Problems using memcpy with mmap

Question

Trying to copy the file using these functions, everything will be fine until the program hits the memcpy function, which gives a bus error and terminates the process.

void copy_mmap(char* in, char* out){ int input_fd, output_fd; input_fd = open (in, O_RDONLY); if (input_fd == -1) { printf("Error opening input file.\n"); exit(2); } output_fd = open(out, O_RDWR | O_CREAT, S_IWUSR | S_IRUSR); if(output_fd == -1){ printf("Error opening output file.\n"); exit(3); } struct stat st; fstat(input_fd, &st); char* target; target=mmap(0, st.st_size+1, PROT_READ, MAP_SHARED, input_fd, 0); if (target==(void*) -1){ printf("Error mapping target with errno: %d.\n", errno); exit(6); } char* destination; destination=mmap(0, st.st_size+1, PROT_READ | PROT_WRITE, MAP_SHARED, output_fd, 0); if (destination==(void*) -1){ printf("Error mapping destination with errno: %d.\n", errno); exit(5); } memcpy(destination, target, st.st_size); munmap(destination, st.st_size); } 

It was not possible to find out what was wrong, as "Bus Error" is not a descriptive error message, and this article does not have a lot of material on the Internet.

Answer 1

When you create a destination file as a new file, its size is 0 bytes. memcpy crashes because it tries to write data outside of the file.

You can do this job by pre-setting the destination file to the size of the source file (using ftruncate() ) before mmap() it.

In addition, you should pass st.st_size as the second argument to mmap , not st.st_size+1 . st.st_size+1 tries to display a range that is larger than the file size, which is invalid.

Answer 2

Perhaps try using memmove? You do not read and write data from the same place in memory / file / device? Memcpy does not work in such situations.

Answer 3

you can also view the file off_t fileposition = lseek (output_fd, st.st_size-1, SEEK_SET) and write a blank character in output_fd.