Issue with sscanf not parsing correctly

Issue with sscanf not parsing correctly

[Alexandre Raymond]

Greetings!

I have been developing on a Teensy 4.1 platform, which I'm told uses
newlib under the hood, and encountered a strange issue with sscanf,
where different parameter orderings yield different results with
stdint types.

Here is a minimal code sample which exhibits the issue:
---8<---

#include <stdio.h>
#include <stdint.h>

void setup() {
    uint8_t i2c_addr = 1;
    uint16_t mem_addr = 1;
    uint8_t len = 1;
    int ret;

    Serial.println(String(i2c_addr) + "," + String(mem_addr) + "," +
String(len));
    //prints: 1,1,1

    const char *cmd = "1 2 3\n";

    // This is the order I intended to scan the string in
    ret = sscanf(cmd, "%hhu %hu %hhu", &i2c_addr, &mem_addr, &len);
    Serial.println(String(ret) + "- " + String(i2c_addr) + "," +
String(mem_addr) + "," + String(len));
    // prints: 3- 1,0,3

    // I just swap the order of the target variables around
    ret = sscanf(cmd, "%hhu %hhu %hu", &i2c_addr, &len, &mem_addr);
    Serial.println(String(ret) + "- " + String(i2c_addr) + "," +
String(len) + "," + String(mem_addr));
    // prints: 3- 1,2,3
}

void loop() { }

--8<---

Would you have any idea what may be causing this?

Thanks in advance for your help!
Alexandre

Re: Issue with sscanf not parsing correctly

[Keith Packard]

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Alexandre Raymond <alexandre.j.raymond@gmail.com> writes:

> Greetings!
>
> I have been developing on a Teensy 4.1 platform, which I'm told uses
> newlib under the hood, and encountered a strange issue with sscanf,
> where different parameter orderings yield different results with
> stdint types.

Odd. I've just tested this using the qemu model for the
thumb_v7e_m_dp_hard gcc target for the picolibc build set to use the
legacy stdio bits from newlib and it seems to work just fine (it also
works fine with the new stdio bits from picolibc).

Could this libc be built to not support C99 formats? That would cause
all kinds of issues as sscanf would fail when it hit the '%hhu' format.

Here's the regular C test code I used for this:


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#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <stdlib.h>

void __assert_func(const char *file, int line, const char *func, const char *expr)
{
    printf("%s:%d (%s) assertion failed: %s\n", file, line, func, expr);
    exit(1);
}

int main(void) {
    uint8_t i2c_addr = 1;
    uint16_t mem_addr = 1;
    uint8_t len = 1;
    int ret;

    printf("%d, %d, %d\n", i2c_addr, mem_addr, len);
    //prints: 1,1,1

    const char *cmd = "1 2 3\n";

    // This is the order I intended to scan the string in
    ret = sscanf(cmd, "%hhu %hu %hhu", &i2c_addr, &mem_addr, &len);

    printf("%d- %d, %d, %d\n", ret, i2c_addr, mem_addr, len);
    assert(i2c_addr ==1 && mem_addr == 2 && len == 3);
    // prints: 3- 1,0,3

    // I just swap the order of the target variables around
    ret = sscanf(cmd, "%hhu %hhu %hu", &i2c_addr, &len, &mem_addr);

    printf("%d- %d, %d, %d\n", ret, i2c_addr, len, mem_addr);
    assert(i2c_addr ==1 && len == 2 && mem_addr == 3);
    // prints: 3- 1,2,3

    return 0;
}

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I've added this as a picolibc test so it will be regularly checked
against both newlib and picolibc stdio bits going forward.

-- 
-keith

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