PMOPENLOG(3) Library Functions Manual PMOPENLOG(3)

pmOpenLog - create a log file for diagnostics and debug output

#include <pcp/pmapi.h>

FILE *pmOpenLog(const char *progname, const char *logname, FILE *oldstream, int *status);

cc ... -lpcp

pmOpenLog reassigns the standard I/O stream oldstream (normally stderr) to be associated with the file logname.

If logname is not "-" and the file already exists, it will be renamed to logname.prev else removed. Due to permissions restrictions, the rename or removal may not succeed, but in the common use cases logname.prev remains with the contents of the previous version of logname. Then logname is recreated if possible (to ensure correct ownership and permissions from the caller to pmOpenLog).

As a special case, if logname is "-" then no renaming, removal or reopening is performed and the function simply sets status to 1 and returns oldstream. This is useful when the caller wants diagnostics on oldstream stream (normally stderr) rather than a file, e.g. pmlogger -l- or pmcd -f -l-. Logging to stderr is also useful for PMDAs in a containerized environment where it is beneficial for all PMDA logs to be written to pmcd's stderr stream (and thus to a single destination), whether that is a file such as pmcd.log or the original stream inherited from the shell.

On return, the function value is the standard I/O stream, possibly replacing oldstream. In the event of an error, the return value will be oldstream unchanged and status will be 0.

For success, status is 1, and a standard preamble is written to logname Log for progname on hostname started date and time
and an atexit(3) handler is installed to write the postscript message to logname Log finished date and time
when the processes exits.

progname is only used to annotate messages.

atexit(3) and freopen(3).

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