is: %s\n", f.lastname);

    free(f.firstname);
    free(f.lastname);
    return EXIT_SUCCESS;

on_error:
    perror("read_line");
    free(f.firstname);
    free(f.lastname);
    return EXIT_FAILURE;
}