Re: [PATCH] tools: Use fputc() calls in some functions

Re: [PATCH] tools: Use fputc() calls in some functions

[Andi Kleen]

On Thu, Jul 23, 2026 at 12:25:25PM +0200, Markus Elfring wrote:
> From: Markus Elfring <elfring@users.sourceforge.net>
> Date: Thu, 23 Jul 2026 11:13:30 +0200
> Subject: [PATCH] tools: Use fputc() calls in some functions
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Single characters should occasionally be transferred into the output.
> Thus use the corresponding function “fputc” (or even “putchar”).
> 
> The source code was transformed by using the Coccinelle software.

gcc already does this transformation automatically.

Re: [PATCH] tools: Use fputc() calls in some functions

[Steven Rostedt]

On Thu, 23 Jul 2026 06:47:00 -0700
Andi Kleen <ak@linux.intel.com> wrote:

> > The source code was transformed by using the Coccinelle software.  
> 
> gcc already does this transformation automatically.

Yep, I showed this in my talk[1] that looked at what printf("Hello World!")
does. Doing an objdump of:

#include <stdio.h>

int main(void) {
	printf("Hello World!\n");
	return 0;
}

has:

0000000000001139 <main>:
    1139:       55                      push   %rbp
    113a:       48 89 e5                mov    %rsp,%rbp
    113d:       48 8d 05 c0 0e 00 00    lea    0xec0(%rip),%rax        # 2004 <_IO_stdin_used+0x4>
    1144:       48 89 c7                mov    %rax,%rdi
    1147:       e8 e4 fe ff ff          call   1030 <puts@plt>  <<<--- puts and not printf
    114c:       b8 00 00 00 00          mov    $0x0,%eax
    1151:       5d                      pop    %rbp
    1152:       c3                      ret

-- Steve

[1] https://www.youtube.com/watch?v=JRyrhsx-L5Y

Re: [PATCH] tools: Use fputc() calls in some functions

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 28649 bytes --]

> diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c
> --- a/tools/bpf/bpftool/common.c
> +++ b/tools/bpf/bpftool/common.c
> @@ -53,7 +53,7 @@ void p_err(const char *fmt, ...)
>  	} else {
>  		fprintf(stderr, "Error: ");
>  		vfprintf(stderr, fmt, ap);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');
                     ^^^^^^

The fputc() arguments are reversed. The C standard signature is
`int fputc(int c, FILE *stream)` - the character comes first, the stream
second. This passes the FILE* stderr as the int c parameter and '\n' as
the FILE* stream parameter.

Correct form is `fputc('\n', stderr)`.

This constraint violation triggers a -Werror=int-conversion diagnostic and
breaks the build. If it compiled, at runtime fputc would dereference
(FILE*)10, crashing the program.

The same argument reversal occurs in 67 conversions across this patch.

> @@ -67,7 +67,7 @@ void p_info(const char *fmt, ...)
>
>  	va_start(ap, fmt);
>  	vfprintf(stderr, fmt, ap);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
                     ^^^^^^

Same reversed-argument bug.

> diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
> --- a/tools/bpf/bpftool/main.c
> +++ b/tools/bpf/bpftool/main.c
> @@ -273,7 +273,7 @@ void fprint_hex(FILE *f, void *arg, unsigned int n, const char *sep)
>  		if (!i)
>  			/* nothing */;
>  		else if (!(i % 16))
> -			fprintf(f, "\n");
> +			fputc(f, '\n');

Arguments reversed: `fputc(f, '\n')` should be `fputc('\n', f)`.

> diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> --- a/tools/bpf/bpftool/prog.c
> +++ b/tools/bpf/bpftool/prog.c
> @@ -1266,7 +1266,7 @@ static void hex_print(void *data, unsigned int size, FILE *f)
>  			fprintf(f, "%c%s", c, j == i + 7 ? " " : "");
>  		}
>
> -		fprintf(f, "\n");
> +		fputc(f, '\n');

Arguments reversed.

> diff --git a/tools/include/nolibc/err.h b/tools/include/nolibc/err.h
> --- a/tools/include/nolibc/err.h
> +++ b/tools/include/nolibc/err.h
> @@ -27,7 +27,7 @@ void vwarnx(const char *fmt, va_list args)
>  {
>  	fprintf(stderr, "%s: ", program_invocation_short_name);
>  	vfprintf(stderr, fmt, args);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed. This is in a nolibc header - nolibc's fputc
(tools/include/nolibc/stdio.h) does `write(fileno(stream), &ch, 1)`.
With stream==(FILE*)10 fileno dereferences a bogus pointer and crashes.

