Better ways to parse the Content-Length from the HTTP header

Question

I have a buffer that contains the HTTP header, which contains the Content-Length string indicating how big the file is that needs to be downloaded.

I am looking to write a generic function for parsing a substring and eventually a value that needs to be converted to an int from a string.

The following approach seems to work but seems more like a "brute-force" approach. Any better ideas?

void parse(char *src, char *dst, const char *firstKey, const char *secKey)
{
    char *start = strstr(src, firstKey) + 2;
    char *end = strstr(start, secKey);
    size_t bytes = end - start;
   
    memcpy(dst, start, bytes);  
}

int main()
{
    char httpHeader[] = {72, 84, 84, 80, 47, 49, 46, 49, 32, 50, 48, 49, 32, 67, 114, 101, 97, 116, 101, 100, 13, 10, 68, 97, 116, 101, 58, 32, 77, 111, 110, 44, 32, 49, 55, 32, 74, 97, 110, 32, 50, 48, 50, 50, 32, 50, 49, 58, 53, 56, 58, 52, 51, 32, 71, 77, 84, 13, 10, 83, 101, 114, 118, 101, 114, 58, 32, 65, 112, 97, 99, 104, 101, 47, 50, 46, 52, 46, 50, 57, 32, 40, 85, 98, 117, 110, 116, 117, 41, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 76, 101, 110, 103, 116, 104, 58, 32, 48, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 84, 121, 112, 101, 58, 32, 116, 101, 120, 116, 47, 104, 116, 109, 108, 13, 10, 13, 10};   
    
    /*
    what httpHeader above stores:
    HTTP/1.1 201 Created
    Date: Mon, 17 Jan 2022 21:58:43 GMT
    Server: Apache/2.4.29 (Ubuntu)
    Content-Length: 0
    Content-Type: text/html
    */
    
    char dst[100] = {0};
    parse(httpHeader, dst, "Content-Length", "\n");    // dst => Content-Length: 0
    
    char dst1[100] = {0};    
    parse(dst, dst1, ":", "\r");
    
    // get the int value
    int contentLenVal = atoi(dst1);

    printf ("dst: %s\ndst1: %s\n\nContent value: %d\n", dst, dst1, contentLenVal);

}

Answer 1

Any better ideas?

Consume the buffer

Rather than leave the buffer "as-is" while parsing, adjust its start and end while parsing.

char *s = httpHeader;
s = parse(s, "Content-Length", "\n");
if (s == NULL) Handle_Error();
s = parse(s, ":", "\r");
if (s == NULL) Handle_Error();
long contentLenVal = strtol(s, ...);   // See below  for details.

---

Avoid bugs.

Avoid going off the end

Below may + 2 past the end of the string. Better to walk carefully.

// char *start = strstr(src, firstKey) + 2;
char *start = strstr(src, firstKey);
if (start == NULL || start[0] == '\0' || start[1] == '\0') {
  Handle_Error();
}
start += 2;
....
if (end == NULL) {
  Handle_Error();
}

IAC, the + 2 is strange. I'd expect:

char *start = strstr(src, firstKey);
if (start == NULL) {
  Handle_Error();
}
start += strlen(firstKey);

dst lacks a certain null character

Presently code relies on the caller to pre-fill with '\0. Better to pass in buffer size and append a '\0' explicitly in the function.

// void parse(char *src, char *dst, ...
void parse(const char *src, size_t sz, char *dst, ...

  ...
  ptrdiff_t diff = end - start;
  if (diff > sz) {
    Handle_Error();
  } else {
    size_t bytes = (size_t) diff;
    memcpy(dst, start, bytes);
    dest[bytes] = '\0';
  }
  

Use strtol()

Robust code looks for errors,

// int contentLenVal = atoi(dst1);
char *endptr;
errno = 0;
long contentLenVal = strtol(dst1, &endptr, 10);
if (dst1 == endptr || errno || contentLenVal < LEN_MIN || contentLenVal >  LEN_MAX) {
  Handle_Error();
}

Minor: Use const

// void parse(char *src, char *dst, ...
void parse(const char *src, char *dst, ...

If code is to consume src, leave as is.

Minor: Wrap

// char httpHeader[] = {72, 84, 84, 80, 47, 49, 46, 49, 32, 50, 48, 49, 32, 67, 114, 101, 97, 116, 101, 100, 13, 10, 68, 97, 116, 101, 58, 32, 77, 111, 110, 44, 32, 49, 55, 32, 74, 97, 110, 32, 50, 48, 50, 50, 32, 50, 49, 58, 53, 56, 58, 52, 51, 32, 71, 77, 84, 13, 10, 83, 101, 114, 118, 101, 114, 58, 32, 65, 112, 97, 99, 104, 101, 47, 50, 46, 52, 46, 50, 57, 32, 40, 85, 98, 117, 110, 116, 117, 41, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 76, 101, 110, 103, 116, 104, 58, 32, 48, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 84, 121, 112, 101, 58, 32, 116, 101, 120, 116, 47, 104, 116, 109, 108, 13, 10, 13, 10};   

Versus

char httpHeader[] = {72, 84, 84, 80, 47, 49, 46, 49, 32, 50, 48, 49, 32, 67,
    114, 101, 97, 116, 101, 100, 13, 10, 68, 97, 116, 101, 58, 32, 77, 111, 110,
    44, 32, 49, 55, 32, 74, 97, 110, 32, 50, 48, 50, 50, 32, 50, 49, 58, 53, 56,
    58, 52, 51, 32, 71, 77, 84, 13, 10, 83, 101, 114, 118, 101, 114, 58, 32, 65,
    112, 97, 99, 104, 101, 47, 50, 46, 52, 46, 50, 57, 32, 40, 85, 98, 117, 110,
    116, 117, 41, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 76, 101, 110,
    103, 116, 104, 58, 32, 48, 13, 10, 67, 111, 110, 116, 101, 110, 116, 45, 84,
    121, 112, 101, 58, 32, 116, 101, 120, 116, 47, 104, 116, 109, 108, 13, 10,
    13, 10};

Tip: Use an auto-formatter.