It seems to me that Linux has it easy with /proc/self/exe. But I'd like to know if there is a convenient way to find the current application's directory in C/C++ with cross-platform interfaces. I've seen some projects mucking around with argv[0], but it doesn't seem entirely reliable.
If you ever had to support, say, Mac OS X, which doesn't have /proc/, what would you have done? Use #ifdefs to isolate the platform-specific code (NSBundle, for example)? Or try to deduce the executable's path from argv[0], $PATH and whatnot, risking finding bugs in edge cases?
Some OS-specific interfaces:
Mac OS X: _NSGetExecutablePath() (man 3 dyld)
Linux: readlink /proc/self/exe
Solaris: getexecname()
FreeBSD: sysctl CTL_KERN KERN_PROC KERN_PROC_PATHNAME -1
FreeBSD if it has procfs: readlink /proc/curproc/file (FreeBSD doesn't have procfs by default)
NetBSD: readlink /proc/curproc/exe
DragonFly BSD: readlink /proc/curproc/file
Windows: GetModuleFileName() with hModule = NULL
There are also third party libraries that can be used to get this information, such as whereami as mentioned in prideout's answer, or if you are using Qt, QCoreApplication::applicationFilePath() as mentioned in the comments.
The portable (but less reliable) method is to use argv[0]. Although it could be set to anything by the calling program, by convention it is set to either a path name of the executable or a name that was found using $PATH.
Some shells, including bash and ksh, set the environment variable "_" to the full path of the executable before it is executed. In that case you can use getenv("_") to get it. However this is unreliable because not all shells do this, and it could be set to anything or be left over from a parent process which did not change it before executing your program.
The use of /proc/self/exe is non-portable and unreliable. On my Ubuntu 12.04 system, you must be root to read/follow the symlink. This will make the Boost example and probably the whereami() solutions posted fail.
This post is very long but discusses the actual issues and presents code which actually works along with validation against a test suite.
The best way to find your program is to retrace the same steps the system uses. This is done by using argv[0] resolved against file system root, pwd, path environment and considering symlinks, and pathname canonicalization. This is from memory but I have done this in the past successfully and tested it in a variety of different situations. It is not guaranteed to work, but if it doesn't you probably have much bigger problems and it is more reliable overall than any of the other methods discussed. There are situations on a Unix compatible system in which proper handling of argv[0] will not get you to your program but then you are executing in a certifiably broken environment. It is also fairly portable to all Unix derived systems since around 1970 and even some non-Unix derived systems as it basically relies on libc() standard functionality and standard command line functionality. It should work on Linux (all versions), Android, Chrome OS, Minix, original Bell Labs Unix, FreeBSD, NetBSD, OpenBSD, BSD x.x, SunOS, Solaris, SYSV, HP-UX, Concentrix, SCO, Darwin, AIX, OS X, NeXTSTEP, etc. And with a little modification probably VMS, VM/CMS, DOS/Windows, ReactOS, OS/2, etc. If a program was launched directly from a GUI environment, it should have set argv[0] to an absolute path.
Understand that almost every shell on every Unix compatible operating system that has ever been released basically finds programs the same way and sets up the operating environment almost the same way (with some optional extras). And any other program that launches a program is expected to create the same environment (argv, environment strings, etc.) for that program as if it were run from a shell, with some optional extras. A program or user can setup an environment that deviates from this convention for other subordinate programs that it launches but if it does, this is a bug and the program has no reasonable expectation that the subordinate program or its subordinates will function correctly.
Possible values of argv[0] include:
/path/to/executable — absolute path
../bin/executable — relative to pwd
bin/executable — relative to pwd
./foo — relative to pwd
executable — basename, find in path
bin//executable — relative to pwd, non-canonical
src/../bin/executable — relative to pwd, non-canonical, backtracking
bin/./echoargc — relative to pwd, non-canonical
Values you should not see:
~/bin/executable — rewritten before your program runs.
