Question

I have the following code where strcat is causing problem.

      char* tokens = strtok(buf, "+");
      int n = 0;
      int type = 0;
      char* name = "";
      char* lifetime = "";
      char* data = "";
      for(n=0; tokens!=NULL; n++) {
          if(n==0)
            type = atoi(tokens);
          if(n==1)
            name = tokens;
          if(n == 2) {
             if(type == 1)
                lifetime = tokens;
             else
                data = tokens;
          }
          if(n == 3)
             lifetime = tokens;
          tokens = strtok(NULL, "+");
      }

      if(type == 2) {
         printf("Received Data with Name: %s, Data: \"%s\" and lifetime: %s seconds\n", name, data, lifetime);

         strncat(name, "+", 1);
         printf("Data: %s\n", data);
         strncat(name, data, strlen(data));
         printf("Full Name: %s\n", name);
      }

In the line where I print the name, data and lifetime everything prints correctly. But after the strcat operations I find that data value is empty. After adding debug printf statements I have found that data value becomes empty right after strncat(name,"+",1). What could be the reason? Please help.

Answer 1

Both the name and data pointers point somewhere into the buf string. When concatenating into the name string, you probably happen to overwrite the data string with a null character. You need to allocate a new buffer for name before writing to it:

char resultingName[BUFFER_LENGTH];
strcpy(resultingName, name);
strncat(resultingName, "+", 1);
[...]