C - Replace $$ in string with ID from getpid()? - c

Question

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"?

Answer 1

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$$bar";
pid_t pid = 1234;
size_t format_length = strlen(input);
char *format = calloc(format_length + 2, 1);
if (!format) {
fputs("Couldn't allocate memory :(\n\n", stderr);
return EXIT_FAILURE;
}
// copy input to format replacing "$$" 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 :(\n\n", stderr);
return EXIT_FAILURE;
}
char *result = malloc(1);
if (!result) {
free(format);
fputs("Couldn't allocate memory :(\n\n", 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 :(\n\n", stderr);
return EXIT_FAILURE;
}
char *temp = realloc(result, ++bytes_needed);
if (!temp) {
free(format);
free(result);
fputs("Couldn't allocate memory :(\n\n", 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 :(\n\n", stderr);
return EXIT_FAILURE;
}
puts(result); // done.
free(format);
free(result);
}

Answer 2

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 $$ ?