System Limits and Options

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Unix implementations must provide limits on system features and resources, like:

• How many files can a process keep open at one time?
• Does the system have realtime signals?
• What is the largest value for a variable?
• How big of an argument list can a program have?
• How long can a pathname be?

Some values should be determined at compile time and some at runtime. For the runtime cases, sysconf, pathconf, and fpathconf can be used to check for implementation limits and options.

System Limits

Retrieving System Limits (and Options) at Run Time

To retrieve limits, use sysconf.

#include <unistd.h>
long sysconf(int name); // Returns value of limit specified by name, or –1 if limit is indeterminate or an error occurred

Retrieving File-Related Limits (and Options) at Run Time

For file related limits, pathconf and fpathconf can be used:

#include <unistd.h>
long pathconf(const char *pathname, int name);
long fpathconf(int fd, int name); // Both return value of limit specified by name, or –1 if limit is indeterminate or an error occurred

Indeterminate Limits

System Options

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