c语言 - 分配中的分割错误

Question

我目前正在学习C语言，我来自Java。我们的作业要求我们从可以添加的文件中计算字符串，或者要求用户输入字符串。我们刚刚开始使用指针，我查找了发生分段错误的不同原因，但我不知道如何检查它是哪个问题。我初始化了所有指向 NULL 的指针，但它仍然不起作用，从我读到的内容来看，这是发生分段错误的最常见原因。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int debug = 0;
int
main(int argc, char **argv)
{
extern char *optarg;
extern int optind;
FILE* infile = NULL;
int c, err = 0; 
int lflag = 0, sflag = 0, count = 0; //flags and count
char *shortWord = NULL, *longWord = NULL; //variable for shortest and longest word
int shortest = 100, longest = 0; //longest char 100, shortest 0
char *string = NULL;
char *pch = NULL;
static char usage[] = "usage: %s [-l] [-s] [filename]n";
while ((c = getopt(argc, argv, "ls")) != -1)
switch (c) 
{
case 'l':
lflag  = 1;
break;
case 's':
sflag = 1;
break; 
case '?':
err = 1;
break;
}
if (err) 
{
fprintf(stderr, usage, argv[0]);
exit(1);
}
//checks for file and then runs loop for word count
infile = fopen("myfile.txt","r");
if (infile != NULL)
{
fgets(string, 100, infile);
pch = strtok (string, " ,.-");
while(pch != NULL)
{
count++;
if (strlen(pch) > longest)
longWord = pch;
if (strlen(pch) < shortest)
shortWord = pch;
pch = strtok (NULL, " ,.");
}
} 
//else, asks for string
else
{
printf("Enter your string: n");
fgets(string, 100, stdin);
int len = strlen(string);
count = len;
pch = strtok ( string, " ,.-");
while(pch != NULL)
{
count++;
if (strlen(pch) > longest)
longWord = pch;
if (strlen(pch) < shortest)
shortWord = pch;
pch = strtok (NULL, " ,.");
}   
}
//following lines compute value based on arguments
if(lflag == 1)
{
printf("Longest word is %s", longWord);
}
if(sflag ==  1)
{
printf("Shortest word is %s", shortWord);
}
printf("Word count = %.2dn", count);
exit(0);
}

Answer 1

它们是代码中的一些问题：

• 您将string初始化为NULL，然后将其用作fgets()的输入缓冲区。fgets()re需要指向字符数组的指针，这些数组可以在堆栈上声明或使用malloc(3)动态分配。您可以设置输入缓冲区，例如char string[100]。
• 必须检查fgets()，因为它在无法读取行时返回NULL。
• strtok()的分隔符不考虑fgets()附加的n字符。您可以删除此换行符，也可以将其包含在 delimter 中。如果要将其包含在分界符中，请确保您的分隔符" ,.-n"。
• 你可以创建函数来解析你的输入，用strtok()，因为这会让你的main()更短，减少代码中的重复性。一个示例函数原型可以void longest_shortest_words(char line[], char **longest, char **shortest, size_t *word_count);，即通过指针将最长、最短的单词以及单词数传递回main()。您也可以只在 2D 数组或指针数组中存储最长和最短的单词。

• 您还应该明确检查您的文件是否已正确打开。应该包括这样的东西：

  infile = fopen("myfile.txt", "r");
  if (infile == NULL) {
  fprintf(stderr, "Failed to open filen");
  exit(EXIT_FAILURE);
  }
  

• 检查opt时，?作为 switch 语句中的字符进行检查是不正确的。而不是：

  case '?':
  err = 1;
  break;
  

  使用default，它涵盖输入的任何其他无效选项。以下是使用它的方法：

  default:
  fprintf(stderr, "usage: %s [-l] [-s] [filename]n", argv[0]);
  exit(EXIT_FAILURE);
  

• 最后检查sflag和lflag是不够的。您应该检查longWord和shortWord是否未NULL。

下面是一些演示这些要点的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define LINESIZE 100
void longest_shortest_words(char line[], char **longest, char **shortest, size_t *wordcount);
void print_output(int lflag, int sflag, char *longword, char *shortword, size_t wordcount);
void remove_newline(char line[]);
int main(int argc, char * const argv[]) {
FILE *infile;
char line[LINESIZE] = {''};
int opt, sflag = 0, lflag = 0;
size_t wordcount = 0;
const char *optstr = "ls";
char *longword = NULL, *shortword = NULL;
while ((opt = getopt(argc, argv, optstr)) != -1) {
switch(opt) {
case 'l':
lflag = 1;
break;
case 's':
sflag = 1;
break;
default:
fprintf(stderr, "usage: %s [-l] [-s] [filename]n", argv[0]); 
exit(EXIT_FAILURE);
}
}
/* Checking if file is in directory */
infile = fopen("myfile.txt", "r");
if (infile == NULL) {
fprintf(stderr, "Failed to open filen");
exit(EXIT_FAILURE);
}
/* checking if line exists in file */
if (fgets(line, LINESIZE, infile) == NULL) {
fprintf(stderr, "No line found in file.n");
printf("nEnter string instead:n");
if (fgets(line, LINESIZE, stdin) != NULL) {
remove_newline(line);
longest_shortest_words(line, &longword, &shortword, &wordcount);   
/* checking that longWord, shortWord and word_count are valid */
if (longword != NULL && shortword != NULL && wordcount > 0) {
print_output(lflag, sflag, longword, shortword, wordcount);
}
}
/* file has line, do stuff with it */
} else {
remove_newline(line);
longest_shortest_words(line, &longword, &shortword, &wordcount);
print_output(lflag, sflag, longword, shortword, wordcount);
}
exit(EXIT_SUCCESS);
}
/* function for printing output, can be improved */
void print_output(int lflag, int sflag, char *longword, char *shortword, size_t wordcount) {
if (lflag) {
printf("Longest word: %sn", longword);
}
if (sflag) {
printf("Shortest word: %sn", shortword);
}
if (wordcount > 0) {
printf("Word count = %zun", wordcount);
}
}
/* function for removing newline, and checking that input hasnt exceeded limit */
void remove_newline(char line[]) {
size_t slen;
slen = strlen(line);
if (slen > 0 && line[slen-1] == 'n') {
line[slen-1] = '';
} else {
fprintf(stderr, "nToo many characters in input.n");
exit(EXIT_FAILURE);
}
}
/* function which parses line, and saves longWord and shortWord in pointers */
void longest_shortest_words(char line[], char **longword, char **shortword, size_t *wordcount) {
char *word = NULL;
const char *delim = " ,.";
word = strtok(line, delim);
if (word != NULL) {
*longword = word;
*shortword = word;
*wordcount = 1;
}
while ((word = strtok(NULL, delim)) != NULL) {
(*wordcount)++;
if (strlen(word) > strlen(*longword)) {
*longword = word;
} else if (strlen(word) < strlen(*shortword)) {
*shortword = word;
} 
}
}

注意：上面显示的代码可以改进，它只是向您展示解决问题的另一种方法。