从函数返回后，为什么我的变量损坏了？

Question

Below is my c code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

char *username;
char *password;

void set_credentials(char*, char*);

int main(void)
{
  set_credentials();
  printf("%s\n", username);     //look here 3
  printf("%s\n", password);     //look here 4
  return EXIT_SUCCESS;
}

void set_credentials(char *username, char *password)
{
  char c;
  char lines[2][100];
  char * tmp = * lines;
  char * user = "user";
  int i = 0;
  FILE *fp = fopen("/netnfork/config/netnfork_credentials.properties", "r");

  if (fp == NULL)
    exit(EIO);
  while ((c = fgetc(fp)) != EOF)
  {
    if (c != '\n')
    {
      *tmp = c;
      tmp++;
    } else {
      *tmp = '\0';
      i++;
      tmp = lines[i];
    }
  }
  fclose(fp);
  i = 0;
  while (i < 2)
  {
    if (strncmp(user, lines[i], 4) == 0)
    {
      username = lines[i] + 5;
      printf("%s\n", username); //look here 1
    } else {
      password = lines[i] + 9;
      printf("%s\n", password);  //look here 2
    }
    i++;
  }
}

Now, when I run the code, I get this:

myname //for 1
mypassword //for 2
myname // for 3
mypasswo // for 4

I can't understand why that behavior. Have anyone ideea about why that?

Answer 1

Your problem is that you are assigning username and password to parts of the lines array. The lines array is a local variable and the memory allocated to it is no longer safe after set_credentials finishes. To fix it, use malloc to get the memory for your username and password, or make lines a global variable like username and password are.

Answer 2

You have created two pointers username and password which point to function local variables created on that particulars functions stack frame. When the function returns, these pointers will point to garbage, since the stack frame has been destroyed and so the function local variables.

You need to pass the variables as args to your set_credentials function or you need create a struct which you can return from your function filled up properly. Return the struct by value. No need for pointers.

Answer 3

David identifies your problem. One solution is to allocate two character arrays that the function can populate.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>



void set_credentials(char*, char*);

int main(void)
{
  /* to hold the username and password obtained from a text file */
  char username[200] = { '\0' };
  char password[200] = { '\0' };
  set_credentials(username,password);
  printf("%s\n", username);     //look here 3
  printf("%s\n", password);     //look here 4
  return EXIT_SUCCESS;
}

void set_credentials(char *username, char *password)
{
  char c;
  char lines[2][100];
  int i = 0;
  char *tmp = *lines;
  char *user = "user";
  char *pass = "password";


  const char *filename = "/netnfork/config/netnfork_credentials.properties";
  FILE *fp = fopen(filename, "r");

  if (fp == NULL)
    exit(EIO);
  else
    printf("successfully opened %s\n",filename);
                                /* in case more than 2 lines */
                                /* or increase array first dimension */ 
  while ((c = fgetc(fp)) != EOF && i < 2)
  {
    if (c != '\n')
    {
      *tmp = c;
      tmp++;
    } else {
      *tmp = '\0';
      i++;
      tmp = lines[i];
    }
  }

  fclose(fp);
  i = 0;
  while (i < 2)
  {
    if (strncmp(user, lines[i], 4) == 0)
    {
      strcpy(username,lines[i] + 5);
    }
    else if  ( strncmp (pass, lines[i],8) == 0 ) {
      strcpy(password,lines[i] + 9);
    }
    else {
      /* can't assume file is correct so fatal error if not */
      printf("ERROR: invalid line in file:\n");
      printf("%s\n",lines[i]);
      exit(1);
    }
    i++;
  }
}

Answer 4

You can use a static variable in the function which allows you to return a pointer to it, since a static variable will be around even after the function has exited.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>


/* 
  In order to modify a caller's char in a function it must be passed as pointer to char 
  In order to modify a caller's "char *" in a function it must be passed as pointer to "char *" 
*/ 
void set_credentials(char**, char**);

int main(void)
{
  /* to point to the username and password obtained from a text file */
  char *username = NULL;
  char *password = NULL;
  /* pass the addresses of the char *'s so their contents can be modified */
  set_credentials(&username,&password);
  printf("%s\n", username);     //look here 3
  printf("%s\n", password);     //look here 4
  return EXIT_SUCCESS;
}

void set_credentials(char **username, char **password)
{
  char c;
#define MAX_FILE_LINES 2
  static char lines[MAX_FILE_LINES][100];
  int i = 0;
  char *tmp = *lines;
  char *user = "user";
  char *pass = "password";