> diff --git a/tools/mm/page_owner_sort.c b/tools/mm/page_owner_sort.c
> --- a/tools/mm/page_owner_sort.c
> +++ b/tools/mm/page_owner_sort.c
> @@ -867,7 +867,7 @@ struct option longopts[] = {
>  			}
>  			if (cull & CULL_STACKTRACE)
>  				fprintf(fout, ":\n%s", list[i].stacktrace);
> -			fprintf(fout, "\n");
> +			fputc(fout, '\n');

Arguments reversed: should be `fputc('\n', fout)`.

> diff --git a/tools/net/ynl/ynltool/main.c b/tools/net/ynl/ynltool/main.c
> --- a/tools/net/ynl/ynltool/main.c
> +++ b/tools/net/ynl/ynltool/main.c
> @@ -156,7 +156,7 @@ void p_err(const char *fmt, ...)
>  	} else {
>  		fprintf(stderr, "Error: ");
>  		vfprintf(stderr, fmt, ap);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');
>  	}
>  	va_end(ap);
>  }
> @@ -170,7 +170,7 @@ void p_info(const char *fmt, ...)
>
>  	va_start(ap, fmt);
>  	vfprintf(stderr, fmt, ap);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Both conversions reverse the arguments.

> diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
> --- a/tools/objtool/builtin-check.c
> +++ b/tools/objtool/builtin-check.c
> @@ -297,7 +297,7 @@ int make_backup(void)
>  			fprintf(stderr, " %s", arg);
>  	}
>
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed. objtool runs during the kernel build, so a build
failure or crash here breaks the build.

> diff --git a/tools/objtool/disas.c b/tools/objtool/disas.c
> --- a/tools/objtool/disas.c
> +++ b/tools/objtool/disas.c
> @@ -537,7 +537,7 @@ void disas_print_insn(FILE *stream, struct disas_context *dctx,
>  		return;
>
>  	if (strcmp(format, "\n") == 0) {
> -		fprintf(stream, "\n");
> +		fputc(stream, '\n');

Arguments reversed.

> diff --git a/tools/perf/builtin-c2c.c b/tools/perf/builtin-c2c.c
> --- a/tools/perf/builtin-c2c.c
> +++ b/tools/perf/builtin-c2c.c
> @@ -2511,7 +2511,7 @@ struct perf_hpp hpp = {
>  		hists__fprintf_headers(&c2c_hists->hists, out);
>  		once = true;
>  	} else {
> -		fprintf(out, "\n");
> +		fputc(out, '\n');
>  	}

[ ... ]

> @@ -2590,15 +2590,15 @@ static void perf_c2c__hists_fprintf(FILE *out, struct perf_session *session)
>  	setup_pager();
>
>  	print_c2c__display_stats(out);
> -	fprintf(out, "\n");
> +	fputc(out, '\n');
>  	print_shared_cacheline_info(out);
> -	fprintf(out, "\n");
> +	fputc(out, '\n');
>  	print_c2c_info(out, session);
>
>  	if (c2c.stats_only)
>  		return;
>
> -	fprintf(out, "\n");
> +	fputc(out, '\n');

[ ... ]

> @@ -2606,7 +2606,7 @@ static void perf_c2c__hists_fprintf(FILE *out, struct perf_session *session)
>
>  	hists__fprintf(&c2c.hists.hists, true, 0, 0, 0, stdout, true);
>
> -	fprintf(out, "\n");
> +	fputc(out, '\n');

All four conversions reverse the arguments.

> diff --git a/tools/perf/builtin-daemon.c b/tools/perf/builtin-daemon.c
> --- a/tools/perf/builtin-daemon.c
> +++ b/tools/perf/builtin-daemon.c
> @@ -691,7 +691,7 @@ static int cmd_session_list(struct daemon *daemon, union cmd *cmd, FILE *out)
>  			/* session up time */
>  			csv_sep, (uint64_t)((curr - daemon->start) / 60));
>
> -		fprintf(out, "\n");
> +		fputc(out, '\n');
>  	} else {

[ ... ]

> @@ -730,7 +730,7 @@
>  				/* session up time */
>  				csv_sep, (uint64_t)((curr - session->start) / 60));
>
> -			fprintf(out, "\n");
> +			fputc(out, '\n');

Both conversions reverse the arguments.

> diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> --- a/tools/perf/builtin-lock.c
> +++ b/tools/perf/builtin-lock.c
> @@ -1319,9 +1319,9 @@ static void print_result(void)
>
>  		list_for_each_entry(key, &lock_keys, list) {
>  			key->print(key, st);
> -			fprintf(lock_output, " ");
> +			fputc(lock_output, ' ');
>  		}
> -		fprintf(lock_output, "\n");
> +		fputc(lock_output, '\n');

Both `fputc(lock_output, ' ')` and `fputc(lock_output, '\n')` reverse the
arguments. Correct form: `fputc(' ', lock_output)` and
`fputc('\n', lock_output)`.

