Mmap for writing to MacOSX 10.8.2 with Xcode 4.6 will crash the program

Question

I am trying to run a simple mmap test under MacOSX 10.8.2 with Xcode 4.6. This program looks as follows: the file displayed for reading is in order, and access to the "target" write pointer will cause the program to crash. The error message is "EXC_BAD_ACCESS".

Do I have the same case with me? Many thanks.

#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;

}

Answer 1

I think you need ftruncate(output, size);to make the output file big enough. I do not believe that the kernel will automatically grow a file to accommodate data written to the mapped addresses.

Answer 2

PROT_EXEC. PROT_READ | PROT_WRITE, , PROT_EXEC .