我的 C 代码中的分段错误需要帮助

Question

我一直试图确定我的程序在哪里生成分段，但无济于事。 我需要帮助来确定哪些字符串操作或字符指针在运行时导致了分段错误。程序编译成功，但运行时出现分段错误。

#include<curses.h>
#include<strings.h>
#include<unistd.h> 
#include<stdlib.h>

/*Implements a Scrolling headband that takes a string argument and continously scrolls*/               
int main(int argc, char* argv[])
{
    /*A Substring function to return substring of a string*/
    char *substr(const char *src, int start, int len);
    /*Appends a character to the Given string*/
    void append(char* s, char c);
    /***Check if the argument is invalid***/
    if(argc!=2)
    {
        puts("Invalid number of arguments: Usage:headband <String>");
    }
    /**Get headtext from the string argument argv[1]**/
    char *headtext = argv[1];

    /*headband(headtext);*/
    /*temporary variable to store strings as execution progresses*/
    char temp[100];
    /*Counter for streaming and scolling headband text*/
    int count = 0;
    /*Placeholder for temporary headband text*/
    char hold[100];
    int i;
    /*maximum x and y co-ordinates of the Screen*/
    int max_x,max_y;
    /*Initialize screen for ncurses*/
    initscr();
    /*Don't show cursor*/
    curs_set(0);
    /*Get terminal console dimensions*/
    getmaxyx(stdscr, max_y, max_x);
    /*Get console width set to default console screen width=80*/
    int consolewidth = max_x;
    /*Clear the screen*/
    clear();
    /*Set the first value as end of String for the momment*/
    temp[0] = '\0';
    /*Run this loop continuously to keep scrolling*/
    for (;;)
    {
        for(i=0; i < strlen(headtext); i++)
        {
            count++;
            /*Append headband text character by character to string hold*/
            append(temp, headtext[i]);
            move(0,consolewidth - count);
            if (consolewidth - count > 0)
            {
                mvaddstr(0,console_width-count,temp);
                refresh();
            }
            else if (consolewidth - count == 0)
            {
                strcpy(hold, temp);
                char q;
                int com = i;
                for(;;)
                {
                    /*Scroll text by triming one character at a time*/ 
                    /*from the left, then adding that character to the*/
                    /*right side of the text*/
                    com = com + 1;
                    if (com < strlen(headtext))
                    {
                        q = headtext[com];
                    }
                    else
                    {
                        com = 0;
                        q = headtext[com];
                        //q = hold[0];
                    }
                    strcpy(hold, substr(hold, 1, strlen(hold) - 1));
                    append(hold, q);
                    move(0,0);
                    clear();
                    mvaddstr(0,0,hold);
                    refresh();
                    usleep(50);
                }
            }
            usleep(50);
        }
    }
    return 0;
}

/*A Substring function to return substring of a string*/
char * substr(const char *src, int start, int len) 
{   
    char *dest = malloc(len+1);   
    if (dest) 
    {
        memcpy(dest, src+start, len);     
        dest[len] = '\0';   
    }   
    return dest;
}

/*Appends a character to the Given string*/
void append(char s[], char c)
{
    int len = strlen(s);
    s[len] = c;
    s[len+1] = '\0';
}

Answer 1

我不知道您使用的是哪个编译器，但这里有一个如何使用 GCC 和 GDB 进行调试的快速指南：

$ gcc -g file.c -o myexec   # the -g flag enables debug info

$ gdb ./myexec

现在您处于 GDB 提示符下。如果您需要设置命令行参数，请使用：

set args <arg1> <arg2> ...

然后运行你的程序

run

一旦它崩溃，你可以做各种各样的事情。它还显示您的错误发生在程序中的哪个位置。您可能想要使用这些命令：

bt            # prints the backtrace / call stack
print <expr>  # print value of an expression

有几个cheat sheets on the web，因此您可以快速了解可用的命令类型。

Answer 2

使用手动调试方法；

#define DEBUG(A) fprintf(stderr, "%d step\n", (A))
#define PRINT(A) fprintf(stderr, "%s\n", (A))

#include<curses.h>
#include <strings.h>
#include<unistd.h> 
#include<stdlib.h>

int main(int argc, char* argv[]){

  char *substr(const char *src, int start, int len);
  PRINT("at top of the main" ) ; /***/
  DEBUG(1);
  void append(char* s, char c);

  if(argc!=2)
  {
    puts("Invalid number of arguments: Usage:headband <String>");
  }
  char *headtext = argv[1];
  DEBUG (2); 
  char temp[100];

  int count = 0;

  char hold[100];
  int i;

  int max_x,max_y;

  initscr();
  DEBUG (3);
  curs_set(0);

  getmaxyx(stdscr, max_y, max_x);
  DEBUG (4);
  int consolewidth = max_x;

  clear();
  DEBUG(5);
  temp[0] = '\0';

  for (;;)
  {
    for(i=0; i < strlen(headtext); i++)
    {
      count++;