> @@ -1598,7 +1598,7 @@ static void print_header_csv(const char *sep)
>  		fprintf(lock_output, "%s%s %s", "type", sep, "caller");
>  		if (verbose > 0)
>  			fprintf(lock_output, "%s %s", sep, "stacktrace");
> -		fprintf(lock_output, "\n");
> +		fputc(lock_output, '\n');

[ ... ]

> @@ -1629,7 +1629,7 @@ static void print_lock_stat_stdio(struct lock_contention *con, struct lock_stat *st)
>
>  	list_for_each_entry(key, &lock_keys, list) {
>  		key->print(key, st);
> -		fprintf(lock_output, " ");
> +		fputc(lock_output, ' ');
>  	}

[ ... ]

> @@ -1687,7 +1687,7 @@ static void print_lock_stat_csv(struct lock_contention *con, struct lock_stat *st,
>  	case LOCK_AGGR_CALLER:
>  		fprintf(lock_output, "%s%s %s", get_type_flags_name(st->flags), sep, st->name);
>  		if (verbose <= 0)
> -			fprintf(lock_output, "\n");
> +			fputc(lock_output, '\n');

[ ... ]

> @@ -1721,7 +1721,7 @@ static void print_lock_stat_csv(struct lock_contention *con, struct lock_stat *st,
>  			get_symbol_name_offset(kmap, sym, ip, buf, sizeof(buf));
>  			fprintf(lock_output, "%s %#lx %s", i ? ":" : sep, (unsigned long) ip, buf);
>  		}
> -		fprintf(lock_output, "\n");
> +		fputc(lock_output, '\n');
>  	}
>  }

[ ... ]

> @@ -1781,7 +1781,7 @@ static void print_footer_csv(int total, int bad, struct lock_contention_fails *fails,
>  		for (i = 0; i < BROKEN_MAX; i++)
>  			fprintf(lock_output, "%s bad_%s=%d", sep, name[i], bad_hist[i]);
>  	}
> -	fprintf(lock_output, "\n");
> +	fputc(lock_output, '\n');

All six conversions in builtin-lock.c reverse the arguments.

> diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
> --- a/tools/perf/builtin-script.c
> +++ b/tools/perf/builtin-script.c
> @@ -2561,7 +2561,7 @@
>
>  	perf_sample__fprintf_ipc(sample, evsel, fp);
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

[ ... ]

> @@ -2815,7 +2815,7 @@
>  				    cursor, symbol_conf.bt_stop_list, fp);
>  	}
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

[ ... ]

> @@ -3271,11 +3271,11 @@ static int list_available_languages_cb(struct scripting_ops *ops, const char *spec)
>
>  static void list_available_languages(void)
>  {
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  	fprintf(stderr, "Scripting language extensions (used in "
>  		"perf script -s [spec:]script.[spec]):\n\n");
>  	script_spec__for_each(&list_available_languages_cb);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

All four conversions reverse the arguments.

> diff --git a/tools/perf/ui/gtk/util.c b/tools/perf/ui/gtk/util.c
> --- a/tools/perf/ui/gtk/util.c
> +++ b/tools/perf/ui/gtk/util.c
> @@ -37,7 +37,7 @@ static int perf_gtk__error(const char *format, va_list args)
>  	    vasprintf(&msg, format, args) < 0) {
>  		fprintf(stderr, "Error:\n");
>  		vfprintf(stderr, format, args);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');
>  		return -1;
>  	}
>
> @@ -62,7 +62,7 @@ static int perf_gtk__warning_info_bar(const char *format, va_list args)
>  	    vasprintf(&msg, format, args) < 0) {
>  		fprintf(stderr, "Warning:\n");
>  		vfprintf(stderr, format, args);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');
>  		return -1;
>  	}
>
> @@ -83,7 +83,7 @@ static int perf_gtk__warning_statusbar(const char *format, va_list args)
>  	    vasprintf(&msg, format, args) < 0) {
>  		fprintf(stderr, "Warning:\n");
>  		vfprintf(stderr, format, args);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');

All three conversions reverse the arguments.

