Using fwrite() to write from pointers

Question

This question was asked quite a lot, but specifically in regards to structs containing pointers, and never helped my situation fully. What I'm trying to do, is strtok() the first and only command line argument based on the "|" character. For example, it will be something like: "ls -l | grep ^d | wc -l." After that is complete, I want to write to a LOGFILE the items I tokenized. Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
void main(void)
{
FILE *logfp =  fopen("LOGFILE", "w");   
char * commands; 
char * container[4];
char commandLine[] = ("test|test2|test3|test4\n");
int i = 0;  
commands = strtok(commandLine, "|");
while(commands != NULL) 
{       
    container[i] = commands; 
    printf("Being stored in container: %s\n", container[i]);    
    i++;  
    commands = strtok(NULL, "|");
}   
printf("This is the size of the container: %d\n", (int)sizeof(container));  
fwrite(container,1,sizeof(container),logfp);
fclose(logfp);

}

sizeof() on a pointer returns 8 instead of the correct amount for a char too, so that's another problem. In addition to that, the logfile is full of what I'm guessing is memory addresses to where the pointers are pointing. I want to write the tokenized strings into the LOGFILE. How would I do that?

score 3 · Answer 1 · answered Apr 01 '13 at 03:06

Using sizeof(container) just gives you the size of a pointer, like you said. That will be 8 regardless of what it's pointing to. If you wanted to get the size of a char (1), you would dereference the pointer by using sizeof(*container). However, this still isn't what you're looking for.

The problem with your approach is that, in order to fwrite() all the strings at once, they would need to be stored sequentially in memory, and they're not. The only things that are stored sequentially are the char*s in your container array. Those pointers point to the actual string data, which are all in completely different memory locations.

That being said, the solution is simple: just fwrite() the strings one at a time.

while(commands != NULL) 
{       
    container[i] = commands;
    printf("Being stored in container: %s\n", container[i]);

    //Write one string, using strlen() to calculate the length
    fwrite(container[i], 1, strlen(container[i]), logfp);

    i++;  
    commands = strtok(NULL, "|");
}

Keep in mind, though, these strings will all be mashed together. The file will look like "testtest2test3test4" unless you explicitly add spaces or newlines between them.

Using fwrite() to write from pointers

1 Answers1