sys/string_utils: add string_writer helper

[benpicco]

Contribution description

Assembling a string through multiple writes if often open-coded like this

char buffer[64];
size_t len = 0;
for (unsigned i = 0; i < ARRAY_SIZE(names); ++i) {
    len += snprintf(&buffer[len], sizeof(buffer) - len, "%u: %s\n", i, names[i]);
}

This is error prone (the above example foregoes any error handling) and ugly.

This introduces a small helper function to achieve the same in a much more convenient fasion:

char buffer[64];
string_writer_t sw;
string_writer_init(&sw, buffer, sizeof(buffer));

for (unsigned i = 0; i < ARRAY_SIZE(names); ++i) {
    swprintf(&sw, "%u: %s\n", i, names[i]);
}

Testing procedure

A new unit test has been added.

Issues/PRs references

[maribu]

That API indeed disarms the usual footguns :)

I wonder if there is some BSD / whatever API already that we could steal?

[Teufelchen1]

All in all looks good. Some nits, some ideas.

You mentioned that this is often done manually in RIOT, could you also replace such instances with this helper right away?

[Teufelchen1]

Great improvements.

[Teufelchen1]

Squash & Ci?

[riot-ci]

Murdock results

✔️ PASSED

db6196e tests/pkg/relic: blacklist ATXmega boards

Success	Failures	Total	Runtime
10242	0	10246	13m:37s

Artifacts

• Documentation preview

[derMihai]

you can add

__attribute__ ((format (printf, 2, 3)))

to enable static printf-like compiler checks for the format string and arguments, see https://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Function-Attributes.html.

[benpicco]

GCC doesn't like it if the variable args are empty

/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c: In function ‘test_libc_swprintf’:
/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c:42:5: error: format not a string literal and no format arguments [-Werror=format-security]
     res = swprintf(&sw, "Hello World!");
     ^~~

[derMihai]

Odd, I tried now and works for me:

__attribute__ ((format (printf, 2, 3)))
int __swprintf(string_writer_t *sw, FLASH_ATTR const char *restrict format, ...);

Then:

    res = swprintf(&sw, "Hello World!");

works just fine, while:

    res = swprintf(&sw, "Hello World%s!");

results in

/home/mihai.renea@ml-pa.loc/proj/RIOT/tests/unittests/tests-libc/tests-libc.c: In function ‘test_libc_s
wprintf’:
/home/mihai.renea@ml-pa.loc/proj/RIOT/tests/unittests/tests-libc/tests-libc.c:42:38: error: format ‘%s’
 expects a matching ‘char *’ argument [-Werror=format=]
   42 |     res = swprintf(&sw, "Hello World%s!");
      |                                     ~^
      |                                      |
      |                                      char *

Which compiler version are you using? Mine:

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (15:10.3-2021.07-4) 10.3.1 20210621 (release)

[benpicco]

This is only happens with avr-gcc (GCC) 7.3.0

/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c: In function ‘test_libc_swprintf’:
/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c:42:5: error: format not a string literal and no format arguments [-Werror=format-security]
     res = swprintf(&sw, "Hello World!");
     ^~~
/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c:49:5: error: format not a string literal and no format arguments [-Werror=format-security]
     res = swprintf(&sw, "0123456789");
     ^~~
/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c:54:5: error: format not a string literal and no format arguments [-Werror=format-security]
     res = swprintf(&sw, "01234567891");
     ^~~
/home/benpicco/dev/RIOT/tests/unittests/tests-libc/tests-libc.c:59:5: error: format not a string literal and no format arguments [-Werror=format-security]
     res = swprintf(&sw, "###");
     ^~~

[benpicco]

Ah I think it's because the TO_FLASH() is interfering