> diff --git a/tools/perf/ui/stdio/hist.c b/tools/perf/ui/stdio/hist.c
> --- a/tools/perf/ui/stdio/hist.c
> +++ b/tools/perf/ui/stdio/hist.c
> @@ -691,7 +691,7 @@
>  		}
>
>  next_line:
> -		fprintf(fp, "\n");
> +		fputc(fp, '\n');
>  	}

[ ... ]

> @@ -783,7 +783,7 @@ hists__fprintf_standard_headers(struct hists *hists,
>  		if (line)
>  			fprintf(fp, "# ");
>  		fprintf_line(hists, hpp, line, fp);
> -		fprintf(fp, "\n");
> +		fputc(fp, '\n');
>  	}

[ ... ]

> @@ -806,10 +806,10 @@ hists__fprintf_standard_headers(struct hists *hists,
>
>  		width = fmt->width(fmt, hpp, hists);
>  		for (i = 0; i < width; i++)
> -			fprintf(fp, ".");
> +			fputc(fp, '.');
>  	}
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

All four conversions reverse the arguments.

> diff --git a/tools/perf/util/debug.c b/tools/perf/util/debug.c
> --- a/tools/perf/util/debug.c
> +++ b/tools/perf/util/debug.c
> @@ -350,7 +350,7 @@ void __dump_stack(FILE *file, void **stackdump, size_t stackdump_size)
>  			fprintf(file, "    #%zd %p ", i, stackdump[i]);
>
>  		map__fprintf_srcline(al.map, al.addr, "", file);
> -		fprintf(file, "\n");
> +		fputc(file, '\n');

Arguments reversed.

> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -2454,7 +2454,7 @@ static void __print_pmu_caps(FILE *fp, int nr_caps, char **caps, char *pmu_name)
>  		delimiter = ", ";
>  	}
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');
>  }

[ ... ]

> @@ -2513,7 +2513,7 @@ static void print_pmu_mappings(struct feat_fd *ff, FILE *fp)
>  		pmu_num--;
>  	}
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

[ ... ]

> @@ -4349,7 +4349,7 @@ int perf_header__fprintf_info(struct perf_session *session, FILE *fp, bool full)
>  			fprintf(fp, "%s ", feat_ops[bit].name);
>  	}
>
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

All three conversions reverse the arguments.

> diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-log.c b/tools/perf/util/intel-pt-decoder/intel-pt-log.c
> --- a/tools/perf/util/intel-pt-decoder/intel-pt-log.c
> +++ b/tools/perf/util/intel-pt-decoder/intel-pt-log.c
> @@ -70,14 +70,14 @@ static void intel_pt_print_data(const unsigned char *buf, int len, uint64_t pos,
>  	int i;
>
>  	for (i = 0; i < indent; i++)
> -		fprintf(f, " ");
> +		fputc(f, ' ');
>
>  	fprintf(f, "  %08" PRIx64 ": ", pos);
>  	for (i = 0; i < len; i++)
>  		fprintf(f, " %02x", buf[i]);
>  	for (; i < 16; i++)
>  		fprintf(f, "   ");
> -	fprintf(f, " ");
> +	fputc(f, ' ');
>  }
>
>  static void intel_pt_print_no_data(uint64_t pos, int indent)
> @@ -85,12 +85,12 @@ static void intel_pt_print_no_data(uint64_t pos, int indent)
>  	int i;
>
>  	for (i = 0; i < indent; i++)
> -		fprintf(f, " ");
> +		fputc(f, ' ');
>
>  	fprintf(f, "  %08" PRIx64 ": ", pos);
>  	for (i = 0; i < 16; i++)
>  		fprintf(f, "   ");
> -	fprintf(f, " ");
> +	fputc(f, ' ');
>  }

All four conversions reverse the arguments.

> diff --git a/tools/perf/util/libbfd.c b/tools/perf/util/libbfd.c
> --- a/tools/perf/util/libbfd.c
> +++ b/tools/perf/util/libbfd.c
> @@ -603,7 +603,7 @@
>  		} else
>  			srcline = NULL;
>
> -		fprintf(s, "\n");
> +		fputc(s, '\n');

Arguments reversed. The variable s is an open_memstream() stream.

> diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
> --- a/tools/perf/util/stat-display.c
> +++ b/tools/perf/util/stat-display.c
> @@ -529,7 +529,7 @@ static void print_metric_json(struct perf_stat_config *config __maybe_unused,
>  		}
>  	}
>  	if (!config->metric_only)
> -		fprintf(out, "}");
> +		fputc(out, '}');
>  }

[ ... ]

> @@ -1316,7 +1316,7 @@ static void print_header_std(struct perf_stat_config *config,
>  	FILE *output = config->output;
>  	int i;
>
> -	fprintf(output, "\n");
> +	fputc(output, '\n');
>  	fprintf(output, " Performance counter stats for ");

[ ... ]

> @@ -1333,7 +1333,7 @@ static void print_header_std(struct perf_stat_config *config,
>  	else
>  		fprintf(output, "thread id \'%s", _target->tid);
>
> -	fprintf(output, "\'");
> +	fputc(output, '\'');

