如何使用 fgets 读取未知长度的输入 [英] How to read input of unknown length using fgets

问题描述

我应该如何使用 fgets() 读取长输入，我不太明白.

How am I supposed to read long input using fgets(), I don't quite get it.

我写的

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

int main()
{
    char buffer[10];
    char *input;
    while (fgets(buffer,10,stdin)){
        input = malloc(strlen(buffer)*sizeof(char));
        strcpy(input,buffer);
    }
    printf("%s [%d]",input, (int)strlen(input));
    free(input);
    return 0;
}

推荐答案

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

int main(void)
{
    char buffer[10];
    char *input = 0;
    size_t cur_len = 0;
    while (fgets(buffer, sizeof(buffer), stdin) != 0)
    {
        size_t buf_len = strlen(buffer);
        char *extra = realloc(input, buf_len + cur_len + 1);
        if (extra == 0)
            break;
        input = extra;
        strcpy(input + cur_len, buffer);
        cur_len += buf_len;
    }
    printf("%s [%d]", input, (int)strlen(input));
    free(input);
    return 0;
}

这是关于将为您提供完整输入行的最小更改集.这一次最多可增加 9 个字节的空间；这不是最好的方法，但是需要额外的簿记以更好的方式来做(将分配的空间加倍，并记录分配了多少与使用了多少).注意cur_len记录的是input指向的空间中字符串的长度，不包括终端空值.另请注意，使用 extra 可避免分配失败时的内存泄漏.

This is about the minimal set of changes that will give you the complete line of input. This grows the space by up to 9 bytes at a time; that isn't the best way to do it, but there's extra bookkeeping involved doing it the better ways (doubling the space allocated, and keeping a record of how much is allocated vs how much is in use). Note that cur_len record the length of the string in the space pointed to by input excluding the terminal null. Also note that the use of extra avoids a memory leak on failure to allocate.

strcpy() 操作可以合法地替换为 memmove(input + cur_len, buffer, buf_len + 1)(在这种情况下，您可以使用 memcpy() 而不是 memmove()，但它并不总是有效而 memmove() 总是有效，因此使用起来更可靠memmove()).

The strcpy() operation could be legitimately replaced by memmove(input + cur_len, buffer, buf_len + 1) (and in this context, you could use memcpy() instead of memmove(), but it doesn't always work while memmove() does always work, so it is more reliable to use memmove()).

长度加倍——cur_max 变量记录分配了多少空间，cur_len 记录了正在使用的空间量.

With length-doubling — the cur_max variable records how much space is allocated, and cur_len records how much space is in use.

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

int main(void)
{
    char buffer[10];
    char *input = 0;
    size_t cur_len = 0;
    size_t cur_max = 0;
    while (fgets(buffer, sizeof(buffer), stdin) != 0)
    {
        size_t buf_len = strlen(buffer);
        if (cur_len + buf_len + 1 > cur_max)
        {
            size_t new_len = cur_max * 2 + 1;
            if (buf_len + 1 > new_len)
                new_len = buf_len + 1;
            char *extra = realloc(input, new_len);
            if (extra == 0)
                break;
            input = extra;
            cur_max = new_len;
        }
        strcpy(input + cur_len, buffer);
        cur_len += buf_len;
    }
    printf("%s [%d]", input, (int)strlen(input));
    free(input);
    return 0;
}

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