      append(temp, headtext[i]);
      DEBUG(6);
      move(0,consolewidth - count);
      DEBUG(7);
      if (consolewidth - count > 0)
      {
        mvaddstr(0,console_width-count,temp);
        refresh();
      }
      else if (consolewidth - count == 0)
      {                
        char q;
        int com = i;
        strcpy(hold, temp);
        for(;;)
        {
          com = com + 1;
          if (com < strlen(headtext)){
            q = headtext[com];
            DEBUG (10);
          }else{
            com = 0;
            q = headtext[com];
            //q = hold[0];
          }
          strcpy(hold, substr(hold, 1, strlen(hold) - 1));
          append(hold, q);
          move(0,0);
          clear();
          mvaddstr(0,0,hold);
          refresh();
          usleep(50);
        }
      }
      usleep(50);
    }
  }
  return 0;
}

char * 
   substr(const char *src, int start, int len) 
{   
  char *dest = malloc(len+1);   
  PRINT ( "in substr" );
  if (dest) 
  {
    memcpy(dest, src+start, len);     
    dest[len] = '\0';   
  }   
  PRINT ("at the end of the sub);
  return dest;
}

void append(char s[], char c)
{       
  int len = strlen(s);
  PRINT( "in append function" );
  s[len] = c;
  s[len+1] = '\0';
  PRINT ( "at the end of the append function" );
}

编译它，然后你可以很容易地看到你在哪里出现分段错误

Answer 3

我看到的可能导致分段错误的最明显问题是在以下代码中：

if(argc!=2)
{
    puts("Invalid number of arguments: Usage:headband <String>");
}
char *headtext = argv[1];

确定提供了无效数量的参数后，您继续分配指向argv[1] 的指针。如果用户没有提供您的 参数，则结果是将NULL 传递给strlen()。这会导致未定义的行为，strlen(NULL) 通常会在 Linux\Unix 上崩溃。在我的简短测试中，提供 参数确实会导致头带滚动而没有任何崩溃。

您可以考虑将您的代码更改为类似于以下内容：

if(argc<2)
{
    puts("Invalid number of arguments: Usage:headband <String>");
    exit(EXIT_FAILURE);
}
char *headtext = argv[1];

---

更新
对于我认为的预期目的，您当前的代码似乎不必要地复杂（例如嵌套的无限循环）。当前形式的代码也有一些问题需要解决。有一个memory leak 源自在substr() 函数中对malloc() 的调用，随后丢失了指向已分配内存的指针。如果用户提供的文本大于temp[100] 缓冲区的大小，您将在append() 函数中溢出缓冲区。也不必多次计算headtext 的长度，因为它的长度不会改变。

我认为 curses 应该在 raw terminal escape codes 上使用。 curses 库实际上是原始终端代码的包装器，并提供灵活高效的 API，因此应用程序无需担心底层终端功能。也许NCURSES Programming HOWTO 或X/Open Curses, Reference Pages 可以派上用场。

我自己不是 curses 专家，但是，以下示例显示了另一种基于 curses 的方法，它松散地基于原始代码并且似乎具有相同的输出^*：

#include<curses.h>
#include<signal.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h> 

/* maximum x and y co-ordinates of the window */
int max_x, max_y;
/* pointer to headband buffer */
char * headband = NULL;
/* counter for streaming and scolling headband text */
unsigned int count;

/* Handler for window resize */
void handle_winch(int sig)
{
    /* Function for initializing curses & headband buffer */
    void my_init();

    /* Disable signal handler during reinitialization */
    signal(SIGWINCH, SIG_IGN);

    /* Reinitialize the window to update data structures */
    endwin();
    my_init();
}

/* Function for initializing curses & headband buffer */
void my_init()
{
    /* Initialize / Reinitialize screen for ncurses */
    initscr();
    curs_set(0);
    clear();
    refresh();
    getmaxyx(stdscr, max_y, max_x);

    /* Allocate / Reallocate and initialize scrolling headband buffer */
    free(headband);
    headband = (char *)malloc(max_x+1);
    memset(headband, ' ', max_x);
    headband[max_x] = '\0';
    count = 0;

    /* Setup signal handler for window resizing */
    signal(SIGWINCH, handle_winch);
}

/* Implements a scrolling headband that takes a 
 * string argument and scrolls continously */

int main(int argc, char* argv[])
{
    char * headtext;
    int headtext_len;
    int size;

    if(argc<2)
    {
        puts("Invalid number of arguments: Usage:headband <String>");
        exit(EXIT_FAILURE);
    }
    /* Get headtext from the argument argv[1] and compute length */
    headtext = argv[1];
    headtext_len = strlen(headtext);

    /* Call common initialization / reinitialization routine */
    my_init();

    /* Run this loop continuously to keep scrolling */
    for(;;)
    {
        /* Store & use copy of max_x in case original max_x is 
         * modified in signal handler by window resize event */
        size = max_x;

        /* Rotate headband by shifting entire buffer and then
         * appending next character from headtext buffer */
        memmove(&headband[0], &headband[1], size-1);
        headband[size-1] = headtext[count++ % headtext_len];

        /* Update window */
        move(0,0);
        mvaddstr(0,0,headband);
        refresh();
        usleep(50000);
    }

    /* Unreachable, but included for completeness */
    endwin();
    free(headband);
    exit(EXIT_SUCCESS);
}

_{* 包括窗口大小调整处理}