Malloc fails with errno 12

Question

I read/write data into a custom raw file where I used to write

version (int)
data

I implemented a more generic way to write my data and now the expected format is

headersize (size_t)
header (headersize)
data

Therefore, I manually added 4 (sizeof(int)) to an old raw file (with ghex) to make it compatible with the new implementation.

Now, my program is failing when malloc-ing space for a header_p on which I want to read the header from the raw file.

size_t headersize;
fread(&headersize, sizeof(size_t), 1, fd);
printf("%p\t%d\n", header_p, (int) headersize);
header_p = malloc(headersize);
printf("%p\t%d\t%d\t%s\n", header_p, (int) headersize, errno,strerror(errno));

Returns

(nil)   4
(nil)   4   12  Cannot allocate memory

Why would malloc fail on such operation? The headersize seems correctly hard-written in the raw file since it's equal to 4 and errno of 12 seems to indicate that I don't have enough memory but when I hard-code sizeof(int) at the malloc call, the failure doesn't occur anymore.

size_t headersize;
fread(&headersize, sizeof(size_t), 1, fd);
printf("%p\t%d\n", header_p, (int) headersize);
header_p = malloc(sizeof(int));
printf("%p\t%d\t%d\n", header_p, (int) headersize, errno);

Returns

(nil)   4
0x8e6e90    4   0

I suspect that the errno of 12 hides something else but I don't understand what.

read from file : `fread(&headersize, sizeof(size_t), 1, multifile->fd)` — Jav, Feb 23 '15 at 13:55
You can you `perror()` in order to get you error message in stderr. — Parham Alvani, Feb 23 '15 at 13:56
are you checking that `fread()` returned `1`? With @mch comment I now suspect that `headersize` is not initialized because `fread()` failed. — Iharob Al Asimi, Feb 23 '15 at 13:57
I updated the code with the casting of `size_t headersize` into an `int` for printing — Jav, Feb 23 '15 at 14:00
If you want a better test, assign headersize = sizeof(int), and then try the malloc — Kieveli, Feb 23 '15 at 14:01
@Jav, try using `ssize_t` for the `headersize` variable, and use the appropriate specifier `"%ld"`, and see if the value is negative. — Iharob Al Asimi, Feb 23 '15 at 14:07
to be even more correct you need to use `(void*)header_p` inside printf — Giorgi Moniava, Feb 23 '15 at 14:10
convert to `unsigned long` if you're stuck with a C89 compiler: `printf("%p\t%lu\n", (void *) header_p, (unsigned long) headersize);`. If you have a C99 compiler use `"%zu"` instead: `printf("%p\t%zu\n", (void *) header_p, headersize);` — pmg, Feb 23 '15 at 14:17
`fread(&headersize, sizeof(ssize_t), 1, fd); printf("%ld,headersize);` returns `12884901892` — Jav, Feb 23 '15 at 14:18

score 1 · Accepted Answer · answered Feb 23 '15 at 15:50

There is your problem. sizeof(size_t) must take the pointer size, and may/may not be sizeof(int). If you print it with the (int) cast, it looks okay, because only the least significant 4 bytes (in your case) are evaluated. If you use it for malloc() then the whole size(size_t) is used, and it's either garbage or part of your header.

When you're pickling numbers, you should:

Worry about endianness, and convert it to either little/big endian (or leverage hton* from arpa/inet.h)
Use specific-width integers from stdint.h, never ever pickle a type without specified width

Malloc fails with errno 12

1 Answers1