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Naming Threads under Linux

Under Linux ( and BSD, MacOS, …) it is possible to name threads of a process via pthread_setname_np or prctl(PR_SET_NAME). This can be handy for example when debugging multi-threaded code in gdb or looking at top and figuring out which thread consumes 100% CPU.

In the following this feature is used to name OpenMP threads and make them distinguishable inside gdb. For OpenMP it has to be performed inside a parallel region:

[c]
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, v2, as
// published by the Free Software Foundation
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
//
// File: ThreadName.c
// Setting thread name via pthread_setname_np.
// Compile with:
// gcc -g -fopenmp ThreadName.c -o thread-name
// icc -g -openmp ThreadName.c -o thread-name
//
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#ifdef _OPENMP
#include <openmp.h>
#endif

int main(int argc, char * argv[])
{

#pragma omp parallel
{
int threadId = 0;
char name[16] = { 0 };

#ifdef _OPENMP
threadId = omp_get_thread_num();
#endif

snprintf(name, sizeof(name), „omp-%02d“, threadId);

pthread_setname_np(pthread_self(), name);

sleep(10);
}

return 0;
}
[/c]

[c]
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, v2, as
// published by the Free Software Foundation
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
//
// File: ThreadNamePrctl.c
// Setting thread name via prctl(PR_SET_NAME).
// Compile with:
// gcc -g -fopenmp ThreadName.c -o thread-name-prctl
// icc -g -openmp ThreadName.c -o thread-name-prctl
//
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#ifdef _OPENMP
#include <omp.h>
#endif
#include <sys/prctl.h>

int main(int argc, char * argv[])
{
#pragma omp parallel
{
int threadId = 0;
char name[16] = { 0 };

#ifdef _OPENMP
threadId = omp_get_thread_num();
#endif

snprintf(name, sizeof(name) – 1, „omp-%02d“, threadId);

prctl(PR_SET_NAME, name, 0, 0, 0);

sleep(10);
}

return 0;
}
[/c]

Compile ThreadNamePrctl.c with gcc via:
gcc -g -fopenmp ThreadName.c -o thread-name-prctl
Note: -g is only needed for convience when using gdb.

$ env OMP_NUM_THREADS=4 gdb ./thread-name-prctl
> break sleep
> r
> info threads
  Id   Target Id         Frame
  4    Thread 0x7ffff642d700 (LWP 26499) "omp-03" 0x00007ffff76e1590 in sleep () from /lib64/libc.so.6
  3    Thread 0x7ffff6c2e700 (LWP 26498) "omp-02" 0x00007ffff76e1590 in sleep () from /lib64/libc.so.6
  2    Thread 0x7ffff742f700 (LWP 26497) "omp-01" 0x00007ffff76e1590 in sleep () from /lib64/libc.so.6
* 1    Thread 0x7ffff7fb9760 (LWP 26493) "omp-00" 0x00007ffff76e1590 in sleep () from /lib64/libc.so.6

Thne named threads will also show up with the same name in top (when pressing „H“):

...
26504 user-name  20   0 34940  636  500 S      0  0.0   0:00.00 omp-00
26508 user-name  20   0 34940  636  500 S      0  0.0   0:00.00 omp-01
26509 user-name  20   0 34940  636  500 S      0  0.0   0:00.00 omp-02
26510 user-name  20   0 34940  636  500 S      0  0.0   0:00.00 omp-03
...

Notes:

  • The length of a thread name is limited to 16 characters.
  • Not all versions of gdb support this feature.
  • pthread_setname_np
    • It is only supported in glibc since version 2.12.
    • Compile with -pthread if no OpenMP is used.
  • prctl(PR_SET_NAME) could be used as a fallback, which is supported since Linux kernel version 2.6.9.

C Default Argument Promotions

Have you ever wondered why your application never crashes, shows strange behavior, or does not print the correct double precision numbers, when you write something like

float  f = 1.1f;
double d = 1.1;

printf("float  = %f\n", f);
printf("double = %f\n", d);

This would print following result

float  = 1.100000
double = 1.100000

and works like you normally expect it.

But then why the question above? The %f format specifier for the printf/scanf class of functions denotes a variable of type float which consumes 32 bits. In the example above d is of type double which uses 64 bits and actually requires %lf. The provided argument d (64 bits) would be interpreted as a variable of type float (32 bits). If this would happen, then the output would look more like that (at least on architectures using little endian byte order):

float  = 1.100000
double = -0.000000

The double variable d is stored in memory as:

0x9a 99 99 99 99 99 f1 3f

As little endian byte order is used dwill finally be interpreted as:

0x3f f1 99 99 99 99 99 9a

When only the first four bytes would be considered by printf then these bytes would be 0x9a 99 99 99 (from memory) and interpreted as float which would represent the number (ca.) -1.58818684e-23 (0x99 99 99 9a). Printed with %f gives -0.000000. But nothing of this happens and everything works like expected.

The reason is that ONLY the printf class of functions treat %f as %lf, which denotes a double precision argument. Accordingly this is also done for %g and %e. So why can printf safely do this?

The reason are the default argument promotions from C. During this following casts take place (without guarantee to be complete):

char, short   to   int
unsigned char, unsigned short   to   unsigned int
float   to   double

The promotions are applied to

  • all parameters of a function, when no prototype for this function exists and
  • to all non-declared parameters of variadic functions, i.e. functions which take a variable number of arguments.

As the printf class of functions are variadic (denoted by the elipsis „…“ in the function signature) the default argument promotions take place. Finally the call printf("float = %f\n", f) will after promotion look like printf("float = %f\n", (double)f) and printf can safely assume that every %f, %g, and %e denotes a parameter of type double.

References:

  • Michael L. Overton. Numerical Computing with IEEE Floating Point Arithmetic, SIAM, 2001. Chapter 10: Floating Point in C.
  • C Standard. Chapter 6.5.2.2 (http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1124.pdf)
  • Stackoverflow – Default argument promotions in C function calls (http://stackoverflow.com/questions/1255775).