[ ... ]

> @@ -1406,9 +1406,9 @@ static void print_table(struct perf_stat_config *config, FILE *output, double avg)
>  		fprintf(output, " %17.9f (%+.9f) ", run, run - avg);
>
>  		for (h = 0; h < n; h++)
> -			fprintf(output, "#");
> +			fputc(output, '#');
>
> -		fprintf(output, "\n");
> +		fputc(output, '\n');
>  	}

[ ... ]

> @@ -1428,7 +1428,7 @@ static void print_footer(struct perf_stat_config *config)
>  		return;
>
>  	if (!config->null_run)
> -		fprintf(output, "\n");
> +		fputc(output, '\n');

All six conversions reverse the arguments.

> diff --git a/tools/perf/util/values.c b/tools/perf/util/values.c
> --- a/tools/perf/util/values.c
> +++ b/tools/perf/util/values.c
> @@ -217,7 +217,7 @@ static void perf_read_values__display_pretty(FILE *fp,
>  	fprintf(fp, "# %*s  %*s", pidwidth, "PID", tidwidth, "TID");
>  	for (j = 0; j < values->num_counters; j++)
>  		fprintf(fp, "  %*s", counterwidth[j], evsel__name(values->counters[j]));
> -	fprintf(fp, "\n");
> +	fputc(fp, '\n');

[ ... ]

> @@ -225,7 +225,7 @@ static void perf_read_values__display_pretty(FILE *fp,
>  		for (j = 0; j < values->num_counters; j++)
>  			fprintf(fp, "  %*" PRIu64,
>  				counterwidth[j], values->value[i][j]);
> -		fprintf(fp, "\n");
> +		fputc(fp, '\n');

Both conversions reverse the arguments.

> diff --git a/tools/power/acpi/tools/acpidump/apdump.c b/tools/power/acpi/tools/acpidump/apdump.c
> --- a/tools/power/acpi/tools/acpidump/apdump.c
> +++ b/tools/power/acpi/tools/acpidump/apdump.c
> @@ -171,7 +171,7 @@ ap_dump_table_buffer(struct acpi_table_header *table,
>  	acpi_ut_dump_buffer_to_file(gbl_output_file,
>  				    ACPI_CAST_PTR(u8, table), table_length,
>  				    DB_BYTE_DISPLAY, 0);
> -	fprintf(gbl_output_file, "\n");
> +	fputc(gbl_output_file, '\n');

Arguments reversed.

> diff --git a/tools/power/x86/intel-speed-select/isst-display.c b/tools/power/x86/intel-speed-select/isst-display.c
> --- a/tools/power/x86/intel-speed-select/isst-display.c
> +++ b/tools/power/x86/intel-speed-select/isst-display.c
> @@ -122,7 +122,7 @@ static void format_and_print(FILE *outf, int level, char *header, char *value)
>
>  	if (level == 0) {
>  		if (header)
> -			fprintf(outf, "{");
> +			fputc(outf, '{');

[ ... ]

> @@ -138,7 +138,7 @@ static void format_and_print(FILE *outf, int level, char *header, char *value)
>
>  		if (value) {
>  			if (last_level != level)
> -				fprintf(outf, "\n");
> +				fputc(outf, '\n');

[ ... ]

> @@ -156,7 +156,7 @@ static void format_and_print(FILE *outf, int level, char *header, char *value)
>  					fprintf(outf, "\n%s}", delimiters);
>  			}
>  			if (abs(last_level - level) < 3)
> -				fprintf(outf, "\n");
> +				fputc(outf, '\n');

All three conversions reverse the arguments.

> diff --git a/tools/power/x86/turbostat/turbostat.c b/tools/power/x86/turbostat/turbostat.c
> --- a/tools/power/x86/turbostat/turbostat.c
> +++ b/tools/power/x86/turbostat/turbostat.c
> @@ -9099,7 +9099,7 @@ void dump_cpuid_hypervisor(void)
>  	dump_word_chars(ebx);
>  	dump_word_chars(ecx);
>  	dump_word_chars(edx);
> -	fprintf(outf, "\n");
> +	fputc(outf, '\n');
>  }

[ ... ]

> @@ -9749,7 +9749,7 @@
>  		fprintf(outf, " siblings");
>  		for (ht_id = 0; ht_id <= MAX_HT_ID; ++ht_id)
>  			fprintf(outf, " %d", cpus[i].ht_sibling_cpu_id[ht_id]);
> -		fprintf(outf, "\n");
> +		fputc(outf, '\n');

Both conversions reverse the arguments.

