keep in mind that you can't do that, here's a quote that explains it well:
Some programs imprudently rely on code such as the following
sprintf(buf, "%s some further text", buf);
to append text to buf. However, the standards explicitly note that the results
are undefined if source and destination buffers overlap when calling sprintf(),
snprintf(), vsprintf(), and vsnprintf(). Depend‐ ing on the version of gcc(1)
used, and the compiler options employed, calls such as the above will not
produce the expected results.
The glibc implementation of the functions snprintf() and vsnprintf() conforms
to the C99 standard, that is, behaves as described above, since glibc 2.1.
Until glibc 2.0.6, they would return -1 when the output was truncated.
- printf(3), Linux man-pages 6.04, 2023-04-01
Some programs imprudently rely on code such as the following
to append text to buf. However, the standards explicitly note that the results are undefined if source and destination buffers overlap when calling sprintf(), snprintf(), vsprintf(), and vsnprintf(). Depend‐ ing on the version of gcc(1) used, and the compiler options employed, calls such as the above will not produce the expected results.The glibc implementation of the functions snprintf() and vsnprintf() conforms to the C99 standard, that is, behaves as described above, since glibc 2.1. Until glibc 2.0.6, they would return -1 when the output was truncated.
- printf(3), Linux man-pages 6.04, 2023-04-01