I am getting this error in my program…
mprotect: Cannot allocate memory
ulimit -a gives the output:
core file size (blocks, -c) 0 data seg size (kbytes, -d) unlimited scheduling priority (-e) 20 file size (blocks, -f) unlimited pending signals (-i) 16382 max locked memory (kbytes, -l) 64 max memory size (kbytes, -m) unlimited open files (-n) 1024 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) unlimited virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
The amount of memory I’m trying to protect is 60 MB. Can someone tell me what is the problem and how it can be solved?
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Answer
Return Value On success, mprotect() returns zero. On error, -1 is returned, and errno is set appropriately. Errors EACCES The memory cannot be given the specified access. This can happen, for example, if you mmap(2) a file to which you have read-only access, then ask mprotect() to mark it PROT_WRITE. EFAULT The memory cannot be accessed. EINVAL addr is not a valid pointer, or not a multiple of PAGESIZE. ENOMEM Internal kernel structures could not be allocated. Or: addresses in the range [addr, addr+len] are invalid for the address space of the process, or specify one or more pages that are not mapped.
Given the error message, you probably got an ENOMEM error, and looking at the error code, this does not necessarily mean that memory could not be allocated. You might have an invalid address range, or (most likely) you have pages that aren’t mapped.
Don’t try to protect such a big hunk of memory in one swell foop. Given how virtual memory works, the odds are just too high that some page in that huge chunk will not be mapped. You need to ensure that the page (pages) in question are mapped before calling mprotect.
When you are using system functions it is always a good idea to read the man page on that function. Then re-read it. The man pages can be a bit terse at times.