Extra characters added during string concatenation

Question

I am working on a C++ code that concatenates values together into a string of chars for an I2C message system. I have the messaging properly worked out, but when I concatenate the strings together, the code incorrectly concatenates more than just the three values I want onto the string. The code I have written is below:

void concatint(int value1, char address1, char address2) { int alive1 = static_cast<int>(address1); int alive2 = static_cast<int>(address2); char* alive3 = (char*)malloc(sizeof(address1)); char* alive4 = (char*)malloc(sizeof(address2)); //alive3 = address1; //alive4 = address2; sprintf(alive3, "%2d", address1); sprintf(alive4, "%2d", address2); if (value1 < 10) readlength = 1; if (value1 >= 10 && value1 < 100) readlength = 2; if (value1 >= 100 && value1 < 1000) readlength = 3; if (value1 >= 1000 && value1 < 10000) readlength = 4; if (value1 >= 10000 && value1 < 100000) readlength = 5; if (value1 >= 100000 && value1 < 1000000) readlength = 6; if (value1 >= 1000000 && value1 < 10000000) readlength = 7; if (value1 >= 10000000 && value1 < 100000000) readlength = 8; *writedata = 0; itoa(value1, writedata, 10); strcpy(writeaddress, &address1); strcat(writeaddress, &address2); strcat(writeaddress, writedata); strcpy(readaddress, address1); strcat(readaddress, address2); typevalue = 1; }

This function has the input of:

concatint(5, ' ', ' ');

where the two address values are two ASCii characters.

The result of this code should be: ' '' '5 with the ASCii characters being concatenated before the value. However, when I run the code, the result I get is:

" \005 \0055"

My code seems to concatenate an extra character in between my characters and I am not sure where my above code adds that. I've stepped through the code and everything should work fine, not sure where my problem is.

Are you allowed to use c++? This (using cstrings, malloc and cstring functions) is certainly not the way to do this when using c++. — drescherjm
– drescherjm, Commented May 16, 2016 at 15:26
What are all the undefined variables you are using? Where are they defined? How are they initialized? Why are you checking such large ranges for value1 when you know it can't be more than -128 to 127 (or 0 to 255, depending on the signedness of char)? And lastly, where do you free value3 and value4? — Some programmer dude
– Some programmer dude, Commented May 16, 2016 at 15:29
this code is so wrong that its hard to know where to start. What do you think that the first static_cast is doing? — pm100
– pm100, Commented May 16, 2016 at 15:31

Martin Bonner supports Monica · Accepted Answer · 2016-05-16 15:28:28Z

The first eight lines have undefined behaviour:

void concatint(int value1, char address1, char address2) { int alive1 = static_cast<int>(address1); int alive2 = static_cast<int>(address2); // Note that address1 is a char, not a char*, and as such sizeof(address1) // is guaranteed to be 1. Thus we allocate one byte of storage. char * alive3 = (char*) malloc(sizeof(address1)); char * alive4 = (char*) malloc(sizeof(address2)); // Here we write two digits and a terminating NUL to that one byte // => undefined behaviour. Cannot reason further about the program. sprintf(alive3, "%2d", address1); sprintf(alive4, "%2d", address2);

Furthermore:

 strcpy(writeaddress, &address1);

won't work either. address1 is a single character. &address1 is a pointer to this character, but there is no trailing NUL character following it, so it is not a valid pointer to pass to strcpy either.

Use std::string

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