How can I write a file with mmap in C?

Here’s an example of writing to a file using mmap.

#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <string.h>

int main(void) {
  int fd = open("test_file", O_RDWR | O_CREAT, (mode_t)0600);
  const char *text = "hello";
  size_t textsize = strlen(text) + 1;
  lseek(fd, textsize-1, SEEK_SET);
  write(fd, "", 1);
  char *map = mmap(0, textsize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
  memcpy(map, text, strlen(text));
  msync(map, textsize, MS_SYNC);
  munmap(map, textsize);
  return 0;
I just released Vidrio, a free app for macOS and Windows to make your screen-sharing awesomely holographic. Vidrio shows your webcam video on your screen, just like a mirror. Then you just share or record your screen with Zoom, QuickTime, or any other app. Vidrio makes your presentations effortlessly engaging, showing your gestures, gazes, and expressions. #1 on Product Hunt. Available for macOS and Windows.

With Vidrio

With generic competitor

More by Jim

Tagged . All content copyright James Fisher 2017. This post is not associated with my employer. Found an error? Edit this page.