有效读取标准输入c编程 [英] Effective stdin reading c programming

问题描述

有人能帮助我optimalize code读取标准输入。这是我现在有：

 无符号字符*味精;
为size_t msgBytes = 0;
为size_t inputMsgBuffLen = 1024;
如果（（味精=（无符号字符*）malloc的（sizeof的（无符号字符）* inputMsgBuffLen））== NULL）{
    quitErr（无法分配memmory！，EXIT_FAILURE）;
}
对于（INT℃;（C =的getchar（））= EOF;！msgBytes ++）{
    如果（msgBytes＆GT; =（inputMsgBuffLen））{
        inputMsgBuffLen＆所述;＆下; = 1;
        如果（（味精=（无符号字符*）的realloc（味精，sizeof的（无符号字符）* inputMsgBuffLen））== NULL）{
            免费（MSG）;
            quitErr（无法分配更多memmory！，EXIT_FAILURE）;
        }
    }
    味精[msgBytes] =（unsigned char型）C;
}
 

解决方案

问：你从标准输入读取二进制或文本数据？如果文本，你为什么使用 unsigned char型？

的几点建议：

1. 删除所有石膏的malloc 和的realloc ;他们是没有必要的，凌乱的code;


2. 而不是反复调用的getchar ，使用 FREAD 或与fgets （取决于你正在阅读二进制或文本）;


3. 记住的realloc 有可能返回NULL，所以你要的结果分配给一个临时的值，否则你会失去跟踪原指针和风能泄漏内存;


4. 使用为每个输入块静态分配的缓冲区;


5. 使用的sizeof 的对象，而不是类型;这是一个少许清洁剂，它可以保护你的情况下，类型更改（例如， T * p =的malloc（sizeof的* P * number_of_elements）;

清理后的版本假设你打算使用无符号的字符：

 的#define inputBufSize 1024无符号字符*味精= NULL;
为size_t msgBytes = 0;
为size_t inputMsgBufSize = 0;
unsigned char型INPUTBUFFER [inputBufSize]
为size_t读取动作= 0;而（（读取动作= FREAD（
    INPUTBUFFER，//目标缓冲区
    sizeof的INPUTBUFFER，//缓冲区的字节数
    1，//缓冲器大小的元素的数目，以读
    标准输入））＆GT; 0）
{
  无符号字符* TMP = realloc的（味精，inputMsgBufSize +读取动作））;
  如果（TMP）
  {
    味精= tmp目录;
    memmove与（安培;味精[inputMsgBufSize]，INPUTBUFFER，读取动作）;
    inputMsgBufSize + =读取动作;
  }
  其他
  {
    的printf（冉出内存的\\ n）;
    免费（MSG）;
    打破;
  }
}
 

can anyone help me optimalize code for reading standard input. Here it is what I have now:

unsigned char *msg;
size_t msgBytes = 0;
size_t inputMsgBuffLen = 1024;
if ( (msg = (unsigned char *) malloc(sizeof(unsigned char) * inputMsgBuffLen) ) == NULL ) {
    quitErr("Couldn't allocate memmory!", EXIT_FAILURE);
}
for (int c; (c = getchar()) != EOF; msgBytes++) {
    if (msgBytes >= (inputMsgBuffLen)) {
        inputMsgBuffLen <<= 1;
        if ( ( msg = (unsigned char *)realloc(msg, sizeof(unsigned char) * inputMsgBuffLen) ) == NULL) {
            free(msg);
            quitErr("Couldn't allocate more memmory!", EXIT_FAILURE);
        }
    }
    msg[msgBytes] = (unsigned char)c;
}

解决方案

Question: are you reading binary or text data from stdin? If text, why are you using unsigned char?

Some advice:

1. Drop all the casts on malloc and realloc; they aren't necessary and clutter up the code;
2. Instead of repeatedly calling getchar, use fread or fgets (depending on whether you're reading binary or text);
3. Remember that realloc can potentially return NULL, so you want to assign the result to a temporary value, otherwise you'll lose track of the original pointer and wind up leaking memory;
4. Use a statically allocated buffer for each chunk of input;
5. Use sizeof on objects, not types; it's a little cleaner, and it protects you in case the types change (e.g., T *p = malloc(sizeof *p * number_of_elements);.

Cleaned-up version assuming you intend to use unsigned chars:

#define inputBufSize 1024

unsigned char *msg = NULL;
size_t msgBytes = 0;
size_t inputMsgBufSize = 0;
unsigned char inputBuffer[inputBufSize];
size_t bytesRead = 0;

while ((bytesRead = fread(
    inputBuffer,            // target buffer
    sizeof inputBuffer,     // number of bytes in buffer
    1,                      // number of buffer-sized elements to read
    stdin)) > 0)
{
  unsigned char *tmp = realloc(msg, inputMsgBufSize + bytesRead));
  if (tmp)
  {
    msg = tmp;
    memmove(&msg[inputMsgBufSize], inputBuffer, bytesRead);
    inputMsgBufSize += bytesRead;
  }
  else
  {
    printf("Ran out of memory\n");
    free(msg);
    break;
  }
}

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