mmap fails with errno EBADF

Elkvis · 09-10-2009, 09:27 AM

In my program I am opening a file as follows:

Code:

std::string filename = "somefile";
struct stat st;
stat(filename.c_str(), &st);
int fd = open(filename.c_str(), O_RDONLY);

the open succeeds, and gives me a valid file number.
I then attempt to memory-map the file using mmap() as follows:

Code:

unsigned char* buf = (unsigned char*)mmap(NULL, st.st_size, PROT_READ, fd, MAP_PRIVATE | MAP_POPULATE, 0);

I then check the return value of mmap() and find that it is equal to MAP_FAILED. I print the return value of strerror(errno) to stdout and it says "Bad file descriptor."

the file is exactly 51200000 bytes long (an integer multiple of the 4096 byte page size), and I'm not asking it to map to a specific or non-aligned memory address.

not sure what I'm doing wrong here, but I just can't seem to make this work.

system is OpenSuse 10.3 x86 kernel version 2.6.22.5-31-default
2GB of system memory + 4GB swap
GCC version 4.2.1

Codeplug · 09-10-2009, 09:51 AM

You got your parameters to mmap() mixed up.

gg

Elkvis · 09-10-2009, 09:58 AM

thanks! it would have taken me many hours to spot that, being inexperienced with mmap and its friends.

MK27 · 09-10-2009, 10:03 AM

This is all I've ever done with mmap(), but it worked:

seeking for exercises

Looking at that, it appears I used a file opened O_RDWR|O_CREAT, not O_RDONLY, but since your mmap call is PROT_READ I guess that should be okay. The other difference is in the flags at the end...which it looks to me like you have the flags parameter and the fd parameter backward!

09-10-2009, 09:27 AM	#1
Elkvis Registered User Join Date: Oct 2006 Posts: 263	mmap fails with errno EBADF In my program I am opening a file as follows: Code: std::string filename = "somefile"; struct stat st; stat(filename.c_str(), &st); int fd = open(filename.c_str(), O_RDONLY); the open succeeds, and gives me a valid file number. I then attempt to memory-map the file using mmap() as follows: Code: unsigned char* buf = (unsigned char)mmap(NULL, st.st_size, PROT_READ, fd, MAP_PRIVATE \| MAP_POPULATE, 0); I then check the return value of mmap() and find that it is equal to MAP_FAILED. I print the return value of strerror(errno) to stdout and it says "Bad file descriptor." the file is exactly 51200000 bytes long (an integer multiple of the 4096 byte page size), and I'm not asking it to map to a specific or non-aligned memory address. not sure what I'm doing wrong here, but I just can't seem to make this work. system is OpenSuse 10.3 x86 kernel version 2.6.22.5-31-default 2GB of system memory + 4GB swap GCC version 4.2.1 Last edited by Elkvis; 09-10-2009 at 09:30 AM. Reason: add more info*
Elkvis is offline

09-10-2009, 09:51 AM	#2
Codeplug Registered User Join Date: Mar 2003 Posts: 3,844	You got your parameters to mmap() mixed up. gg __________________ How to Ask Questions The Smart Way How to Report Bugs Effectively
Codeplug is offline

09-10-2009, 09:58 AM	#3
Elkvis Registered User Join Date: Oct 2006 Posts: 263	thanks! it would have taken me many hours to spot that, being inexperienced with mmap and its friends.
Elkvis is offline

09-10-2009, 10:03 AM	#4
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	This is all I've ever done with mmap(), but it worked: seeking for exercises Looking at that, it appears I used a file opened O_RDWR\|O_CREAT, not O_RDONLY, but since your mmap call is PROT_READ I guess that should be okay. The other difference is in the flags at the end...which it looks to me like you have the flags parameter and the fd parameter backward! __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS
MK27 is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
errno function, error TLS/non TLS	fran.b	C Programming	13	03-25-2009 08:06 AM
BSD mmap for shared memory - am I right?	sean	Linux Programming	21	03-09-2009 01:57 PM
using mmap for copying large files	rohan_ak1	C Programming	6	05-13-2008 08:12 AM
Function argument assignment between types "unsigned int" and "unsigned long"	nadeer78	C Programming	8	03-10-2008 11:57 AM
simulate Grep command in Unix using C	laxmi	C Programming	6	05-10-2002 04:10 PM