Question

我试图在C中编写一个函数，将一个文件读入传递给它的char数组。

 SELECT * FROM USERS U
 JOIN login_history L
 ON U.id = L.user_id
 WHERE login_at NOT BETWEEN '2017-09-25' AND '2017-10-2'

我的想法是，我会用这样的东西：

void read_file(char* path, char** bufptr, char* buffer){
  /* open the file, and exit on errors */
  FILE *fp = fopen(path, "r");
  if(fp == NULL){
    perror("Failed to open file");
    exit(1);
  }
  /* figure out how long the file is to allocate that memory */
  fseek(fp, 0, SEEK_END);
  long length = ftell(fp);
  rewind(fp);
  /* allocate memory */
  *bufptr = realloc(*bufptr, length+1);
  fread(buffer, length, 1, fp);
  buffer[length] = 0;
}

但是每当我使用它时，程序编译时，printf都不会向控制台打印任何内容。我开始时有点理解＆＃34;间接＆＃34;在一个非常非常基本的程度，但有人可以向我解释为什么我的代码没有按照我期望的方式工作，以及我应该如何实现这个功能。

（这不是家庭作业，所以任何有效的解决方案或方法都是完美的）

Answer 1

read_file的最后两行应为：

  fread(*bufptr, length, 1, fp);
  (*bufptr)[length] = 0;

新分配的缓冲区中的指针位于*bufptr，而不是buffer。

但是你的程序过于复杂，你不需要传递三个参数read_file。两个就够了，就像这样：

void read_file(char* path, char** bufptr) {
  /* open the file, and exit on errors */
  FILE *fp = fopen(path, "r");
  if (fp == NULL) {
    perror("Failed to open file");
    exit(1);
  }
  /* figure out how long the file is to allocate that memory */
  fseek(fp, 0, SEEK_END);
  long length = ftell(fp);
  rewind(fp);
  /* allocate memory */
  *bufptr = realloc(*bufptr, length + 1);
  fread(*bufptr, length, 1, fp);
  (*bufptr)[length] = 0;
}    

int main(int argc, char** argv) {
  char* somestring = malloc(1024);  //  char* somestring = NULL would be even better here
  read_file("readme.txt", &somestring);
  printf("In main(): %s\n", somestring);
  free(somestring);
  return 0;
}

为简洁起见，此处仍然没有错误检查realloc。

Answer 2

如果要在函数内部分配内存 - 则无需在外部分配。但是应该有记录的协议 - 如果失败 - 没有什么是不成功的，如果成功的话 - 调用函数负责释放缓冲区指针。

您的读取功能必须进行更多错误检查，并且还必须关闭任何分支上的文件。

以下是执行阅读的示例代码：

#define _CRT_SECURE_NO_WARNINGS

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

long read_file(const char* path, char** bufptr) {
    char* buffer;
    int res;
    size_t read;
    FILE *fp;
    if (path == NULL || bufptr == NULL) {
        perror("Invalid parameters");
        return -6;
    }
    /* open the file, and exit on errors */
    fp = fopen(path, "rb");
    if (fp == NULL) {
        perror("Failed to open file");
        return -1;
    }
    /* figure out how long the file is to allocate that memory */
    res = fseek(fp, 0, SEEK_END);
    if (res != 0) {
        perror("Failed to seek file");
        fclose(fp);
        return -2;
    }
    long length = ftell(fp);
    if (length <= 0) {
        perror("Failed ftell");
        fclose(fp);
        return -3;
    }
    rewind(fp);
    /* allocate memory */
    buffer = malloc(length + 1);
    if (buffer == NULL) {
        perror("Out of memory");
        fclose(fp);
        return -4;
    }
    read = fread(buffer, 1, length, fp);
    fclose(fp);

    if ((long)read != length) {
        perror("Failed to read whole file");
        free(buffer);
        return -5;
    }
    buffer[length] = 0;
    *bufptr = buffer;
    return length;
}
int main() {
    char* somestring;
    long res = read_file("c:/key.txt", &somestring);
    if (res < 0) {
        //nothing is leaked - everything opened or allocated was closed and freed by the function
        exit(res);
    }
    printf("In main(): %s\n", somestring);
    free(somestring);
    return 0;
}

C - realloc（）指针然后在函数内设置值

2 个答案: