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Top Forums Programming sizeof an array of structure without using 'sizeof' operator Post 302302724 by shamrock on Tuesday 31st of March 2009 07:37:33 PM
Old 03-31-2009
The pragma directive lets you alter the packing of structures but has no affect on ordinary variables. Also it is non-portable and should not be used in code that will be deployed to multiple OSes AIX HPUX Solaris Linux. Output below is from an AIX machine...
Code:
#include <stdio.h>

#pragma pack(1)

int main(void)
{
    char beg;
    struct ptr
    {
        int a;
        char b;
        int *p;
    } abc;
    char end;

    printf("Using sizeof operator = %u\n", sizeof(abc));

    printf("beg address = %lu\n", (unsigned long) &beg);
    printf("end address = %lu\n", (unsigned long) &end);
    printf("first element address = %lu\n", (unsigned long) &abc.a);
    printf("last element address = %lu\n", (unsigned long) &abc.p);
    printf("size of structure = %lu\n", (unsigned long) &end - (unsigned long) &abc.a);
}

Output...
Code:
using sizeof operator = 9
beg address = 104399808
end address = 104399821
first element address = 104399812
last element address = 104399817
size of structure = 9


Last edited by shamrock; 03-31-2009 at 08:45 PM..
 

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SYSINFO(2)						     Linux Programmer's Manual							SYSINFO(2)

NAME
sysinfo - return system information SYNOPSIS
#include <sys/sysinfo.h> int sysinfo(struct sysinfo *info); DESCRIPTION
sysinfo() returns certain statistics on memory and swap usage, as well as the load average. Until Linux 2.3.16, sysinfo() returned information in the following structure: struct sysinfo { long uptime; /* Seconds since boot */ unsigned long loads[3]; /* 1, 5, and 15 minute load averages */ unsigned long totalram; /* Total usable main memory size */ unsigned long freeram; /* Available memory size */ unsigned long sharedram; /* Amount of shared memory */ unsigned long bufferram; /* Memory used by buffers */ unsigned long totalswap; /* Total swap space size */ unsigned long freeswap; /* Swap space still available */ unsigned short procs; /* Number of current processes */ char _f[22]; /* Pads structure to 64 bytes */ }; In the above structure, the sizes of the memory and swap fields are given in bytes. Since Linux 2.3.23 (i386) and Linux 2.3.48 (all architectures) the structure is: struct sysinfo { long uptime; /* Seconds since boot */ unsigned long loads[3]; /* 1, 5, and 15 minute load averages */ unsigned long totalram; /* Total usable main memory size */ unsigned long freeram; /* Available memory size */ unsigned long sharedram; /* Amount of shared memory */ unsigned long bufferram; /* Memory used by buffers */ unsigned long totalswap; /* Total swap space size */ unsigned long freeswap; /* Swap space still available */ unsigned short procs; /* Number of current processes */ unsigned long totalhigh; /* Total high memory size */ unsigned long freehigh; /* Available high memory size */ unsigned int mem_unit; /* Memory unit size in bytes */ char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding to 64 bytes */ }; In the above structure, sizes of the memory and swap fields are given as multiples of mem_unit bytes. RETURN VALUE
On success, sysinfo() returns zero. On error, -1 is returned, and errno is set to indicate the cause of the error. ERRORS
EFAULT info is not a valid address. VERSIONS
sysinfo() first appeared in Linux 0.98.pl6. CONFORMING TO
This function is Linux-specific, and should not be used in programs intended to be portable. NOTES
All of the information provided by this system call is also available via /proc/meminfo and /proc/loadavg. SEE ALSO
proc(5) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 SYSINFO(2)
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