Segmentation Fault when accessing char array outside function but noth within - c

Question

so basically, thats the code producing my segmentation fault
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CORRUPT_KEY_FILE "s64581-source-key-corrupt.bin" /* ContainsKey and IV */
#define DEBUG 1
/**
* Reads content of a given file and writes it into a buffer
*
* #param filename Path of the file to be read
* #param buffer Pointer to the buffer to write into
* #param length Variable to store the filelength into
*
* #return EXIT_FAILURE if an error occurred opening th File, otherwise EXIT_SUCCESS
*/
int readFiletoBuffer(const char* filename, unsigned char* buffer, int* length) {
int i;
if(DEBUG) printf("\n\nFunction readFileToBuffer(..) called\n");
FILE *file = fopen(filename, "rb");
if(!file) {
printf("Error opening file %s", filename);
return EXIT_FAILURE;
}
fseek(file, 0 , SEEK_END);
*length = ftell(file);
rewind(file);
if(DEBUG) printf("\tLength of File %s: %d\n",filename ,*length);
buffer = (unsigned char*) malloc(*length+1);
fread(buffer, *length, 1, file);
fclose(file);
for(i=0; i<*length; i++) printf("%d:\t%X\n",14, buffer[14]);
return EXIT_SUCCESS;
}
int main(int argc, char **argv)
{
unsigned char *tmp_char;
int tmp_length, i;
tmp_char=NULL;
readFiletoBuffer(CORRUPT_KEY_FILE, tmp_char, &tmp_length);
for(i=0; i<tmp_length; i++) printf("%d:\t%X\n",i, tmp_char[i]);
return 0;
}
Notice the two for loops printing the content of the buffer. The one inside the function works fine, the one in the main-function produces a segmentations fault. Why is that, and how could I fix this while allocating the needed memory within the function readFiletoBuffer ?
Any help is appreciated! :D

Answer 1

To take the char array you want from the function you have to pass a char** array as argument.
In that way your char* pointer will be returned.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CORRUPT_KEY_FILE "s64581-source-key-corrupt.bin" /* ContainsKey and IV */
#define DEBUG 1
/**
* Reads content of a given file and writes it into a buffer
*
* #param filename Path of the file to be read
* #param buffer Pointer to the buffer to write into
* #param length Variable to store the filelength into
*
* #return EXIT_FAILURE if an error occurred opening th File, otherwise EXIT_SUCCESS
*/
int readFiletoBuffer(const char* filename, unsigned char** buffer, int* length) {
int i;
if(DEBUG) printf("\n\nFunction readFileToBuffer(..) called\n");
FILE *file = fopen(filename, "rb");
if(!file) {
printf("Error opening file %s", filename);
return EXIT_FAILURE;
}
fseek(file, 0 , SEEK_END);
*length = ftell(file);
rewind(file);
if(DEBUG) printf("\tLength of File %s: %d\n",filename ,*length);
*buffer = (unsigned char*) malloc(*length+1);
fread(*buffer, *length, 1, file);
fclose(file);
for(i=0; i<*length; i++) printf("%d:\t%X\n",14, buffer[14]);
return EXIT_SUCCESS;
}
int main(int argc, char **argv)
{
unsigned char *tmp_char;
int tmp_length, i;
tmp_char=NULL;
readFiletoBuffer(CORRUPT_KEY_FILE, &tmp_char, &tmp_length);
for(i=0; i<tmp_length; i++) printf("%d:\t%X\n",i, tmp_char[i]);
return 0;
}
Here is my suggestion.

Answer 2

The problem is, when you call your function readFiletoBuffer with parameter tmp_char, this parameter will be copied! Your tmp_char is NULL after call this function!
You need to work with the pointer to this parameter to change it like this:
tmp_char=NULL;
readFiletoBuffer(CORRUPT_KEY_FILE, &tmp_char, &tmp_length);
So your function will look like this:
int readFiletoBuffer(const char* filename, unsigned char** buffer, int* length)