Problem to acess to a dynamic string

collinm · #1 25-Mar-2005, 09:19

hi

in my main i do:

CPP / C++ / C Code:

char *led_line=NULL;
searchFile(mnt_dir_led, "txt", led_line);

in my searchFile i do:

CPP / C++ / C Code:

int searchFile(char *directory, char *ext, char *led_line)
{
  DIR *dirh;
  struct dirent *dirinfo;
  char *file_ext;
  int num = 0;
  
  dirh = opendir(directory);
  
  while(dirh)
  {
    if((dirinfo = readdir(dirh)) != NULL)
    {
      if((file_ext = strrchr(dirinfo->d_name, '.')) != NULL) 
        file_ext++;
      else 
        file_ext = dirinfo->d_name;
      
      if(!strcmp(file_ext, ext))
      {
        analyzeFilename(dirinfo->d_name, strlen(dirinfo->d_name), led_line);
      }
      num++;
    }
    else break;
  }
  closedir(dirh);  
  return num;
}

in my analyzeFileName i do:

CPP / C++ / C Code:

void analyzeFilename(char *filename, int size, char *led_line)
{
    readFile(tmp_mnt_dir_led, size, mnt_dir_led, filename, led_line);
    printf("led_line: %s\n",led_line);

in my readFile i do:

CPP / C++ / C Code:

void readFile(char *tmp_dir_led, int size, char *directory, char *filename, char *led_line)
{
    FILE *fp;
    char line[LINE_MAX];
    snprintf(tmp_mnt_dir_led, sizeof(tmp_mnt_dir_led),"%s/%s", directory,filename);        
    fp = fopen(tmp_mnt_dir_led, "r");
    if(fp!=NULL)
    {
        fgets(line, LINE_MAX, fp);
        if(led_line!=NULL)
             free(led_line);
        led_line=(char *) malloc (sizeof(line));
        strcpy(led_line,line);
    }
}

why in my analyzeFilename function, after the call function to readFile i get NULL when i printf led_line?

i want to do some action to led_line after the readFile... but i get NULL

how to resolve that?

thanks

davekw7x · #2 25-Mar-2005, 10:21

Quote:

Originally Posted by collinm

hi

in my main i do:

in my analyzeFileName i do:

CPP / C++ / C Code:

void analyzeFilename(char *filename, int size, char *led_line)
{
    readFile(tmp_mnt_dir_led, size, mnt_dir_led, filename, led_line);
    printf("led_line: %s\n",led_line);

in my readFile i do:

CPP / C++ / C Code:

void readFile(char *tmp_dir_led, int size, char *directory, char *filename, char *led_line)
{

        led_line=(char *) malloc (sizeof(line));
        strcpy(led_line,line);

why in my analyzeFilename function, after the call function to readFile i get NULL when i printf led_line?

i want to do some action to led_line after the readFile... but i get NULL

how to resolve that?

thanks

You pass led_line (a pointer to char) to readFile(). Inside readFile(), you set the value of readFile()'s local copy of led_line to the allocated storage. This does not affect the value of led_line in the calling program.

One way around this is to pass the address of led_line (therefore a pointer to a pointer to char).

An example:

CPP / C++ / C Code:

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

void print_something(char *);

int main()
{
  int do_something(char **);

  char *led_line = NULL;

  if (!do_something(&led_line)) {
    printf("do_something() failed. Terminating program\n");
    return 1;
  }

  printf("\nIn main().                led_line = <%s>\n", led_line);
  printf("\nIn main().                Calling print_something()\n");
  print_something(led_line);

  if (led_line) {
    free(led_line);
  }

  return 0;
}

int do_something(char **line)
{
  int length;
  char *this_is_it = "This is from do_something";
  length = strlen(this_is_it);

  *line = malloc(length + 1); /* an extra byte for the terminating zero byte */
  if (!*line) {
    printf("Error: malloc() couldn't allocate %d bytes\n", length+1);
    return 0; /* error return */
  }
  else {
    strcpy(*line, this_is_it);
    printf("\nIn do_something().        Calling print_something)()\n");
    print_something(*line);
    return 1; /*OK return */
  }
}

void print_something(char *s)
{
  printf("\nIn print_something().     s = <%s>\n", s);
}

Regards,

Dave

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