How do you determine the amount of Linux system RAM in C++? - c++

Question

I just wrote the following C++ function to programmatically determine how much RAM a system has installed. It works, but it seems to me that there should be a simpler way to do this. Am I missing something?
getRAM()
{
FILE* stream = popen("head -n1 /proc/meminfo", "r");
std::ostringstream output;
int bufsize = 128;
while( !feof(stream) && !ferror(stream))
{
char buf[bufsize];
int bytesRead = fread(buf, 1, bufsize, stream);
output.write(buf, bytesRead);
}
std::string result = output.str();
std::string label, ram;
std::istringstream iss(result);
iss >> label;
iss >> ram;
return ram;
}
First, I'm using popen("head -n1 /proc/meminfo") to get the first line of the meminfo file from the system. The output of that command looks like
MemTotal: 775280 kB
Once I've got that output in an istringstream, it's simple to tokenize it to get at the information I want. Is there a simpler way to read in the output of this command? Is there a standard C++ library call to read in the amount of system RAM?

Answer 1

On Linux, you can use the function sysinfo which sets values in the following struct:
#include <sys/sysinfo.h>
int sysinfo(struct sysinfo *info);
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 for libc5 */
};
If you want to do it solely using functions of C++ (I would stick to sysinfo), I recommend taking a C++ approach using std::ifstream and std::string:
unsigned long get_mem_total() {
std::string token;
std::ifstream file("/proc/meminfo");
while(file >> token) {
if(token == "MemTotal:") {
unsigned long mem;
if(file >> mem) {
return mem;
} else {
return 0;
}
}
// Ignore the rest of the line
file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
return 0; // Nothing found
}

Answer 2

There isn't any need to use popen(). You can just read the file yourself.
Also, if their first line isn't what you're looking for, you'll fail, since head -n1 only reads the first line and then exits. I'm not sure why you're mixing C and C++ I/O like that; it's perfectly OK, but you should probably opt to go all C or all C++. I'd probably do it something like this:
int GetRamInKB(void)
{
FILE *meminfo = fopen("/proc/meminfo", "r");
if(meminfo == NULL)
... // handle error
char line[256];
while(fgets(line, sizeof(line), meminfo))
{
int ram;
if(sscanf(line, "MemTotal: %d kB", &ram) == 1)
{
fclose(meminfo);
return ram;
}
}
// If we got here, then we couldn't find the proper line in the meminfo file:
// do something appropriate like return an error code, throw an exception, etc.
fclose(meminfo);
return -1;
}

Answer 3

Remember /proc/meminfo is just a file. Open the file, read the first line, and close the file. Voilà!

Answer 4

Even top (from procps) parses /proc/meminfo. See here.