CODEWITHSUNDEEP
c++c
martin s
martin s

Reputation: 1131

Map char pointer to in-memory file with file descriptor

I try to map a char * to an in-memory FILE* with fmemopen. I just found out that fmemopen will not give me a file descriptor. Is there a way to get an in-memory FILE with FD? 

char * data = ... data ...;
FILE *fid = fmemopen(data, 172, "r");

int fd =  fileno(fid);
if(fd < 0)
    std::cout << "BAD" << std::endl;
else
    std::cout << "GOOD" << std::endl;

Upvotes: 1

Views: 1400

Answers (2)

maha
maha

Reputation: 607

Have a look and see if shm_open() in conjunction with fdopen() does what you want.

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

int main() {
    FILE *f = NULL;
    int fd = 0, n = 0;
    char buffer[6] = "hello";

    fd = shm_open("myfile", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
    if(fd<0) {
       printf("failed to shm_open\n");
       exit(1);
    }
    f = fdopen(fd,"w+");
    if(!f) {
       printf("failed to fdopen\n");
       exit(1);
    }
    n = fwrite(buffer, sizeof(char), sizeof(buffer), f);
    printf("wrote %d bytes", n);
    fclose(f);
}

Upvotes: 3

Olaf Dietsche
Olaf Dietsche

Reputation: 74018

fopen is based on the open system call and therefore has a corresponding file descriptor.

fmemopen on the other side is a pure library implementation and doesn't need a system file descriptor. See also the NOTES section on fileno.

Upvotes: 0

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