C-string manipulation

Question

I'd just like to get some pointers on my newbie-ish C here. The intention is to have an environment variable provide the "prefix" for any paths this app needs.

For example, APP_PREFIX could be /opt/app, a path the app wanted to get could be /etc/app.conf, and so app_path("/etc/app.conf") should return /opt/app/etc/app.conf.

I realise this is currently sensitive to leading/trailing slashes; I just wanted to make sure I'm on the right track with this basic stuff. Be as pedantic as you like – from outright bugs down to style and convention.

It compiles with no warnings on Mac OS X with gcc -Wall -pedantic ..., and does what I expect it to. On Linux, I had to compile with libbsd, and #include <bsd/string.h> for strlcpy() and strlcat().

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define APP_PREFIX_ENV_VAR "APP_PREFIX"
const char * app_path_prefix() {
 char * env_prefix = getenv(APP_PREFIX_ENV_VAR);
 if (env_prefix == NULL) {
 return "";
 } else {
 return env_prefix;
 }
}
char * app_path(char * in_path) {
 const char * prefix = app_path_prefix();
 size_t prefix_len = strlen(prefix);
 size_t out_size = prefix_len + strlen(in_path) + 1;
 char * out = malloc(out_size);
 strlcpy(out, prefix, prefix_len + 1);
 strlcat(out, in_path, out_size);
 return out;
}
int main(int argc, char ** argv) {
 char * path = app_path(argv[1]);
 printf("result: '%s'\n", path);
 free(path);
 return EXIT_SUCCESS;
}

Answer 1

All seems good.

As you mentioned it is sensitive to trailing slash being missing. I would just force that issue and always append one between the prefix and the path. An extra slash will not hurt but a missing one will.

Apart from not using C in the first place :-) all my other issues are simple stylistic ones that only 30% of people would agree (I always divide stylistic camps as 30% Side A, 30% side ~A, 40% don't care).

A tiny bit more white space to make it easier to read. But let me just say your code is not bad in that regards (just slightly less than I would use).

I don't want to recommend any particular style. But I can show you how I would have written it:

// Keep you includes sorted in alphabetical order
// Here it is not such a big deal but with big projects the list can get big
// and alphabetical order helps you find things in large lists.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define APP_PREFIX_ENV_VAR "APP_PREFIX"
const char * app_path_prefix()
{
 char * env_prefix = getenv(APP_PREFIX_ENV_VAR);
 return (env_prefix == NULL)
 ? ""
 : env_prefix;
}
char * app_path(char * in_path)
{
 /*
 * Some people really hate this.
 *
 * Others like myself really like lining up the '=' sign
 * Just do it locally with a set of calculations. I find it easier to see
 * both the lhs and the rhs of expression quickly.
 */
 char const* prefix = app_path_prefix();
 size_t prefix_len = strlen(prefix);
 size_t out_size = prefix_len + strlen(in_path) + 1;
 /* 
 * Dynamically allocated. Released to outside world it is the
 * responsibility of the caller to free this memory
 */
 char * out = malloc(out_size);
 strlcpy(out, prefix, prefix_len + 1);
 strlcat(out, in_path, out_size);
 return out;
}
int main(int argc, char ** argv)
{
 /* test */
 char * path = app_path(argv[1]);
 printf("result: '%s'\n", path);
 free(path);
 return EXIT_SUCCESS;
}

• Dan B: Thanks! Do feel free to expand on the style points more if you like though, with reasons. Then I can form my own opinions :) Whitespace: blank lines between variable defs and the rest of the function?
• Loki Astari: @DanB: Added more comments.
• Anonymous: The conditional in app_path_prefix can be simplified to env_prefix ? env_prefix : "". Don't hesitate to exploit C's generalized Boolean conditionals.