How to replace multiple occurrences of substring with C Programming Language

Question

I am having a very difficult time with this issue and need some help. I have a variable that is declared as this:

char *var_val = "";

This variable is later defined (in Asterisk) as:

var_val = S_OR(pair, "");

In this case, the pair variable represents a valid string (ie: SIP/221[|]Custom:DND221[|]Custom:USER221).

I am wanting to replace all occurrences of [|] with &

In PHP, I would just write var_val = str_replace("[|]", "&", var_val), which would yield SIP/221&Custom:DND221&Custom:USER221

I have tried to do this in C, but am failing miserably. Either the compiler throws all types of warnings, outright errors, or compiles fine but the var_val is left unchanged when the script runs. Can someone please provide me the C code needed to accomplish this task?

Given that I know var_val="SIP/221[|]Custom:DND221[|]Custom:USER221", my attempts have all been derived around some form of the following:

// create a buffer to hold the new string
char buffer[strlen(var_val)];

// in C strings are basically arrays so we need an index to run through
// the original string
int i = 0;

// loop through original string
while (*var_val) {
    
    // does the needle exist in the string?     
    if (strstr(var_val, "[|]") == (char *)"[|]") { 

        // it does so we want to replace [|] with & and 
        // skip ahead 3 positions in the original string    
        strcpy(&buffer[i], "&");
                    
        i += 1;
                        
        var_val += 3;
    }

    // needle is not found at this spot so copy one character from 
    // source string into buffer
    else
        buffer[i++] = *var_val++;
    }

}

// finish our buffer with trailing 0 to indicate end of string
buffer[i] = '\0';


// point our original string to our new string
var_val = (char *)buffer;

---

EDIT

---

Thanks to comments from both @Jonathan Leffler and @Dante Culaciati, here is a function that does work ( What function is to replace a substring from a string in C? ), although it generates one compiler warning about placing parenthesis around the while (needle =.... construct. I also had to replace malloc() with ast_malloc() and realloc() with ast_realloc() since this is being used within Asterisk.

static char *dull_replace(const char *in, const char *pattern, const char *by)
{
    size_t outsize = strlen(in) + 1;
    // TODO maybe avoid reallocing by counting the non-overlapping occurences of pattern
    char *res = malloc(outsize);
    // use this to iterate over the output
    size_t resoffset = 0;

    char *needle;
    while (needle = strstr(in, pattern)) {
        // copy everything up to the pattern
        memcpy(res + resoffset, in, needle - in);
        resoffset += needle - in;

        // skip the pattern in the input-string
        in = needle + strlen(pattern);

        // adjust space for replacement
        outsize = outsize - strlen(pattern) + strlen(by);
        res = realloc(res, outsize);

        // copy the pattern
        memcpy(res + resoffset, by, strlen(by));
        resoffset += strlen(by);
    }

    // copy the remaining input
    strcpy(res + resoffset, in);

    return res;
}

To use the function, I pass var_val to it, and it returns the updated or unmodified original string:

var_val = dull_replace(var_val, "[|]", "&");

Anyone know how to interpret this compiler warning and correct the code?

res_config_curl.c: In function ‘dull_replace’:
res_config_curl.c:526:5: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
     while (needle = strstr(in, pattern)) {
     ^

Answer 1

Thanks to several commentators, I was able to get this working. The function used came from the link provided by Dante. The link and the working function are in my original question (edited).