mremap function failed to allocate new memory - c

I have write the following code , but the code is still fiven me EEERROR message , which tells that the mremap failed to extend the memory.
int main()
{
int size_of_mem = 1024
int fd = shm_open("/myregion", O_CREAT | O_RDWR, S_IRWXO | S_IRUSR | S_IWUSR);
if (fd == -1)
printf("ERROR in shm_open \n") ;
if (ftruncate(fd, size_of_mem) == -1)
printf("ERROR in ftruncate \n") ;
int shm_address = mmap(0 , size_of_mem , PROT_READ | PROT_WRITE | PROT_EXEC ,MAP_SHARED , fd , 0) ;
if (shm_address == MAP_FAILED)
{
printf("Error mmapping the file \n");
exit(EXIT_FAILURE);
}
int temp = mremap(shm_address , size_of_mem ,4000 , MREMAP_MAYMOVE) ;
if( temp < 0)
{
printf("EEEEEEEERROR\n") ;
}
return 0 ;
}

There are a couple of things wrong here.
First, mmap() and mremap() return a void* pointer, which you must not just cast to an int.
Second, the mremap() man page states:
RETURN VALUE
On success mremap() returns a pointer to the new virtual memory area. On error, the value MAP_FAILED (that is, (void *) -1) is returned, and errno is set appropriately.
So your check temp < 0 is wrong. It should be temp == (void*)-1. It is entirely possible that mremap() returns a valid pointer on success that is smaller than zero when cast to an int.
Third, both mmap() and mremap() set the errno (man page) variable when an error occurs. You can read that to get more information about what exactly went wrong. To just output a text error message use the perror() function (man page). Note that you have to #include <errno.h> for that.
Fourth, if you detect an error condition, you always print a message, but you mostly allow execution to continue. That doesn't make sense. If shm_open() failed, you want to return immediately (or call exit(EXIT_FAILURE)). None of the following functions will work if you couldn't even open the SHM file.
Thus, my cleaned up version looks like this:
#include <error.h>
int main()
{
int size_of_mem = 1024;
int fd = shm_open("/myregion", O_CREAT | O_RDWR,
S_IRWXO | S_IRUSR | S_IWUSR);
if (fd == -1)
{
perror("Error in shm_open");
return EXIT_FAILURE;
}
if (ftruncate(fd, size_of_mem) == -1)
{
perror("Error in ftruncate");
return EXIT_FAILURE;
}
void *shm_address = mmap(0, size_of_mem,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_SHARED, fd, 0);
if (shm_address == MAP_FAILED)
{
perror("Error mmapping the file");
return EXIT_FAILURE;
}
void *temp = mremap(shm_address, size_of_mem, 4000, MREMAP_MAYMOVE);
if(temp == (void*)-1)
{
perror("Error on mremap()");
return EXIT_FAILURE;
}
return 0;
}
Note:
Correct data types (void*), correct error checking for mremap(), usage of perror() to print more informative error messages, error paths ending execution of the function.
Correct/consistent indentation.
No spaces before , in function calls.

