A
Arduino•2mo ago
Tari

char to char* adds characters on the end

Hi, I'm using this code to pull data from a section of a char* string, and pass a single character to the U8G2 drawStr() method. The drawStr method will only accept a const char*, so I need to convert the char I get from the char* array into another char*. This, for some reason; adds the characters xV? to the end of the desired output when displaying with U8G2 drawStr. 
char *charset = ".,!?ABCDEFGHIJKLMNOPQRSTUVWXYZ\"\':;()";
const char *key1Char = new char(charset[4]);
const char *key2Char = new char(charset[5]);
const char *key3Char = new char(charset[6]);
const char *key4Char = new char(charset[7]);

u8g2.drawStr(57, 22, key1Char);
u8g2.drawStr(29, 37, key2Char);
u8g2.drawStr(84, 40, key3Char);
u8g2.drawStr(56, 55, key4Char);
char *charset = ".,!?ABCDEFGHIJKLMNOPQRSTUVWXYZ\"\':;()";
const char *key1Char = new char(charset[4]);
const char *key2Char = new char(charset[5]);
const char *key3Char = new char(charset[6]);
const char *key4Char = new char(charset[7]);

u8g2.drawStr(57, 22, key1Char);
u8g2.drawStr(29, 37, key2Char);
u8g2.drawStr(84, 40, key3Char);
u8g2.drawStr(56, 55, key4Char);
2 Replies
sven
sven•2mo ago
The issue is that you're creating single characters, not null-terminated strings. When you use new char(charset[4]), you're allocating a single character without a null terminator (\0). The drawStr() function expects a null-terminated C-string, so it keeps reading memory beyond your single character until it happens to find a null byte, which is why you're seeing garbage characters. - If U8G2 has a drawChar method : // Check if this exists in your U8G2 library u8g2.drawGlyph(57, 22, charset[4]); // or u8g2.drawChar(57, 22, charset[4]); - Use stack-allocated arrays : char *charset = ".,!?ABCDEFGHIJKLMNOPQRSTUVWXYZ"':;()"; char key1Char[2] = {charset[4], '\0'}; char key2Char[2] = {charset[5], '\0'}; char key3Char[2] = {charset[6], '\0'}; char key4Char[2] = {charset[7], '\0'}; u8g2.drawStr(57, 22, key1Char); u8g2.drawStr(29, 37, key2Char); u8g2.drawStr(84, 40, key3Char); u8g2.drawStr(56, 55, key4Char);
Tari
TariOP•2mo ago
Thank you! I ended up going with this solution, which works quite well; although I'm not sure if performance is optimal (I'm incredibly rusty with C, I took a break from programming for a few years and mainly worked with C#). 
int charindex = charsubset * 4;

char key1[2] = "\0";
char key2[2] = "\0";
char key3[2] = "\0";
char key4[2] = "\0";
if (tmode) {
  key1[0] = ABXYcharset[charindex];
  key2[0] = ABXYcharset[charindex + 1];
  key3[0] = ABXYcharset[charindex + 2];
  key4[0] = ABXYcharset[charindex + 3];
} else {
  key1[0] = 'Y';
  key2[0] = 'X';
  key3[0] = 'B';
  key4[0] = 'A';
}
int charindex = charsubset * 4;

char key1[2] = "\0";
char key2[2] = "\0";
char key3[2] = "\0";
char key4[2] = "\0";
if (tmode) {
  key1[0] = ABXYcharset[charindex];
  key2[0] = ABXYcharset[charindex + 1];
  key3[0] = ABXYcharset[charindex + 2];
  key4[0] = ABXYcharset[charindex + 3];
} else {
  key1[0] = 'Y';
  key2[0] = 'X';
  key3[0] = 'B';
  key4[0] = 'A';
}
I forgot to close the topic, but hopefully your answer helps others with a similar problem :)

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