同一C程序在Code::Blocks与gcc编译后执行异常问题求助

问题现象 #

• 使用Code::Blocks（MinGW环境）编译目标代码，生成54KB可执行文件，运行完全符合预期：输入内容后按Ctrl+D/Ctrl+C可正常输出单词长度的垂直直方图；
• 使用gcc在cmd或Emacs中编译同一代码，编译成功（生成41KB文件），但运行时仅部分执行就快速退出，无任何输出；即使将gcc编译的exe放到Code::Blocks中运行，结果依旧如此。

该程序是《C程序设计语言》1.13题的实现，功能为统计输入单词长度并输出垂直直方图。

排查与问题根源 #

多次删除可执行文件重新编译，结果一致。曾怀疑是getchar()函数的问题，最终确认未初始化largestN变量是导致异常的核心原因：

• Code::Blocks默认Debug编译模式下，未初始化的局部变量可能被自动初始化为0，因此largestN能正常参与后续的最大值遍历逻辑；
• gcc默认编译为Release模式，未初始化的局部变量会包含随机垃圾值。在遍历wordLen数组寻找最大值时，if (wordLen[ii] >= largestN)的判断逻辑会因largestN的随机值失效，甚至可能导致后续int level[largestN];定义的数组出现越界、内存非法访问等问题，最终触发程序崩溃退出，无任何输出。

问题代码分析 #

问题代码中largestN仅声明未初始化，导致逻辑异常：

//Declaration of variables for Histogram.
int currentLvl, largestN, ii, iii, IV, V;

//Finds and stores the largest number from the array into var:largestN.
for (ii = 0; ii < MAXLENGTH; ii++) {
    if (wordLen[ii] >= largestN)
        largestN = wordLen[ii];
}

修复方案 #

初始化largestN为0，确保最大值遍历逻辑从合理的初始值开始：

//Declaration of variables for Histogram.
int currentLvl, largestN = 0, ii, iii, IV, V;

//Finds and stores the largest number from the array into var:largestN.
for (ii = 0; ii < MAXLENGTH; ii++) {
    if (wordLen[ii] >= largestN)
        largestN = wordLen[ii];
}

完整修复后的代码：

#include <stdio.h>

#define MAXLENGTH 25  /* Maximum length of input. */

int main(){

    /* Histogram of length of words in its input. Vertical or Horizontal. (Vertical is hardest, so let's do that!).
     * Identify words and count characters within each, store that length in a array that represents the length for each word.
     * Draw based on the number of words, their lengths, and their maximum height (optional).
     */

    /* Identify when inside of a word, start counting. When leaving word, record c-count into array, reset count to 0 after.
     * Until entering new word.
     */
    
    //Declaration of the Counter's variables here
    //The Histogram's variables are farther down below!
    int nw, c, cCount, wordLen[MAXLENGTH];
    int i = nw = cCount = 0;
    
    //Initialize wordLen into 0 to avoid garbage results from any potential unused members of the array.
    for (i = 0; i < 25; i++) {
        wordLen[i] = 0;
    }

    //Counts characters in each word and stores inside of chronological array.
    while ((c = getchar())!= EOF){
        if (c == ' ' || c == '\t' || c == '\n'){ 
            wordLen[nw] = cCount;
            ++nw;
            cCount = 0;
            
        }
        else {
            ++cCount; 
        }
    }
    //Prints first 4 words lengths for comparison purposes.
    printf("%d %d %d %d\n\n", wordLen[0], wordLen[1], wordLen[2], wordLen[3]);

    //Declaration of variables for Histogram.
    int currentLvl, largestN = 0, ii, iii, IV, V;

    //Finds and stores the largest number from the array into var:largestN.
    for (ii = 0; ii < MAXLENGTH; ii++) {
        if (wordLen[ii] >= largestN)
            largestN = wordLen[ii];
    }

    //Declare and initialize array for comparison purposes for later use in the Histogram generation process.
    int level[largestN];
    level[0] = 0;
    for (iii = 1; iii < largestN + 1; iii++){
        level[iii] = iii;
    }
    // printf("\n largest N is %d\n", largestN); //for testing
    // printf("wordLen 0 is: %d\n", wordLen[0]); //for testing
    

    /* Histogram generation:
     * The logic is simple: From Top-left, move rightwards (in chronological order) and check which of the words' lengths
     * reach the current level or height (starting from Top). Print a line at that spot if they ever do, print an empty space if
     * they do not.
     * Then move one step down, and repeat the process but comparing 1 level lower. Until bottom is reached.
     */
    for (V = largestN; V > 0; V--){
        currentLvl = level[V];
        
        for (IV = 0; IV < MAXLENGTH; IV++){
            if (wordLen[IV] >= currentLvl)
                printf("|");
            else {
                printf(" ");
            }
            printf(" ");
        }
        printf("\n");
    }
    
    return 0;
}

内容的提问来源于stack exchange，提问作者nyrx