返回指针时出现分段错误

Question

I'm trying to make a code in C that can get environment variable and then search for a specific word from that result using strstr. I'm using UBUNTU OS and gcc compiler. Here is the code that I've written. The comment are what I expected to happen.

#include <stdio.h>
#include <string.h>
#include <unistd.h>

extern char **environ;
extern char **tmp;
extern char **tmp2;

char *search_string(char *tmp,int x)
{
      char string[]="ABC";          //I'm looking for ABC in the environment variable
      char *pointer;
      pointer=strstr(tmp,string);   //pointer will point to the result of strstr
      if(pointer != NULL)
      {       printf("%s ,",tmp);
              printf("data found : %s \n",pointer);
      } else  {
              //hope to do something
      }
      return (pointer);
}

int main(char *tmp2)
{
      int x = 0;
      for(x=0;environ[x]!='\0';x++){   //I'm expecting it to keep looping until finish
      tmp2=search_string(environ[x],x); //tmp2 will point to the function return value
      printf("%s\n",tmp2);             //print the return value
      }  //If the search_string return NULL, does it consider string or something else?
      return 0;
}

After running the code, it crashes because of core dump. Here are the output.

ABC=/tmp ,data found : ABC=/tmp 
ABC=/tmp
Segmentation fault (core dumped)

From what I see, it can only do the search_string for only 1 time. Then it crashes. Then I use gdb to find out at what line does it actually crash and here are the result:

Starting program: /home/fikrie/a.out 
ABC=/tmp ,data found : ABC=/tmp 
ABC=/tmp

Program received signal SIGSEGV, Segmentation fault.
__strlen_ia32 () at ../sysdeps/i386/i686/multiarch/../../i586/strlen.S:99
99  ../sysdeps/i386/i686/multiarch/../../i586/strlen.S: No such file or directory.

What I dont understand from the debug is that it's receiving error because of SEGV signal. Can someone point me on how to solve this problem? Is it because the search_string returns a NULL value?

Answer 1

The problem is that if search_string() doesn't find the string, it returns NULL . You then pass that NULL to printf() , which crashes.

In main() , you need something like:

if (tmp2)
    printf("%s\n", tmp2);

Also, the tmp2 variable should be of type char * , not char ** . And there's no reason not to declare it local to main() .

Answer 2

A very simple change to your main loop stops the program from crashing:

int main(char *tmp2)
{
      int x = 0;
      for(x=0;environ[x]!='\0';x++){   //I'm expecting it to keep looping until finish
      tmp2=search_string(environ[x],x); //tmp2 will point to the function return value
// >>>>> change these next two lines:
      if(tmp2 != NULL) printf("%s\n",tmp2);             //print the return value
      else printf("%s does not contain ABC\n", environ[x]);
// <<<<< end of change
      }  //If the search_string return NULL, does it consider string or something else?
      return 0;
}

Note that if you only expect one match, you could add a break; when you print the match. The above code prints out all the environment variables - you can see it doesn't stop...