Memory leak / not properly freeing. How to fix?

Question

I am partially reading input into a buffer in a function and then freeing it in main() but it doesn't seem to be working. My code:

char *save_to_buff()
{
    int fd = 0; // set read() to read from STDIN_FILENO, because it's number is 0
    const size_t read_size = 100; // set chunk size
    size_t size = read_size;
    char *buff = malloc(size+1);
    size_t offset = 0;
    size_t res = 0;
    while((res = read(fd, buff + offset, read_size)) > 0) // partial read from stdin and save to buff
    {
        if(res == -1) // check for read errors
        {
            read_error();
            free(buff);
            return NULL;
        }
        
        offset += res;
        if (offset + read_size > size)
        {
            size *= 2;
            buff = realloc(buff, size+1);
        }
        buff[offset] = '\0';
    }
    return buff;
}

main:

char *buff = save_to_buff();
// do sth
free(buff);

Edit: just tried it with a 1 byte read and not a partial read and there is no memory leak.

Answer 1

When you read the whole file once, so the read pointer is at EOF, if you read again, as in your code, the read() will return 0, according to this . This will exit your loop without freeing the memory.

Also, depending on what type of file you are reading: Files that do not support seeking - for example, terminals - always read from the current position. The value of a file offset associated with such a file is undefined

Answer 2

You did not post the full code for the main() function, it is possible there is something fishy we don't see.

The function save_to_buff has some issues:

• if stdin is already at the end of file when it starts, it will return a pointer to an uninitialized block of 101 bytes. It should either return a null pointer or a pointer to an empty string.
• you do not test for memory allocation failure

The calling sequence in the code fragment seems fine as long as you do not modify buff in the // do sth part.

Here is a modified version:

#include <stdio.h>
#include <unistd.h>

char *save_to_buff() {
    int fd = 0; // set read() to read from STDIN_FILENO
    const size_t read_size = 100; // set chunk size
    size_t size = read_size;
    size_t offset = 0;
    size_t res = 0;
    char *buff = malloc(size + 1);

    if (buff == NULL)
        return NULL;
   
    *buff = '\0';
    while ((res = read(fd, buff + offset, read_size)) != 0) {
        // partial read from stdin and save to buff
        if (res == (size_t)-1) { // check for read errors
            read_error();
            free(buff);
            return NULL;
        }
        
        offset += res;
        buff[offset] = '\0';

        if (offset + read_size > size) {
            char *buff0;

            size *= 2;
            buff = realloc(buff0 = buff, size + 1);
            if (buff == NULL) {
                free(buff0);
                return NULL;
            }
        }
    }
    return buff;
}