Related

How to map an empty file using mmap

I am trying to create an empty file if it does not exists. And than map it using mmap() so, that i can pass it to my other program for writing. I am not sure which arguments for mmap are suitable for an empty file. My code works for non empty files but gives error "Invalid argument" if file is empty
Code program1 (only creates an empty file if not exists)
int i;
int fd = open("/home/sungmin/dummy_programs/dummy.txt", O_RDONLY | O_CREAT, 0777);
char *pmap;
pid_t child;
if (fd == -1)
{
perror("Error opening file for writing");
exit(EXIT_FAILURE);
}
struct stat fileInfo = {0};
if (fstat(fd, &fileInfo) == -1)
{
perror("Error getting the file size");
exit(EXIT_FAILURE);
}
/*if (fileInfo.st_size == 0)
{
fprintf(stderr, "Error: File is empty, nothing to do\n");
exit(EXIT_FAILURE);
}*/
pmap = mmap(0, fileInfo.st_size, PROT_READ | PROT_EXEC , MAP_ANONYMOUS, fd, 0);
if (pmap == MAP_FAILED)
{
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
/* Calling fork function */
if((child=fork())==0){
printf("Iam Child process\n\n");
static char *argv[]={"This is some sample text. I need to write this text in my dummy file.","/home/sungmin/dummy_programs/dummy.txt",NULL};
execv("/home/sungmin/dummy_programs/pro2",argv);
exit(127);
}
else {
printf("Iam parent, waiting for child process to exit\n\n");
waitpid(child,0,0);
printf("Existing parent\n\n");
}
/* Don't forget to free the mmapped memory*/
if (munmap(pmap, fileInfo.st_size) == -1)
{
close(fd);
perror("Error un-mmapping the file");
exit(EXIT_FAILURE);
}
/* Un-mmaping doesn't close the file, so we still need to do that.*/
close(fd);
Code program2 (opens same file as program1 and writes text passed by program1)
size_t i;
int fd;
char *pmap;
pid_t child;
struct stat fileInfo = {0};
const char *text = argv[0];
fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
if (fd == -1)
{
perror("Error opening file for writing");
exit(EXIT_FAILURE);
}
size_t textsize = strlen(text) + 1; // + \0 null character
if (lseek(fd, textsize-1, SEEK_SET) == -1)
{
close(fd);
perror("Error calling lseek() to 'stretch' the file");
exit(EXIT_FAILURE);
}
if (write(fd, "", 1) == -1)
{
close(fd);
perror("Error writing last byte of the file");
exit(EXIT_FAILURE);
}
pmap = mmap(0, textsize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (pmap == MAP_FAILED)
{
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
/* Writting users text to file */
for (i = 0; i < textsize; i++)
{
pmap[i] = text[i];
}
// Write it now to disk
if (msync(pmap, textsize, MS_SYNC) == -1)
{
perror("Could not sync the file to disk");
}
/* Don't forget to free the mmapped memory*/
if (munmap(pmap, textsize) == -1)
{
close(fd);
perror("Error un-mmapping the file");
exit(EXIT_FAILURE);
}
/* Un-mmaping doesn't close the file, so we still need to do that.*/
close(fd);
You need to use truncate to extend the file length after creating it before mapping it.
Yes, the function name sounds wrong, but truncate can actually set the file length to any number. Be sure to use a multiple of 4K for best results.
Then, if you want to keep the mapping open to see data between Program 1 and 2, you need to get rid of ANONYMOUS and map with MAP_SHARED in Program 1. A mapping that isn't shared will not show changes made by other programs. Or it might, if it has to reload from disk. It's weird, don't mix SHARED and not-SHARED mappings.
Once you've changed Program 1 to use truncate, take that lseek and write code out of Program 2. The file will already have been created and extended by Program 1.

ftruncate() invalid arguement error

I just started learned about shared memory and we are suppose to create an object of shared memory for my assignment, but I keep getting an error when I run my program.
The error comes from the ftruncate() function and it keeps telling me its an invalid argument, here's my code segment for the shared memory:
struct container* rPtr;
int fd;
/* Creates shared memory object and sets it size */
fd = shm_open("/collatzRegion", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
if (fd == -1)
{ perror("shm_open");
return 1;
}
if ((ftruncate(fd, sizeof(struct container))) == -1)
{ perror("ftruncate");
return 1;
}
/* Maps shared memory object */
rPtr = mmap(NULL, sizeof(struct container), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if ( rPtr == MAP_FAILED)
{ perror("mmap");
return 1;
}
and my structure, that is defined before my main(), is:
/* Defines "structure" of shared memory */
#define MAX_LEN 10000
struct container
{ int length;
int buf[MAX_LEN];
};

Why stat and fstat return the st_size == 0?

