Use mmap in C to write to memory.

Question

I want to use mmap() to create a file containing some integers. I want to write to this file by writing to memory. I know that the data in binary format memory and therefore the data in the file will also be in binary format. Can I use mmap for this purpose? Where can I find good resources on using mmap ? I did not find a good guide to start.

Answer 1

Here is an example:

 #include <sys/types.h> #include <sys/stat.h> #include <sys/mman.h> /* mmap() is defined in this header */ #include <fcntl.h> void err_quit(char *msg) { printf(msg); return 0; } int main (int argc, char *argv[]) { int fdin, fdout; char *src, *dst; struct stat statbuf; int mode = 0x0777; if (argc != 3) err_quit ("usage: a.out <fromfile> <tofile>"); /* open the input file */ if ((fdin = open (argv[1], O_RDONLY)) < 0) {printf("can't open %s for reading", argv[1]); return 0; } /* open/create the output file */ if ((fdout = open (argv[2], O_RDWR | O_CREAT | O_TRUNC, mode )) < 0)//edited here {printf ("can't create %s for writing", argv[2]); return 0; } /* find size of input file */ if (fstat (fdin,&statbuf) < 0) {printf ("fstat error"); return 0; } /* go to the location corresponding to the last byte */ if (lseek (fdout, statbuf.st_size - 1, SEEK_SET) == -1) {printf ("lseek error"); return 0; } /* write a dummy byte at the last location */ if (write (fdout, "", 1) != 1) {printf ("write error"); return 0; } /* mmap the input file */ if ((src = mmap (0, statbuf.st_size, PROT_READ, MAP_SHARED, fdin, 0)) == (caddr_t) -1) {printf ("mmap error for input"); return 0; } /* mmap the output file */ if ((dst = mmap (0, statbuf.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fdout, 0)) == (caddr_t) -1) {printf ("mmap error for output"); return 0; } /* this copies the input file to the output file */ memcpy (dst, src, statbuf.st_size); return 0; } /* main */ 

From here
Another Linux example
An implementation of Windows memory mapping.

Answer 2

Ressources → mmap man 2

Examples: Linux cp from fahmy

  if ((dst = mmap (0, statbuf.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fdout, 0)) == (caddr_t) -1) err_sys ("mmap error for output"); /* this copies the input file to the output file */ memcpy (dst, src, statbuf.st_size); 

And wiki mmap example

 #include <sys/types.h> #include <sys/mman.h> #include <err.h> #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> /* Does not work on OS X, as you can't mmap over /dev/zero */ int main(void) { const char str1[] = "string 1"; const char str2[] = "string 2"; int parpid = getpid(), childpid; int fd = -1; char *anon, *zero; if ((fd = open("/dev/zero", O_RDWR, 0)) == -1) err(1, "open"); anon = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0); zero = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED, fd, 0); if (anon == MAP_FAILED || zero == MAP_FAILED) errx(1, "either mmap"); strcpy(anon, str1); strcpy(zero, str1); printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero); switch ((childpid = fork())) { case -1: err(1, "fork"); /* NOTREACHED */ case 0: childpid = getpid(); printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero); sleep(3); printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero); munmap(anon, 4096); munmap(zero, 4096); close(fd); return (EXIT_SUCCESS); } sleep(2); strcpy(anon, str2); strcpy(zero, str2); printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero); munmap(anon, 4096); munmap(zero, 4096); close(fd); return (EXIT_SUCCESS); } 

Try both options and adapt them for your purpose.