Extracting key=value with scanf in C - c

Question

I need to extract a value for a given key from a string. I made this quick attempt:
char js[] = "some preceding text with\n"
"new lines and spaces\n"
"param_1=123\n"
"param_2=321\n"
"param_3=string\n"
"param_2=321\n";
char* param_name = "param_2";
char *key_s, *val_s;
char buf[32];
key_s = strstr(js, param_name);
if (key_s == NULL)
return 0;
val_s = strchr(key_s, '=');
if (val_s == NULL)
return 0;
sscanf(val_s + 1, "%31s", buf);
printf("'%s'\n", buf);
And it in fact works ok (printf gives '321'). But I suppose the scanf/sscanf would make this task even easier but I have not managed to figure out the formatting string for that.
Is that possible to pass a content of a variable param_name into sscanf so that it evaluates it as a part of a formatting string? In other words, I need to instruct sscanf that in this case it should look for a pattern param_2=%s (the param_name in fact comes from a function argument).

Answer 1

Not directly, no.
In practice, there's of course nothing stopping you from building the format string for sscanf() at runtime, with e.g. snprintf().
Something like:
void print_value(const char **js, size_t num_js, const char *key)
{
char tmp[32], value[32];
snprintf(tmp, sizeof tmp, "%s=%%31s", key);
for(size_t i = 0; i < num_js; ++i)
{
if(sscanf(js[i], tmp, value) == 1)
{
printf("found '%s'\n", value);
break;
}
}
}

Answer 2

OP's has a good first step:
char *key_s = strstr(js, param_name);
if (key_s == NULL)
return 0;
The rest may be simplified to
if (sscanf(&key_s[strlen(param_name)], "=%31s", buf) == 0) {
return 0;
}
printf("'%s'\n", buf);
Alternatively one could use " =%31s" to allow spaces before =.
OP's approach gets fooled by "param_2 321\n" "param_3=string\n".
Note: Weakness to all answers so far to not parse the empty string.

Answer 3

One issue that bears consideration is the difference between finding a 'key=value' setting in the string for a specific key value (such as param_2 in the question), and finding any 'key=value' setting in the string (with no specific key in mind a priori). The techniques to be used are rather different.
Another issue that has not self-evidently been considered is the possibility that you're looking for a key param_2 but the string also contains param_22=xyz and t_param_2=abc. The simple-minded approaches using strstr() to hunt for param_2 will pick up either of those alternatives.
In the sample data, there is a collection of characters that are not in the 'key=value' format to be skipped before the any 'key=value' parts. In the general case, we should assume that such data appears before, in between, and after the 'key=value' pairs. It appears that the values do not need to support complications such as quoted strings and metacharacters, and the value is delimited by white space. There is no comment convention visible.
Here's some workable code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum { MAX_KEY_LEN = 31 };
enum { MAX_VAL_LEN = 63 };
int find_any_key_value(const char *str, char *key, char *value);
int find_key_value(const char *str, const char *key, char *value);
int find_any_key_value(const char *str, char *key, char *value)
{
char junk[256];
const char *search = str;
while (*search != '\0')
{
int offset;
if (sscanf(search, " %31[a-zA-Z_0-9]=%63s%n", key, value, &offset) == 2)
return(search + offset - str);
int rc;
if ((rc = sscanf(search, "%255s%n", junk, &offset)) != 1)
return EOF;
search += offset;
}
return EOF;
}
int find_key_value(const char *str, const char *key, char *value)
{
char found[MAX_KEY_LEN + 1];
int offset;
const char *search = str;
while ((offset = find_any_key_value(search, found, value)) > 0)
{
if (strcmp(found, key) == 0)
return(search + offset - str);
search += offset;
}
return offset;
}
int main(void)
{
char js[] = "some preceding text with\n"
"new lines and spaces\n"
"param_1=123\n"
"param_2=321\n"
"param_3=string\n"
"param_4=param_2=confusion\n"
"m= x\n"
"param_2=987\n";
const char p2_key[] = "param_2";
int offset;
const char *str;
char key[MAX_KEY_LEN + 1];
char value[MAX_VAL_LEN + 1];
printf("String being scanned is:\n[[%s]]\n", js);
str = js;
while ((offset = find_any_key_value(str, key, value)) > 0)
{
printf("Any found key = [%s] value = [%s]\n", key, value);
str += offset;
}
str = js;
while ((offset = find_key_value(str, p2_key, value)) > 0)
{
printf("Found key %s with value = [%s]\n", p2_key, value);
str += offset;
}
return 0;
}
Sample output:
$ ./so24490410
String being scanned is:
[[some preceding text with
new lines and spaces
param_1=123
param_2=321
param_3=string
param_4=param_2=confusion
m= x
param_2=987
]]
Any found key = [param_1] value = [123]
Any found key = [param_2] value = [321]
Any found key = [param_3] value = [string]
Any found key = [param_4] value = [param_2=confusion]
Any found key = [m] value = [x]
Any found key = [param_2] value = [987]
Found key param_2 with value = [321]
Found key param_2 with value = [987]
$
If you need to handle different key or value lengths, you need to adjust the format strings as well as the enumerations. If you pass the size of the key buffer and the size of the value buffer to the functions, then you need to use snprint() to create the format strings used by sscanf(). There is an outside chance that you might have a single 'word' of 255 characters followed immediately by the target 'key=value' string. The chances are ridiculously small, but you might decide you need to worry about that (it prevents this code being bomb-proof).