Why do I get memory errors when mmap()'ing a JPG file?
arrays, c++, file-io, mmap
Solution
You should be checking just `data == -1` (or, even better, `MAP_FAILED`) and not `*data == -1` (dereferencing via `*` is wrong here).
The reason your code fails is because the first byte of every JPEG file is `FF` in hex or -1 in signed decimal.
For more detail on JPEG, Google for "JPEG file format." For example, search for "SOI" on this page.
Problem
I have the following section of code: ``` char* Sender::PrepareData(char* filename, unsigned long long int bytesToTransfer) { int fd, pagesize; char *data; ifstream file(filename, ios::binary | ios::ate); int size = file.tellg(); cout << "File Size: " << size << endl; if(size < bytesToTransfer) {cout << "File smaller than specified number of bytes {" << bytesToTransfer << "} to transfer -- Exiting!\n"; exit(1);} fd = open(filename, O_RDONLY); if (fd==NULL) {fputs ("File error",stderr); exit (1);} cout << "File Open: " << filename << endl; pagesize = getpagesize(); cout << "Pagesize: " << pagesize << endl; data = static_cast<char*>(mmap((caddr_t)0, bytesToTransfer, PROT_READ, MAP_SHARED, fd, 0)); if (*data == -1) {fputs ("Memory error",stderr); exit (2);} return data; } ``` This seemingly works fine for text and .deb files - however, when trying with a ~3MB image file (.jpg), I get memory errors: ``` File Size: 3333840 File Open: t1.jpg Pagesize: 4096 Memory error[Inferior 1 (process 3293) exited with code 02] ``` Am I using `mmap()` wrong? I'm trying to write a simple wrapper that will take any type of file and return a char* containing the specified number of bytes.