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My Thoughts, Trials and Adventures

mmap - one-liner to read file to string

Posted at — Feb 22, 2022 | Last Modified on — Jul 26, 2023


mmap stands for memory map. the declaration1 is given below:

       #include <sys/mman.h>

       void *mmap(void *addr, size_t length, int prot, int flags,
                  int fd, off_t offset);
       int munmap(void *addr, size_t length);

mmap vs. other ways

there are several ways to read files in C, none you could do without several line of code and iteration. the quickest approach would be fgets() within a loop. mmap offers a simple one-liner. recently I’ve been working with OpenCL, which is known for its unconventional practice to load kernel as a string and compile and run it during runtime.

reading file to string with mmap

we need to know the exact size of the file we are going to read. we can do that with stat

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>

int main() {

       int k_fd = open("kernel.cl", O_RDONLY);
       struct stat s;
       fstat(k_fd, &s);
       printf("%d\n", s.st_size);

then we need to allocate memory and call mmap.

       char* kernelSource = (char *)mmap(0, s.st_size, PROT_READ, MAP_PRIVATE, k_fd, 0);
       printf("file content = %s\n", kernelSource);
       return 0;

mmap returns a void pointer, it’s necessary to cast it to a char *.


It looks better and feels good (at least to me 😉) than other ways of reading files. sharing this short example with others who might be looking for it.

  1. https://man7.org/linux/man-pages/man2/mmap.2.html ↩︎