11314：合并多个文件

题目

按给定顺序把多个二进制文件复制到调用方提供的输出流，并保证关闭函数自行打开的每个输入流。

解析

先检查全部路径，再开始任何写入，可以避免因为后面的空路径而留下无意义的部分结果。每次只打开一个输入文件，资源数量保持常数。

fread 返回短块时，需要结合 ferror 和 feof 判断原因。fwrite 也可能只写入一部分，因此用内部循环继续写剩余字节；无法继续时记录写错误。无论复制在哪一步结束，copy_one_file 都执行一次 fclose，并且只在没有更早错误时报告关闭错误。

解析

#include <stddef.h>
#include <stdio.h>

typedef enum {
    MERGE_OK,
    MERGE_INVALID,
    MERGE_OPEN_ERROR,
    MERGE_READ_ERROR,
    MERGE_WRITE_ERROR,
    MERGE_CLOSE_ERROR
} MergeResult;

static MergeResult copy_one_file(
    FILE *output,
    const char *path
) {
    FILE *input = fopen(path, "rb");
    if (input == NULL) {
        return MERGE_OPEN_ERROR;
    }

    MergeResult result = MERGE_OK;
    unsigned char buffer[4096];

    while (result == MERGE_OK) {
        size_t amount = fread(
            buffer,
            1,
            sizeof buffer,
            input
        );

        size_t offset = 0;
        while (offset < amount) {
            size_t written = fwrite(
                buffer + offset,
                1,
                amount - offset,
                output
            );
            if (written == 0) {
                result = MERGE_WRITE_ERROR;
                break;
            }
            offset += written;
        }

        if (result != MERGE_OK) {
            break;
        }

        if (amount < sizeof buffer) {
            if (ferror(input)) {
                result = MERGE_READ_ERROR;
            } else if (!feof(input)) {
                result = MERGE_READ_ERROR;
            }
            break;
        }
    }

    if (fclose(input) == EOF && result == MERGE_OK) {
        result = MERGE_CLOSE_ERROR;
    }
    return result;
}

MergeResult merge_files(
    FILE *output,
    const char *const source_paths[],
    size_t source_count
) {
    if (output == NULL) {
        return MERGE_INVALID;
    }
    if (source_count > 0 && source_paths == NULL) {
        return MERGE_INVALID;
    }

    for (size_t i = 0; i < source_count; ++i) {
        if (source_paths[i] == NULL) {
            return MERGE_INVALID;
        }
    }

    for (size_t i = 0; i < source_count; ++i) {
        MergeResult result = copy_one_file(
            output,
            source_paths[i]
        );
        if (result != MERGE_OK) {
            return result;
        }
    }

    if (fflush(output) == EOF) {
        return MERGE_WRITE_ERROR;
    }
    return MERGE_OK;
}

来源与改编

题型借鉴 MIT 6.087 Practical Programming in C 的 Assignment 3，采用 CC BY-NC-SA 4.0；本题改写为逐资源获取、统一释放并区分错误类别的接口。