I was testing a code from APUE, in chapter 14(Advanced I/O) of memory map file, the fstat() always return the fdin's st_size as zero, and I tried stat() instead, and also get the same result. I list the code below(I have removed the apue.h dependencies):
#include <fcntl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>
#define COPYINCR (1024*1024*1024) /* 1GB */
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("usage: %s <fromfile> <tofile>", argv[0]);
exit(1);
}
int fdin, fdout;
if ((fdin = open(argv[1], O_RDONLY)) < 0) {
printf("can not open %s for reading", argv[1]);
exit(1);
}
if ((fdout = open(argv[2] /* typo fix */, O_RDONLY | O_CREAT | O_TRUNC)) < 0) {
printf("can not open %s for writing", argv[2]);
exit(1);
}
struct stat sbuf;
if (fstat(fdin, &sbuf) < 0) { /* need size fo input file */
printf("fstat error");
exit(1);
}
// always zero, and cause truncate error (parameter error)
printf("input_file size: %lld\n", (long long)sbuf.st_size);
if (ftruncate(fdout, sbuf.st_size) < 0) { /* set output file size */
printf("ftruncate error");
exit(1);
}
void *src, *dst;
off_t fsz = 0;
size_t copysz;
while (fsz < sbuf.st_size) {
if (sbuf.st_size - fsz > COPYINCR)
copysz = COPYINCR;
else
copysz = sbuf.st_size - fsz;
if (MAP_FAILED == (src = mmap(0, copysz, PROT_READ,
MAP_SHARED, fdin, fsz))) {
printf("mmap error for input\n");
exit(1);
}
if (MAP_FAILED == (dst = mmap(0, copysz,
PROT_READ | PROT_WRITE,
MAP_SHARED, fdout, fsz))) {
printf("mmap error for output\n");
exit(1);
}
memcpy(dst, src, copysz);
munmap(src, copysz);
munmap(dst, copysz);
fsz += copysz;
}
return 0;
}
And then I have tried the Python os.stat, it also get the zero result, why this happened? I have tried these and got the same result on Mac OS (Darwin kernel 13.4) and Ubuntu (kernel 3.13).
UPDATE:
Oh, there was a typo error, I should refer to fdout to argv[2], and the O_TRUNC flag certainly make the fdin to zero. Should I close or delete this question?
The reason why Python's os.stat() also return (stat.st_size == 0) is that I passed the same test file (argv[1]) to test, and the file has been previously truncated to zero (I haven't check its size using ls -lh before passing to os.stat()), and certainly os.stat() return zero.
Do not ask SO questions before you go to bed or in a rush.
Ok, the real problem is double open the same input file, and this does not cause any build or runtime error until the ftruncate().
The first open get a read-only fdin, the second open create a new file (fdout and truncated) to copy from fdin via memory map, and the second open truncated the first file (argv[1]), and cleaned all its content. But the fdin still working with fstat (and certainly), this make me hard to find the reason.
The second part is I always use the same file for testing (generated via dd) and have not checking the size, so the os.stat(/path/to/file) and stat(/path/to/file) also return st_size == 0, this makes me believe that this must be some os-level-prolicy defined the behaviour, and I rushed to Mac OS (using the same typo code), and got the same result (they really consistent on POSIX level, event the bug!), and at last, I came to SO for help.