> diff --git a/tools/testing/selftests/bpf/benchs/bench_htab_mem.c b/tools/testing/selftests/bpf/benchs/bench_htab_mem.c
> --- a/tools/testing/selftests/bpf/benchs/bench_htab_mem.c
> +++ b/tools/testing/selftests/bpf/benchs/bench_htab_mem.c
> @@ -148,7 +148,7 @@ static const struct htab_mem_use_case *htab_mem_find_use_case_or_exit(const char *name)
>  	fprintf(stderr, "available use case:");
>  	for (i = 0; i < ARRAY_SIZE(use_cases); i++)
>  		fprintf(stderr, " %s", use_cases[i].name);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c b/tools/testing/selftests/bpf/jit_disasm_helpers.c
> --- a/tools/testing/selftests/bpf/jit_disasm_helpers.c
> +++ b/tools/testing/selftests/bpf/jit_disasm_helpers.c
> @@ -228,7 +228,7 @@ struct bpf_prog_info info = {};
>  	for (pc = 0, i = 0; i < jited_funcs; ++i) {
>  		fprintf(text_out, "func #%d:\n", i);
>  		disasm_one_func(text_out, image + pc, func_lens[i]);
> -		fprintf(text_out, "\n");
> +		fputc(text_out, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/bpf/test_progs.c b/tools/testing/selftests/bpf/test_progs.c
> --- a/tools/testing/selftests/bpf/test_progs.c
> +++ b/tools/testing/selftests/bpf/test_progs.c
> @@ -383,7 +383,7 @@ static void print_test_result(const struct prog_test_def *test, const struct test_state *test_state)
>  	else
>  		fprintf(env.stdout_saved, "OK (SKIP: %d/%d)", skipped_cnt, subtests_cnt);
>
> -	fprintf(env.stdout_saved, "\n");
> +	fputc(env.stdout_saved, '\n');
>  }

[ ... ]

> @@ -391,7 +391,7 @@ static void print_test_log(char *log_buf, size_t log_cnt)
>  	log_buf[log_cnt] = '\0';
>  	fprintf(env.stdout_saved, "%s", log_buf);
>  	if (log_buf[log_cnt - 1] != '\n')
> -		fprintf(env.stdout_saved, "\n");
> +		fputc(env.stdout_saved, '\n');
>  }

[ ... ]

> @@ -409,7 +409,7 @@ static void print_subtest_name(int test_num, int subtest_num,
>  	if (result)
>  		fprintf(env.stdout_saved, ":%s", result);
>
> -	fprintf(env.stdout_saved, "\n");
> +	fputc(env.stdout_saved, '\n');

All three conversions reverse the arguments.

> diff --git a/tools/testing/selftests/drivers/net/hw/ncdevmem.c b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> --- a/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> +++ b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> @@ -135,7 +135,7 @@ static void pr_err(const char *fmt, ...)
>
>  	if (errno != 0)
>  		fprintf(stderr, ": %s", strerror(errno));
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/kvm/s390/keyop.c b/tools/testing/selftests/kvm/s390/keyop.c
> --- a/tools/testing/selftests/kvm/s390/keyop.c
> +++ b/tools/testing/selftests/kvm/s390/keyop.c
> @@ -117,7 +117,7 @@ static void dump_sk(const unsigned char skeys[], const char *descr)
>  		fprintf(stderr, "# %3d: ", i);
>  		for (j = 0; j < 32; j++)
>  			fprintf(stderr, "%02x ", skeys[i + j]);
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
> --- a/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
> +++ b/tools/testing/selftests/net/mptcp/pm_nl_ctl.c
> @@ -208,7 +208,7 @@
>  		}
>  		if (server_side)
>  			fprintf(stderr, ",server_side:1");
> -		fprintf(stderr, "\n");
> +		fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/net/psock_tpacket.c b/tools/testing/selftests/net/psock_tpacket.c
> --- a/tools/testing/selftests/net/psock_tpacket.c
> +++ b/tools/testing/selftests/net/psock_tpacket.c
> @@ -116,7 +116,7 @@ static int pfsocket(int ver)
>  static void status_bar_update(void)
>  {
>  	if (total_packets % 10 == 0) {
> -		fprintf(stderr, ".");
> +		fputc(stderr, '.');

[ ... ]

> @@ -825,7 +825,7 @@ static int test_tpacket(int version, int type)
>  	unmap_ring(sock, &ring);
>  	close(sock);
>
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Both conversions reverse the arguments.

