Converting char array to int with strtol() in C

Question

i have a difficulty with strtol() function in C, here's a piece of code of how i'm trying to use it

char   TempChar;                        
char   SerialBuffer[21];
char   hexVoltage[2];
long   intVoltage;

 do
   {
     Status = ReadFile(hComm, &TempChar, sizeof(TempChar), &NoBytesRead, NULL);
     SerialBuffer[i] = TempChar;
     i++;
    }
  while (NoBytesRead > 0);

memcpy(hexVoltage, SerialBuffer+3, 2);

intVoltage = strtol(hexVoltage, NULL, 16);

So the question is why does strtol() returns 0 ? And how do i convert char array of values in hex to int (long in this particular case)? hexVoltage in my case contains {03, 34} after memcpy(). Thanks in advance. Really appreciate the help here.

You need to make it one character longer and null terminate it, otherwise it's not a string — Baldrick, Sep 09 '16 at 11:48
Please replace the code about reading file with proper [mcve]. It's not relevant to your question, and makes it hard for us to reproduce the issue. — user694733, Sep 09 '16 at 11:50
"in my case contains {03, 34}": then it couldn't be a hex string at all, it is just raw numbers. — Blagovest Buyukliev, Sep 09 '16 at 11:50
Tried it with adding hexVoltage[2] = '\0'; after memcpy, had no luck. I would like the integer to contain 820 i.e 0x0334 in decimal. @BlagovestBuyukliev i'm reading com port, — sentenced, Sep 09 '16 at 11:52
Element `hexVoltage[2]` does not exist. `hexVoltage` has only 2 elements. — user694733, Sep 09 '16 at 11:52
We really don't have enough info in the question to know what the data even represents, where it came from and how it should be converted. — Baldrick, Sep 09 '16 at 12:02

Blagovest Buyukliev · Accepted Answer · 2016-11-02T20:39:19.097

1

strtol and friends expect that you supply them with a printable, ASCII representation of the number. Instead, you are supplying it with a binary sequence that is read from the file (port).

In that case, your intVoltage can be computed by combining the two read bytes into a 2-byte number with bitwise operations, depending on the endianness of these numbers on your platform:

uint8_t binVoltage[2];
...
uint16_t intVoltage = binVoltage[0] | (binVoltage[1] << 8);
/* or */
uint16_t intVoltage = (binVoltage[0] << 8) | binVoltage[1];

edited Nov 02 '16 at 20:39

answered Sep 09 '16 at 12:02

Blagovest Buyukliev

42,498
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Also, if the endianness is known to be common at both ends, you could use a `union` with 2 members, one `uint8_t binVoltage[2]` and the other `uint16_t intVoltage`, and convert it that way. I believe that is string-aliasing safe from C99 onwards. – Baldrick Sep 09 '16 at 12:13
Thanks a lot man, your second variant worked, i still don't quite understand the logic, but you showed me a way where to dig. – sentenced Sep 09 '16 at 12:19
The C standard does not require ASCII as execution character set. – too honest for this site Sep 09 '16 at 12:53
@Baldrick: That has endianess issues, etc. Better use shifts until you can prove this is a bottleneck and the `union` solves it. – too honest for this site Sep 09 '16 at 12:55
The code invokes undefined behaviour for certain values on systems with 16 bit `int`. – too honest for this site Sep 09 '16 at 12:57
@Olaf: care to explain? I suppose it's because of the `int` promotion of the left operand of the shifts? – Blagovest Buyukliev Sep 09 '16 at 13:17

Converting char array to int with strtol() in C

1 Answers1