Segmentation fault when initializing pointer to char array - c

Question

I am trying to write a program that reads text from a file, converts the characters to uppercase and then writes the output on a new file. The code works just fine for the reading and converting to uppercase parts, but for the writing the output part when I create a char* for the name of the output file, I get a segmentation fault.
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
void lowerToUpper(char* temp)
{
char* name;
name = strtok(temp,":");
// Convert to upper case
char* s = name;
while (*s)
{
*s = toupper((unsigned char) *s);
s++;
}
}
int main()
{
int fd;
char* file_name_read = "./test.txt";
fd = open(file_name_read, O_RDONLY);
char* buf_rd;
ssize_t nr;
size_t byte_count = 1000;
off_t offset = 0;
nr = pread(fd, buf_rd, byte_count, offset);
close(fd);
lowerToUpper(buf_rd);
char* file_name_write = "./test_uppercase.txt";
/* CODE FOR WRITING TO THE FILE */
return 0;
}
When I remove the char* file_name_write line, the code works fine. When I include it in the code, I get a segmentation fault.
I have tried
removing the call to the lowerToUpper() inside main
using char file_name_write[] instead of char* file_name_write
using malloc() to allocate space and then assign its value
using different byte_count and offset values
Edit:
The problem was an uninitialized pointer with buf_rd. When I added
char* buf_rd = (char*) malloc(1000 * sizeof(char));
it solved the problem.
Thank you Mr Lister and lurker!

Answer 1

char* buf_rd;
...
nr = pread(fd, buf_rd, byte_count, offset);
You did not allocate memory for buf_rd. It is just a pointer.

Answer 2

You use buf_rd as buffer, but that variable is only declared and never initialized.
The documentation says:
ssize_t pread(int fd , void * buf , size_t count , off_t offset );
pread() reads up to count bytes from file descriptor fd at offset
offset (from the start of the file) into the buffer starting at buf.
The file offset is not changed.
It is expected of you to initialize the a buffer that can be used by the pread function.