  const char *filename = "/netnfork/config/netnfork_credentials.properties";
  FILE *fp = fopen(filename, "r");

  if (fp == NULL)
  {
    printf("ERROR: unable to open %s\n",filename);
    exit(EIO);
  }
  else
    printf("successfully opened %s\n",filename);
                                /* in case more lines than expected */
  while ((c = fgetc(fp)) != EOF && i < MAX_FILE_LINES)
  {
    if (c != '\n')
    {
      *tmp = c;
      tmp++;
    } else {
      *tmp = '\0';
      i++;
      tmp = lines[i];
    }
  }

  fclose(fp);
  i = 0;
  while (i < 2)
  {
    if (strncmp(user, lines[i], 4) == 0)
    {
      /* assign to the char * back in the calling routine */
      *username = lines[i] + 5;
    }
    else if  ( strncmp (pass, lines[i],8) == 0 ) {
      /* assign to the char * back in the calling routine */
      *password = lines[i] + 9;
    }
    else {
      /* can't assume file is correct so fatal error if not */
      printf("ERROR: invalid line in file:\n");
      printf("%s\n",lines[i]);
      exit(1);
    }
    i++;
  }
}

Answer 5

Here is a version that uses malloc() and free().

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>


/* "user " */
#define USERLINE_PREFIX 5
/* "password " */
#define PASSLINE_PREFIX 9



void set_credentials(char**, char**);

int main(void)
{
  /* to point to the username and password obtained from a text file */
  char *username = NULL;
  char *password = NULL;
  set_credentials(&username,&password);
  /* (original scheme) printf("username: %s\n", username);  */
  /* (original scheme) printf("password: %s\n", password);  */
  printf("username: %s\n", username + USERLINE_PREFIX ); // line starts "user "    
  printf("password: %s\n", password + PASSLINE_PREFIX ); // line starts "password "
  /* (original scheme)  free(username - USERLINE_PREFIX); */
  /* (original scheme) free(password - PASSLINE_PREFIX); */
  free(password);
  free(username);
  return EXIT_SUCCESS;
}

void set_credentials(char **username, char **password)
{
  /* file format:
  line 1 ->user <username>
  line 2 ->password <password>
  */
  char c;
  #define FILE_LINES 2
  #define MAX_LINE_LENGTH 100
  int i = 0, j = 0;
  char *lines[FILE_LINES];
  char *tmp = NULL;
  char *user = "user ";
  char *pass = "password ";
  char user_found = 0, password_found = 0;
  for (j = 0; j < FILE_LINES; j++)
  {
    lines[j] = malloc( MAX_LINE_LENGTH + 1 );
    lines[j][0] = '\0';
  }
  tmp = lines[0];
  const char *filename = "/netnfork/config/netnfork_credentials.properties";
  FILE *fp = fopen(filename, "r");

  if (fp == NULL)
  {
    printf("ERROR %d trying to open %s\n",errno,filename);
    /* if not exiting program, would need to free() here */
    exit(EIO);
  }
                                /* in case more lines than expected */
  while ((c = fgetc(fp)) != EOF && i < FILE_LINES)
  {
    if (c != '\n')
    {
      *tmp = c;
      tmp++;
    } else {
      *tmp = '\0';
      i++;
      tmp = lines[i];
    }
  }
  if ( i < 2 )  {
     printf("ERROR: file %s is incomplete needs %d lines (password and user)\n",filename,FILE_LINES);
     /* if not exiting program, would need to free() here */
     exit(1);
  }
  fclose(fp);
  i = 0;
  while (i < FILE_LINES)
  {
    if (strncmp(user, lines[i], USERLINE_PREFIX) == 0)
    {
       user_found = 1;
       /* (original scheme) *username = lines[i] + USERLINE_PREFIX; */
       *username = lines[i];
    }
    else if  ( strncmp (pass, lines[i],PASSLINE_PREFIX) == 0 ) {
       password_found = 1;
       /* (original scheme) *password = lines[i] + PASSLINE_PREFIX; */
       *password = lines[i];
    }
    else {
       printf("ERROR: invalid line in file:\n");
       printf("%s\n",lines[i]);
       /* if not exiting program, would need to free() here */
       exit(1);
    }
    i++;
  }
  /* check for the extremely unlikely event that the two lines are both of the same type */
  if ( ! (password_found && user_found ) )
  {
     printf("ERROR: file %s is invalid, missing %s line\n",filename, (user_found) ? "password" : "user" );  
     /* if not exiting program, would need to free() here */
     exit(1);
  }
}