> diff --git a/tools/testing/selftests/net/txtimestamp.c b/tools/testing/selftests/net/txtimestamp.c
> --- a/tools/testing/selftests/net/txtimestamp.c
> +++ b/tools/testing/selftests/net/txtimestamp.c
> @@ -200,10 +200,10 @@ static void __print_timestamp(const char *name, struct timespec *cur,
>  		ts_delta = timespec_to_ns64(cur) - timespec_to_ns64(&ts_usr);
>  		fprintf(stderr, "  (USR +");
>  		__print_ts_delta_formatted(ts_delta);
> -		fprintf(stderr, ")");
> +		fputc(stderr, ')');
>  	}
>
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  }

[ ... ]

> @@ -254,7 +254,7 @@ static void print_timing_event(char *name, struct timing_event *te)
>  	__print_ts_delta_formatted(te->min);
>  	fprintf(stderr, ", max=");
>  	__print_ts_delta_formatted(te->max);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  }

[ ... ]

> @@ -271,7 +271,7 @@ static void print_payload(char *data, int len)
>  	fprintf(stderr, "payload: ");
>  	for (i = 0; i < len; i++)
>  		fprintf(stderr, "%02hhx ", data[i]);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  }

[ ... ]

> @@ -933,7 +933,7 @@ int main(int argc, char **argv)
>  	fprintf(stderr, "protocol:     %s\n", sock_names[cfg_proto]);
>  	fprintf(stderr, "payload:      %u\n", cfg_payload_len);
>  	fprintf(stderr, "server port:  %u\n", dest_port);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

All four conversions reverse the arguments.

> diff --git a/tools/testing/selftests/powerpc/nx-gzip/gunz_test.c b/tools/testing/selftests/powerpc/nx-gzip/gunz_test.c
> --- a/tools/testing/selftests/powerpc/nx-gzip/gunz_test.c
> +++ b/tools/testing/selftests/powerpc/nx-gzip/gunz_test.c
> @@ -363,7 +363,7 @@
>  			goto err3;
>  		fprintf(stderr, "%02x ", tmp[i]);
>  		if (i == 5)
> -			fprintf(stderr, "\n");
> +			fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/testing/selftests/vfio/lib/include/libvfio/assert.h b/tools/testing/selftests/vfio/lib/include/libvfio/assert.h
> --- a/tools/testing/selftests/vfio/lib/include/libvfio/assert.h
> +++ b/tools/testing/selftests/vfio/lib/include/libvfio/assert.h
> @@ -10,7 +10,7 @@ #define SELFTESTS_VFIO_LIB_INCLUDE_LIBVFIO_ASSERT_H
>
>  #define VFIO_LOG_AND_EXIT(...) do {		\
>  	fprintf(stderr, "  " __VA_ARGS__);	\
> -	fprintf(stderr, "\n");			\
> +	fputc(stderr, '\n');			\

Arguments reversed in a widely-used macro. Every expansion of this macro
is broken.

> diff --git a/tools/testing/selftests/vfio/lib/libvfio.c b/tools/testing/selftests/vfio/lib/libvfio.c
> --- a/tools/testing/selftests/vfio/lib/libvfio.c
> +++ b/tools/testing/selftests/vfio/lib/libvfio.c
> @@ -60,16 +60,16 @@ char **vfio_selftests_get_bdfs(int *argc, char *argv[], int *nr_bdfs)
>  	}
>
>  	fprintf(stderr, "Unable to determine which device(s) to use, skipping test.\n");
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  	fprintf(stderr, "To pass the device address via environment variable:\n");
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  	fprintf(stderr, "    export VFIO_SELFTESTS_BDF=\"segment:bus:device.function\"\n");
>  	fprintf(stderr, "    %s [options]\n", argv[0]);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  	fprintf(stderr, "To pass the device address(es) via argv:\n");
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');
>  	fprintf(stderr, "    %s [options] segment:bus:device.function ...\n", argv[0]);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

All five conversions reverse the arguments.

> diff --git a/tools/testing/vsock/vsock_diag_test.c b/tools/testing/vsock/vsock_diag_test.c
> --- a/tools/testing/vsock/vsock_diag_test.c
> +++ b/tools/testing/vsock/vsock_diag_test.c
> @@ -86,7 +86,7 @@ static void print_vsock_addr(FILE *fp, unsigned int cid, unsigned int port)
>  		fprintf(fp, "%u:", cid);
>
>  	if (port == VMADDR_PORT_ANY)
> -		fprintf(fp, "*");
> +		fputc(fp, '*');

[ ... ]

> @@ -94,7 +94,7 @@ static void print_vsock_addr(FILE *fp, unsigned int cid, unsigned int port)
>  static void print_vsock_stat(FILE *fp, struct vsock_stat *st)
>  {
>  	print_vsock_addr(fp, st->msg.vdiag_src_cid, st->msg.vdiag_src_port);
> -	fprintf(fp, " ");
> +	fputc(fp, ' ');

Both conversions reverse the arguments.

