Trying to copy a file using these functions, everything goes fine until 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); }
Failed to figure out what is wrong, as “Bus Error” isn’t a descriptive error message and there isn’t any much material on the internet regarding this problem.
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Answer
When you create the destination file as a new file, its size is 0 bytes. memcpy
crashes because it tries to write data beyond the end of the file.
You can make this work by pre-sizing the destination file to the size of the source file (using ftruncate()
) before you mmap()
it.
Also, you should pass st.st_size
as the second argument to mmap
, not st.st_size+1
. st.st_size+1
tries to map a range that is larger than the size of the file, which is invalid.