Segmentation fault when initializing pointer to char array

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!

@MrLister It gets initialized in pread(). I also tried initializing it with NULL and other temporary values but the problem still continues. — OmerFY
– OmerFY, Commented Jul 1, 2019 at 13:24
@OmerFY no it doesn't. pread expects you to pass a valid buffer pointer. buf_rd doesn't hold a valid buffer pointer. It's an uninitialized pointer. Initializing it with NULL will cause pread to attempt to write to address NULL, which is bad. Unclear what "other temporary values" means in your context. And you said you have tried using malloc but haven't shown exactly what/how you did that. — lurker
– lurker, Commented Jul 1, 2019 at 13:26
@lurker I tried using malloc for the file_name_write but now I have tried it with buf_rd and it solved the problem. Thank you! — OmerFY
– OmerFY, Commented Jul 1, 2019 at 13:31
You need something like buf_rd = malloc(byte_count * sizeof *buf_rd); before calling pread — 4386427
– 4386427, Commented Jul 1, 2019 at 13:32

virolino · Accepted Answer · 2019-07-01 13:28:30Z

3

char* buf_rd;
...
nr = pread(fd, buf_rd, byte_count, offset);

You did not allocate memory for buf_rd. It is just a pointer.

edited Jul 1, 2019 at 13:28

answered Jul 1, 2019 at 13:26

virolino

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1 Comment

virolino Over a year ago

I noticed that and I already updated the answer. Thanks, though ;) I wrote the answer before re-reading the docs for strtok().

3limin4t0r · Accepted Answer · 2019-07-01 13:33:58Z

1

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.

answered Jul 1, 2019 at 13:33

3limin4t0r

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