mmap for write under MacOSX 10.8.2 with XCode 4.6 will make program crash

Question

I try to run a simple test of mmap under MacOSX 10.8.2, with XCode 4.6. This program is as follows, the file mapped for read is OK while the access to the write pointer "target" will make the program crash. Error message is "EXC_BAD_ACCESS".

Does anyone have the same case with me ? Thanks a lot.

#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, const char * argv[]) {
int input, output;
size_t size;

char *source, *target;

input = open("SEK2.txt", O_RDONLY);
if (input == -1) {
    printf("Open source file failed");
    return -1;
}
output = open("test.txt", O_RDWR|O_CREAT|O_TRUNC);
if (output == -1) {
    printf("Open Output file failed");
    return -1;
}

size = lseek(input, 0, SEEK_END);
printf("File size = %d\n", size);

source = (char*)mmap(0, size, PROT_READ, MAP_PRIVATE, input, 0);
if ( source == (void*)-1) {
    printf("Source MMap Error\n");
    return -1;
}

target = (char*)mmap(0, size, PROT_EXEC, MAP_PRIVATE, output, 0);
if ( target == (void*)-1 ) {
    printf( "Target MMap Error\n");
    return -1;
}

memcpy(target, source, size); // EXC_BAD_ACCESS to "target"
munmap(source, size);
munmap(target, size);

close(input);
close(output);

printf("Successed");
return 0;

}

score 3 · Answer 1 · answered Apr 27 '13 at 18:38

3

I think you need to ftruncate(output, size); to make the output file large enough. I don't believe the kernel with automatically grow a file to accommodate data written to the mapped addresses.

answered Apr 27 '13 at 18:38

Ken Thomases

88,520
7
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This is one of the problems. The other is that using MAP_PRIVATE means that once the mapping has been deleted then the change goes away and are not persisted back to disk – Anya Shenanigans Apr 27 '13 at 18:44
I assume that's what he wanted. – bmargulies Apr 27 '13 at 21:12

score 2 · Answer 2 · answered Apr 27 '13 at 17:05

2

You can't write to a memory map with only PROT_EXEC. You need PROT_READ|PROT_WRITE, and I don't see why you want PROT_EXEC at all.

answered Apr 27 '13 at 17:05

bmargulies

97,814
39
186
310

score 0 · Answer 3 · answered Oct 24 '18 at 17:21

0

Under mac, you cannot use PROT_EXEC, so you cannot use named pthread mutex, cond or etc.

use only PROT_READ | PROT_WRITE flag

answered Oct 24 '18 at 17:21

KoLiBer

194
4
15

mmap for write under MacOSX 10.8.2 with XCode 4.6 will make program crash

3 Answers3

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