> diff --git a/tools/thermal/tmon/sysfs.c b/tools/thermal/tmon/sysfs.c
> --- a/tools/thermal/tmon/sysfs.c
> +++ b/tools/thermal/tmon/sysfs.c
> @@ -522,7 +522,7 @@ int update_thermal_data()
>  	}
>
>  	if (tmon_log) {
> -		fprintf(tmon_log, "\n");
> +		fputc(tmon_log, '\n');

Arguments reversed.

> diff --git a/tools/thermal/tmon/tmon.c b/tools/thermal/tmon/tmon.c
> --- a/tools/thermal/tmon/tmon.c
> +++ b/tools/thermal/tmon/tmon.c
> @@ -199,7 +199,7 @@
>  		fprintf(tmon_log, "%s%d ", ptdata.cdi[i].type,
>  			ptdata.cdi[i].instance);
>
> -	fprintf(tmon_log, "\n");
> +	fputc(tmon_log, '\n');

Arguments reversed.

> diff --git a/tools/tracing/latency/latency-collector.c b/tools/tracing/latency/latency-collector.c
> --- a/tools/tracing/latency/latency-collector.c
> +++ b/tools/tracing/latency/latency-collector.c
> @@ -1999,7 +1999,7 @@
>  "been enabled. Random sleep is intended for the following tracers:\n");
>  			for (i = 0; random_tracers[i]; i++)
>  				fprintf(stderr, "%s\n", random_tracers[i]);
> -			fprintf(stderr, "\n");
> +			fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/tracing/rtla/src/common.c b/tools/tracing/rtla/src/common.c
> --- a/tools/tracing/rtla/src/common.c
> +++ b/tools/tracing/rtla/src/common.c
> @@ -449,7 +449,7 @@ void common_usage(const char *tool, const char *mode,
>  		fprintf(stderr, "%s [-h] ", mode);
>
>  	print_msg_array(start_msgs);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/tracing/rtla/src/utils.c b/tools/tracing/rtla/src/utils.c
> --- a/tools/tracing/rtla/src/utils.c
> +++ b/tools/tracing/rtla/src/utils.c
> @@ -67,7 +67,7 @@ void fatal(const char *fmt, ...)
>  	va_start(ap, fmt);
>  	vfprintf(stderr, fmt, ap);
>  	va_end(ap);
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/usb/ffs-test.c b/tools/usb/ffs-test.c
> --- a/tools/usb/ffs-test.c
> +++ b/tools/usb/ffs-test.c
> @@ -562,7 +562,7 @@ empty_out_buf(struct thread *ignore, const void *buf, size_t nbytes)
>  				fprintf(stderr, "%4zd:", len);
>  			fprintf(stderr, " %02x", *p);
>  			if (31 == (len % 32))
> -				fprintf(stderr, "\n");
> +				fputc(stderr, '\n');

Arguments reversed.

> diff --git a/tools/verification/rv/src/in_kernel.c b/tools/verification/rv/src/in_kernel.c
> --- a/tools/verification/rv/src/in_kernel.c
> +++ b/tools/verification/rv/src/in_kernel.c
> @@ -667,7 +667,7 @@ static void ikm_usage_print_reactors(void)
>  		end = strstr(start, "\n");
>  	}
>
> -	fprintf(stderr, "\n");
> +	fputc(stderr, '\n');

Arguments reversed.

Looking at the lore discussion by Andi Kleen and Steven Rostedt, they
raised a concern that GCC already performs this fprintf->fputc optimization
automatically, making the patch unnecessary code churn. The author has not
responded to this feedback.

Beyond the optimization question, the technical implementation has a
systematic bug: every fputc() conversion in this patch reverses the
argument order.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/30016526959

Re: [PATCH] tools: Use fputc() calls in some functions

[Segher Boessenkool]

On Thu, Jul 23, 2026 at 06:47:00AM -0700, Andi Kleen wrote:
> On Thu, Jul 23, 2026 at 12:25:25PM +0200, Markus Elfring wrote:
> > From: Markus Elfring <elfring@users.sourceforge.net>
> > Date: Thu, 23 Jul 2026 11:13:30 +0200
> > Subject: [PATCH] tools: Use fputc() calls in some functions
> > MIME-Version: 1.0
> > Content-Type: text/plain; charset=UTF-8
> > Content-Transfer-Encoding: 8bit
> > 
> > Single characters should occasionally be transferred into the output.
> > Thus use the corresponding function “fputc” (or even “putchar”).
> > 
> > The source code was transformed by using the Coccinelle software.
> 
> gcc already does this transformation automatically.

Yup.  And the printf() form is much more regular, expected, readable.
The primary consumers of source code are human beings, all programs
should be written with that in mind!


Segher