c - 分段错误 - C

Question

为什么以下代码返回分段错误？当我注释掉第 7 行时，段错误消失了。

int main(void){
      char *s;
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

干杯!

Answer 1

s是一个未初始化的指针；您正在写入内存中的随机位置。这将调用未定义的行为。

您需要为s. 另外，永远不要使用gets; 没有办法防止它溢出您分配的内存。改为使用fgets。

Answer 2

灾难性的糟糕：

int main(void){
      char *s;
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

更好的：

#include <stdio.h>
#define BUF_SIZE 80

int 
main(int argc, char *argv[])
{
      char s[BUF_SIZE];
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

最好的：

#include <stdio.h>
#define BUF_SIZE 80

int 
main(int argc, char *argv[])
{
      char s[BUF_SIZE];
      int ln;
      puts("Enter String");
      fgets(s, BUF_SIZE, stdin); // Use fgets (our "cin"): NEVER "gets()"

      int ln = strlen(s); 
      char *dyn_s = (char*) malloc (ln+1);
      strcpy (dyn_s, s);
      puts(dyn_s);
      return 0;
    }

Answer 3

你scanf("%s", s);的被注释掉了。这意味着 s 未初始化，因此当此行ln = strlen(s);执行时，您会遇到段错误。

它总是有助于将指针初始化为 NULL，然后在使用指针之前测试 null。

Answer 4

甚至更好

#include <stdio.h>
int
main(void)
{
  char *line = NULL;
  size_t count;
  char *dup_line;

  getline(&line,&count, stdin);
  dup_line=strdup(line);

  puts(dup_line);

  free(dup_line);
  free(line);

  return 0;
}

Answer 5

char *s  does not have some memory allocated . You need to allocate it manually in your case . You can do it as follows
s = (char *)malloc(100) ;

这不会导致分段错误错误，因为您将不再引用未知位置