pthread_setname_np.3: Rework ERRORS text layout

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.\" Copyright (C) 2012 Chandan Apsangi <chandan.jc@gmail.com>
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.TH PTHREAD_SETNAME_NP 3 2013-02-04 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setname_np, pthread_getname_np \- set/get the name of a thread
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <pthread.h>
.BI "int pthread_setname_np(pthread_t *" thread ", const char *" name ");
.BI "int pthread_getname_np(pthread_t *" thread ,
.BI "                       const char *" name ", size_t " len );
.fi
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
By default, all the threads created using
.BR pthread_create ()
inherit the program name.
The
.BR pthread_setname_np ()
fuction can be used to set a unique name for a thread,
which can be useful for debugging
multi-threaded applications.
The thread name is a meaningful C language string, whose length is
restricted to 16 characters, including the terminating null byte.
The
.I thread
argument specifies the thread whose name is to be changed;
.I name
specifies the new name.

The
.BR pthread_getname_np ()
function can be used to retrieve the name of the thread.
The
.I thread
argument specifies the thread whose name is to be retriecved.
The buffer
.I name
is used to return the thread name;
.I len
specifies the number of bytes available in
.IR name .
The buffer specified by
.I name
should be at least 16 characters in length.
The returned thread name in the output buffer will be null terminated.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a nonzero error number.
.SH ERRORS
The
.BR pthread_setname_np ()
function can fail with the following errors:
.TP
.B ERANGE
The length of the string specified pointed to by
.I name
exceeds the allowed limit.
.PP
If either of these functions fails to open
.IR /proc/self/task/[tid]/comm ,
then the call may fail with one of the errors described in
.BR open (2).
.SH VERSIONS
These functions first appeared in glibc in version 2.12.
.SH CONFORMING TO
These functions are nonstandard GNU extensions.
.SH NOTES
.BR pthread_setname_np ()
internally writes to the thread specific comm file under
.IR /proc
filesystem:
.IR /proc/self/task/[tid]/comm .
.BR pthread_getname_np ()
retreives it from the same location.

.SH EXAMPLE
.PP
The program below demonstrates the use of
.BR pthread_setname_np ()
and
.BR pthread_getname_np ().

The following shell session shows a sample run of the program:
.in +4n
.nf

.RB "$" " ./a.out"
Created a thread. Default name is: a.out
The thread name after setting it is THREADFOO.
\fB^Z\fP                           # Suspend the program
[1]+  Stopped           ./a.out
.RB "$ " "ps H -C a.out -o 'pid tid cmd comm'"
  PID   TID CMD                         COMMAND
 5990  5990 ./a.out                     a.out
 5990  5991 ./a.out                     THREADFOO
.RB "$ " "cat /proc/5990/task/5990/comm"
a.out
.RB "$ " "cat /proc/5990/task/5991/comm"
THREADFOO
.fi
.in

.SS Program source
\&
.nf
#define _GNU_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>

#define NAMELEN 16

#define errExitEN(en, msg) \\
            do { errno = en; perror(msg); exit(EXIT_FAILURE); \\
        } while (0)

static void *
threadfunc(void *parm)
{
    sleep(5);          // allow main program to set the thread name
    return NULL;
}

int
main(int argc, char **argv)
{
    pthread_t thread;
    int rc;
    char thread_name[NAMELEN];

    rc = pthread_create(&thread, NULL, threadfunc, NULL);
    if (rc != 0)
        errExitEN(rc, "pthread_create");

    rc = pthread_getname_np(thread, thread_name, NAMELEN);
    if (rc != 0)
        errExitEN(rc, "pthread_getname_np");

    printf("Created a thread. Default name is: %s\\n", thread_name);
    rc = pthread_setname_np(thread, (argc > 1) ? argv[1] : "THREADFOO");
    if (rc != 0)
        errExitEN(rc, "pthread_setname_np");

    sleep(2);
    
    rc = pthread_getname_np(thread, thread_name,
                            (argc > 2) ? atoi(argv[1]) : NAMELEN);
    if (rc != 0)
        errExitEN(rc, "pthread_getname_np");
    printf("The thread name after setting it is %s.\\n", thread_name);
    
    rc = pthread_join(thread, NULL);
    if (rc != 0)
        errExitEN(rc, "pthread_join");
    
    printf("Done\\n");
    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.ad l
.nh
.BR prctl (2),
.BR pthread_create (3),
.BR pthreads (7)