system call to populate struct values

问题

I'm trying to populate struct values using system calls. My initial effort follows. However i get junk values from the print statement.

int fd;
int nbytes;
struct message {
  char *from;
  char *to;
  int size;
};
struct message m1={"me","you",10};
struct message m2;

fd=creat("structfile",0644);
nbytes=write(fd,&m1,sizeof(m1));
read(fd,&m2,nbytes);

printf("%s %s %d",m2.from,m2.to,m2.size);

Is there another way to do this? (I'm thinking of the way that structures like hostent and dirent are filled up directly by making calls to gethostbyname and readdir respectively)

回答1:

Problem 1:

Where is fd intialized?

Problem 2:

The line

nbytes=write(fd,&m1,sizeof(m1));

write the numerical values of the pointers m1.from and m1.to. It does not write the strings they point to.

If you want to write the contents of m1, you have to serialize the contents of m1.from and m1.to. There are libraries for that purpose. You can see some references from How to serialize data in C.

回答2:

Problem 3, your struct contains character pointers:

struct message {
  char *from;
  char *to;
  int size;
};

Your sizeof(m1) writes the size of 2 (char *) pointers and an (int). The data pointed to by from and to can vary based on the size of the string. Reading sizeof(m1) does not take into account the size of the strings pointed to.

来源：https://stackoverflow.com/questions/25028553/system-call-to-populate-struct-values

标签

pointers

struct

system-calls