What API do I call to get the system uptime?

Question

I would like to get the system uptime from within a C application running on a linux-based system. I don't want to call uptime(1) and parse the output, I'd like to call the underlying C API I suspect exists. Anyone know if there is such a call, or does uptime(1) simply process records obtained from wtmp?

Answer 1

The system call you're looking for is sysinfo().

It's defined in sys/sysinfo.h

Its signature is: int sysinfo(struct sysinfo *info)

Since kernel 2.4, the structure has looked like this:

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 */ }; 

Have fun!