Ndtyjky

If I have a string like the following:

char* exampleString = "test$$";

Let's say that getpid() returns 1587.

How can I replace the $$ in the string with the result of getpid() such that the result would be a string "test1587"?

Since "test$$" is immutable and a pointer to it should better be char const* instead of char* you'd have to copy it to an array where you then can replace "$$".

Possible solution:

#include <stdbool.h>

#include <stddef.h>

#include <string.h>

#include <stdlib.h>

#include <stdio.h>



#include <unistd.h>



int main(void)

{

    char const *input = "foo$$bar";

    pid_t pid = 1234;



    size_t format_length = strlen(input);

    char *format = calloc(format_length + 2, 1);

    if (!format) {

        fputs("Couldn't allocate memory :(nn", stderr);

        return EXIT_FAILURE;

    }



    // copy input to format replacing "$$" with "%ld" in the process

    bool replacement_done = false;

    for (size_t i = 0; i < format_length + replacement_done; ++i) {

        if (!replacement_done && i + 1 < format_length &&

            input[i] == '$' && input[i + 1] == '$')

        {

            format[  i] = '%';  // just

            format[++i] = 'l';  // being

            format[++i] = 'd';  // safe.

            replacement_done = true;

            continue;

        }



        format[i] = input[i - replacement_done];

    }



    if (!replacement_done) {

        free(format);

        fputs("Nothing to do :(nn", stderr);

        return EXIT_FAILURE;

    }



    char *result = malloc(1);

    if (!result) {

        free(format);

        fputs("Couldn't allocate memory :(nn", stderr);

        return EXIT_FAILURE;

    }



    // there is no guesswork needed, snprintf() will tell the needed size

    int bytes_needed = snprintf(result, 1, format, (long)getpid());

    if (bytes_needed < 0) {

        free(format);

        free(result);

        fputs("snprintf() failed :(nn", stderr);

        return EXIT_FAILURE;

    }



    char *temp = realloc(result, ++bytes_needed);

    if (!temp) {

        free(format);

        free(result);

        fputs("Couldn't allocate memory :(nn", stderr);

        return EXIT_FAILURE;

    }

    result = temp;



    int written = snprintf(result, bytes_needed, format, (long)getpid());  // long should be big enough

    if(written < 0 || written >= bytes_needed ) {

        free(format);

        free(result);

        fputs("snprintf() failed :(nn", stderr);

        return EXIT_FAILURE;

    }



    puts(result); // done.



    free(format);

    free(result);

}

You can't do a direct substitution because your string (character array) is not long enough to hold the pid, and of course, if declared as a string literal is also not mutable.

There are a couple of ways you could go with this, but here's a reasonably elegant one:

/* Modify the existing string to be a pattern string for snprintf */

int len = strlen(exampleString) - 1; /* Can you see why I might do this? */

char* formatString = strdup(exampleString); /* Because we can't modify a literal */

int newLen = len + 12; /* How about this? */

char *pidString = malloc(newLen);

int i;

for (i = 0; i < len; i++) {

  if (formatString[i] == '$' && formatString[i+1] == '$') {

    formatString[i] = '%';

    formatString[i+1] = 'd';

    break;

  }

}

snprintf(pidString, newLen - 1, formatString, getpid());

Can you follow how this works?

How would you enhance this to fail gracefully if the exampleString does not contain $$ ?