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@garcia556
Created December 3, 2017 21:08
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POSIX shared memory IPC example (shm_open, mmap), working on Linux and macOS
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#define STORAGE_ID "/SHM_TEST"
#define STORAGE_SIZE 32
int main(int argc, char *argv[])
{
int res;
int fd;
char data[STORAGE_SIZE];
pid_t pid;
void *addr;
pid = getpid();
// get shared memory file descriptor (NOT a file)
fd = shm_open(STORAGE_ID, O_RDONLY, S_IRUSR | S_IWUSR);
if (fd == -1)
{
perror("open");
return 10;
}
// map shared memory to process address space
addr = mmap(NULL, STORAGE_SIZE, PROT_READ, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED)
{
perror("mmap");
return 30;
}
// place data into memory
memcpy(data, addr, STORAGE_SIZE);
printf("PID %d: Read from shared memory: \"%s\"\n", pid, data);
return 0;
}
#!/bin/bash
function is_linux
{
if [[ "$(uname)" == "Linux" ]]; then
echo 1
return
fi
echo 0
}
SET="set"
GET="get"
CFLAGS=""
if [[ "$(is_linux)" == "1" ]]; then
CFLAGS="-lrt"
fi
cc ${CFLAGS} -o ${SET} ${SET}.c
cc ${CFLAGS} -o ${GET} ${GET}.c
./${SET} &
sleep 1
./${GET}
if [[ "$(is_linux)" == "1" ]]; then
echo "/dev/shm:"
ls -l /dev/shm
fi
sleep 1
rm ${SET} ${GET}
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#define STORAGE_ID "/SHM_TEST"
#define STORAGE_SIZE 32
#define DATA "Hello, World! From PID %d"
int main(int argc, char *argv[])
{
int res;
int fd;
int len;
pid_t pid;
void *addr;
char data[STORAGE_SIZE];
pid = getpid();
sprintf(data, DATA, pid);
// get shared memory file descriptor (NOT a file)
fd = shm_open(STORAGE_ID, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
if (fd == -1)
{
perror("open");
return 10;
}
// extend shared memory object as by default it's initialized with size 0
res = ftruncate(fd, STORAGE_SIZE);
if (res == -1)
{
perror("ftruncate");
return 20;
}
// map shared memory to process address space
addr = mmap(NULL, STORAGE_SIZE, PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED)
{
perror("mmap");
return 30;
}
// place data into memory
len = strlen(data) + 1;
memcpy(addr, data, len);
// wait for someone to read it
sleep(2);
// mmap cleanup
res = munmap(addr, STORAGE_SIZE);
if (res == -1)
{
perror("munmap");
return 40;
}
// shm_open cleanup
fd = shm_unlink(STORAGE_ID);
if (fd == -1)
{
perror("unlink");
return 100;
}
return 0;
}
@aigoncharov
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aigoncharov commented Oct 31, 2020

Thank you! It helped me with my college course!

@jiapengwen
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what is the different betweent shm_open+mmap and shmget? I suggest is the same

@hbwang15
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what is the different betweent shm_open+mmap and shmget? I suggest is the same

一个是posix一个是systemv

@MSK61
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MSK61 commented Jul 29, 2023

I think get.c is missing an munmap call at the end, just like set.c. And both files are missing a close(fd) at the end as well.

@MSK61
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MSK61 commented Jul 30, 2023

And generally it's best to close the file descriptor right after the memory is mapped with mmap.

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