strcpy to mmap address retuns bus error

I created a process which calls mmap with MAP_SHARED flag set,when i attempt to copy a string to that address i receive Bus error core dumped,could some one please explain the reason behind it and how to fix it. Following is my code
int main()
{
int fd=0;
char* ret = NULL;
void *map_addr = NULL;
fd = open("./shared_file.txt", O_RDWR, S_IRUSR | S_IWUSR);
if(fd == -1) {
printf("errno = %d\n",errno);
printf("Aborting process1###########\n");
abort();
}
map_addr = mmap(NULL, 5*sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if(map_addr == MAP_FAILED) {
printf("mmap failed error no =%d\n",errno);
close(fd);
return -1;
}
printf("map_addr = %p#################\n",(int*)map_addr);
printf("processid = %d#################\n",(int)getpid());
ret = strcpy((char*)map_addr,"Stack Overflow");
if(ret == (char*)map_addr)
printf("strcpy success\n");
/*if(msync(map_addr, sizeof(int), MS_SYNC))
printf("msync failed errno = %d\n",errno);*/
close(fd);
sleep(120);
return (0);
}
The cause of a bus error is usually an attempt to dereference a pointer that has not been initialized properly and contains junk data that are not accessible in a multiple of 4 or 1 or as related to datatype sizes.
First you should check if the shared_file.txt file size is >= 20 bytes(assuming sizeof int is 4 bytes) as specified in the mmap() length argument(where you put 5*(sizeof(int))) in the line below:
map_addr = mmap(NULL, 5*sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
If file size is less than 20 bytes, you could use fallocate call to pre allocate the memory.
If shared_file.txt file size is <= 20 bytes and then you ask mmap to map 20 bytes, it could cause a bus error when you write beyond the actual no. of bytes available in file, because that would be access to an undefined portion of a memory. MMAP_FAILED will not be returned in this case, during memory initialization.
A quick check is to see if you can write a single character in the mmap char* pointer. If you can't( you will get a SIGBUS), that means file size is zero.

Change an mmap'd memory region from MAP_SHARED to MAP_PRIVATE

So I have a region of memory that I have allocated with mmap() similar to the code below:
void * ptr = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
The key here is that I'm using the MAP_SHARED flag. Unlike this related question where it was sufficient to simply call mmap() again to get MAP_PRIVATE and Copy-on-Write semantics, I can't have the kernel allocate me a different range of virtual addresses. In addition, I do not want to invoke munmap() and risk the kernel giving part/all of that address range to something else within the process before I can call mmap() again.
Is there an existing mechanism to switch a region of mmap'd memory from MAP_SHARED to MAP_PRIVATE to get copy-on-write semantics without unmapping the block?
Calling mmap() again with MAP_PRIVATE | MAP_FIXED will work. The MMAP(2) man page states that when using MAP_FIXED:
If the specified address cannot be used, mmap() will fail.
So, just use a temporary pointer to store the mmap() result. If mmap() fails, no harm done. If mmap() succeeds you have successfully switched a memory mapped region from MAP_SHARED to MAP_PRIVATE. (see example)
#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main()
{
int fd;
void *shared_0, *shared_1;
void *private_0;
struct stat st;
if((fd = open("filename", O_RDWR, S_IRUSR | S_IWUSR)) < 0) {
fprintf(stderr, "Failed to open(): %s\n", strerror(errno));
}
else if(fstat(fd, &st) < 0) {
fprintf(stderr, "Failed fstat(): %s\n", strerror(errno));
}
else if((shared_0 = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0)) == MAP_FAILED) {
fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
}
else if((shared_1 = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0)) == MAP_FAILED) {
fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
}
else if((private_0 = mmap(shared_0, st.st_size, PROT_READ | PROT_WRITE,
MAP_FIXED | MAP_PRIVATE, fd, 0)) == MAP_FAILED) {
fprintf(stderr, "Failed to mmap(): %s\n", strerror(errno));
}
else if(shared_0 != private_0) {
fprintf(stderr, "Error: mmap() didn't map to the same region");
}
else {
printf("shared_0: %p == private_0: %p\n", shared_0, private_0);
printf("shared_1: %p\n", shared_1);
printf("Shared mapping before write: %d\n", (*(char *)shared_1));
printf("Private mapping before write: %d\n", (*(char *)private_0));
/* write to the private COW mapping and sync changes */
(*(char*)private_0) = 42;
if(msync(private_0, 1, MS_SYNC | MS_INVALIDATE) < 0) {
fprintf(stderr, "Failed msync(): %s\n", strerror(errno));
return(1);
}
printf("Shared mapping after write: %d\n", (*(char *)shared_1));
printf("Private mapping after write: %d\n", (*(char *)private_0));
}
return(0);
}

Resources