Thread: passing pointers outside of functions

I am trying to create a function that reads everything from a file and puts it into memory.

void fileRead(char *filePtr,const char *filename)
Successfully copies the data from the file into the char *buffer variable,
the char *filePtr then points to what char *buffer is pointing to.

For some reason I just cannot seem to access the file data after the function ends.

char *copydatatohere;
fileRead(copydatatohere,"myfile.txt");
printf("%s",copydatatohere); <------------copydatatohere points to nothing.????! why?

Code:

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

void displayUsage(){
	printf("-----------------------------\n");
	printf("Usuage: bwPercentage input.bmp\n");
	printf("-----------------------------\n");
}

void fileRead(char *filePtr,const char *filename){
	FILE * pFile;
	long lSize;
	size_t result;
	char *buffer;

	pFile = fopen ( filename, "rb" );
  	if (pFile==NULL) {
		fputs ("File error",stderr); 
		exit (1);
	}

  	fseek (pFile , 0 , SEEK_END);
  	lSize = ftell (pFile);
  	rewind (pFile);

  	buffer = (char*) malloc (sizeof(char)*lSize);
  	if (buffer == NULL) {
		fputs ("Memory error",stderr); 
		exit (2);
	}

  	result = fread (buffer,1,lSize,pFile);
  	if (result != lSize) {
		fputs ("Reading error",stderr); 
		exit (3);
	}
  	fclose (pFile);
	filePtr = buffer;
	//printf("%s",filePtr); WORKS?!
	//why doesn't the char *data variable point to the buffer after this function finishes?
	}



int main(int argc, char **argv)
{
	char *filename;
        if(argc == 2){
		filename = argv[1];
	}
	else{
		displayUsage();
		exit(EXIT_FAILURE);	
	}
	char *data;
	fileRead(data,filename); //does not transfer into char *data
	printf("%s",data); //prints nothing but an address
        exit(EXIT_SUCCESS);
}

Not working because you are only passing a random adress to your function instead of passing an adress of a char *.

Code:

//...

void fileRead(char **filePtr,const char *filename){
	FILE * pFile;
	long lSize;
	size_t result;
	char *buffer;

	pFile = fopen ( filename, "rb" );
  	if (pFile==NULL) {
		fputs ("File error",stderr); 
		exit (1);
	}

  	fseek (pFile , 0 , SEEK_END);
  	lSize = ftell (pFile);
  	rewind (pFile);

  	buffer = (char*) malloc (sizeof(char)*lSize);
  	if (buffer == NULL) {
		fputs ("Memory error",stderr); 
		exit (2);
	}

  	result = fread (buffer,1,lSize,pFile);
  	if (result != lSize) {
		fputs ("Reading error",stderr); 
		exit (3);
	}
  	fclose (pFile);
	*filePtr = buffer;
	//printf("%s",filePtr); WORKS?!
	//why doesn't the char *data variable point to the buffer after this function finishes?
}



int main(int argc, char **argv)
{
	char *filename;
        if(argc == 2){
		filename = argv[1];
	}
	else{
		displayUsage();
		exit(EXIT_FAILURE);	
	}
	char *data;
	fileRead(&data,filename); //does not transfer into char *data
	printf("%s",data); //prints nothing but an address
        exit(EXIT_SUCCESS);
}

And careful with this, you are reading your file as binary but you are printing it as a string, which is not really semantically good (also, fread() doesn't append a '\0' character...). Anyway, be aware of this.

It works =D.
Well, I still don't understand this whole pointer ordeal, but
thank you very much, I appreciate the help foxman.

I will definitely work on those brackets, Elysia.

I would like to add that if you have some problems understanding why you need a char** as the first argument of your function, well, you could rewrite your function for something more "natural"

Code:

//...

char* fileRead(const char *filename){
	FILE * pFile;
	long lSize;
	size_t result;
	char *buffer;

	//...your original code minus the line "filePtr = buffer;"

	return buffer;
}

int main(int argc, char **argv)
{
	//...
	char *data;
	data = fileRead(filename);
}

You are interested in where buffer points to. When you originally wrote "filePtr = buffer;" in your fileRead function, well, that's exactly like declaring a local char* variable in the fileRead function and doing "thisCharPtrVar = buffer;". That's not making any change outside the function.