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Full Discussion: Parameters placement on stack in C
Top Forums Programming Parameters placement on stack in C Post 303035998 by migurus on Wednesday 12th of June 2019 02:21:00 AM
Old 06-12-2019
Parameters placement on stack in C
I am trying to illustrate the reverse order of parameters on the stack when passed to a function in C:


Code:
#include <stdio.h>

void  p(int p1, int p2, double p3)
{
        printf("params:\n"
                        "1) %p offset = %li\n"
                        "2) %p offset = %li\n"
                        "3) %p\n",
                        (void *)&p1,    (void *)&p1 - (void *)&p2,
                        (void *)&p2,    (void *)&p2 - (void *)&p3,
                        (void *)&p3);
}

int main(int argc, char *argv[])
{
  struct A a = {10,11,12, {1,2,3}};

        p(-1, 0, +1);
        return(0);
}

Result is:
Code:
params:
1) 0x7ffe2f20afac offset = 4
2) 0x7ffe2f20afa8 offset = 8
3) 0x7ffe2f20afa0

This is as expected on the 64 bit system (Ubuntu 19.04)


When I pass a structure as a parameter the stack looks puzzling to me:
Code:
#include <stdio.h>
struct A {
        int     n1;
        int     n2;
        int     n3;
        int     arr[3];
};

void    p(int p1, struct A p2, double p3)
{
        printf("params:\n"
                        "1) %p offset = %li\n"
                        "2) %p offset = %li\n"
                        "3) %p\n",
                        (void *)&p1,    (void *)&p1 - (void *)&p2,
                        (void *)&p2,    (void *)&p2 - (void *)&p3,
                        (void *)&p3);
}

int main(int argc, char *argv[])
{
  struct A a = {10,11,12, {1,2,3}};
        p(-1, a, +1);
        return(0);
}

Now result is:
Code:
params:
1) 0x7ffee5f7ddbc offset = -20
2) 0x7ffee5f7ddd0 offset = 32
3) 0x7ffee5f7ddb0

Parameter 3 is close to 1 and parameter 2 is not between 1 and 3? offsets look wrong to me.Please shed some light here, thanks in advance.
The compiler is gcc 7.4.0
 

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LEARN ABOUT NETBSD

dofileread

DOFILEREAD(9)						   BSD Kernel Developer's Manual					     DOFILEREAD(9)

NAME

     dofileread, dofilereadv, dofilewrite, dofilewritev -- high-level file operations

SYNOPSIS

     #include <sys/file.h>

     int
     dofileread(struct lwp *l, int fd, struct file *fp, void *buf, size_t nbyte, off_t *offset, int flags, register_t *retval);

     int
     dofilewrite(struct lwp *l, int fd, struct file *fp, const void *buf, size_t nbyte, off_t *offset, int flags, register_t *retval);

     int
     dofilereadv(struct lwp *l, int fd, struct file *fp, const struct iovec *iovp, int iovcnt, off_t *offset, int flags, register_t *retval);

     int
     dofilewritev(struct lwp *l, int fd, struct file *fp, const struct iovec *iovp, int iovcnt, off_t *offset, int flags, register_t *retval);

DESCRIPTION

     The functions implement the underlying functionality of the read(2), write(2), readv(2), and writev(2) system calls.  They are also used
     throughout the kernel as high-level access routines for file I/O.

     The dofileread() function attempts to read nbytes of data from the object referenced by file entry fp into the buffer pointed to by buf.  The
     dofilewrite() function attempts to write nbytes of data to the object referenced by file entry fp from the buffer pointed to by buf.

     The dofilereadv() and dofilewritev() functions perform the same operations, but scatter the data with the iovcnt buffers specified by the
     members of the iov array.

     The offset of the file operations is explicitly specified by *offset.  The new file offset after the file operation is returned in *offset.
     If the FOF_UPDATE_OFFSET flag is specified in the flags argument, the file offset in the file entry fp is updated to reflect the new file
     offset, otherwise it remains unchanged after the operation.

     The file descriptor fd is largely unused except for use by the ktrace framework for reporting to userlevel the process's file descriptor.

     Upon successful completion the number of bytes which were transferred is returned in *retval.

RETURN VALUES

     Upon successful completion zero is returned, otherwise an appropriate error is returned.

CODE REFERENCES

     The framework for these file operations is implemented within the file sys/kern/sys_generic.c.

SEE ALSO

     file(9)

BSD
								 December 20, 2005							       BSD
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