~user/bin/executable — rewritten before your program runs
alias — rewritten before your program runs
$shellvariable — rewritten before your program runs
*foo* — wildcard, rewritten before your program runs, not very useful
?foo? — wildcard, rewritten before your program runs, not very useful
In addition, these may contain non-canonical path names and multiple layers of symbolic links. In some cases, there may be multiple hard links to the same program. For example, /bin/ls, /bin/ps, /bin/chmod, /bin/rm, etc. may be hard links to /bin/busybox.
To find yourself, follow the steps below:
Save pwd, PATH, and argv[0] on entry to your program (or initialization of your library) as they may change later.
Optional: particularly for non-Unix systems, separate out but don't discard the pathname host/user/drive prefix part, if present; the part which often precedes a colon or follows an initial "//".
If argv[0] is an absolute path, use that as a starting point. An absolute path probably starts with "/" but on some non-Unix systems it might start with "" or a drive letter or name prefix followed by a colon.
Else if argv[0] is a relative path (contains "/" or "" but doesn't start with it, such as "../../bin/foo", then combine pwd+"/"+argv[0] (use present working directory from when program started, not current).
Else if argv[0] is a plain basename (no slashes), then combine it with each entry in PATH environment variable in turn and try those and use the first one which succeeds.
Optional: Else try the very platform specific /proc/self/exe, /proc/curproc/file (BSD), and (char *)getauxval(AT_EXECFN), and dlgetname(...) if present. You might even try these before argv[0]-based methods, if they are available and you don't encounter permission issues. In the somewhat unlikely event (when you consider all versions of all systems) that they are present and don't fail, they might be more authoritative.
Optional: check for a path name passed in using a command line parameter.
Optional: check for a pathname in the environment explicitly passed in by your wrapper script, if any.
Optional: As a last resort try environment variable "_". It might point to a different program entirely, such as the users shell.
Resolve symlinks, there may be multiple layers. There is the possibility of infinite loops, though if they exist your program probably won't get invoked.
Canonicalize filename by resolving substrings like "/foo/../bar/" to "/bar/". Note this may potentially change the meaning if you cross a network mount point, so canonization is not always a good thing. On a network server, ".." in symlink may be used to traverse a path to another file in the server context instead of on the client. In this case, you probably want the client context so canonicalization is ok. Also convert patterns like "/./" to "/" and "//" to "/".
In shell, readlink --canonicalize will resolve multiple symlinks and canonicalize name. Chase may do similar but isn't installed. realpath() or canonicalize_file_name(), if present, may help.
If realpath() doesn't exist at compile time, you might borrow a copy from a permissively licensed library distribution, and compile it in yourself rather than reinventing the wheel. Fix the potential buffer overflow (pass in sizeof output buffer, think strncpy() vs strcpy()) if you will be using a buffer less than PATH_MAX. It may be easier just to use a renamed private copy rather than testing if it exists. Permissive license copy from android/darwin/bsd:
https://android.googlesource.com/platform/bionic/+/f077784/libc/upstream-freebsd/lib/libc/stdlib/realpath.c
Be aware that multiple attempts may be successful or partially successful and they might not all point to the same executable, so consider verifying your executable; however, you may not have read permission — if you can't read it, don't treat that as a failure. Or verify something in proximity to your executable such as the "../lib/" directory you are trying to find. You may have multiple versions, packaged and locally compiled versions, local and network versions, and local and USB-drive portable versions, etc. and there is a small possibility that you might get two incompatible results from different methods of locating. And "_" may simply point to the wrong program.
A program using execve can deliberately set argv[0] to be incompatible with the actual path used to load the program and corrupt PATH, "_", pwd, etc. though there isn't generally much reason to do so; but this could have security implications if you have vulnerable code that ignores the fact that your execution environment can be changed in variety of ways including, but not limited, to this one (chroot, fuse filesystem, hard links, etc.) It is possible for shell commands to set PATH but fail to export it.
You don't necessarily need to code for non-Unix systems but it would be a good idea to be aware of some of the peculiarities so you can write the code in such a way that it isn't as hard for someone to port later. Be aware that some systems (DEC VMS, DOS, URLs, etc.) might have drive names or other prefixes which end with a colon such as "C:", "sys$drive:[foo]bar", and "file:///foo/bar/baz". Old DEC VMS systems use "[" and "]" to enclose the directory portion of the path though this may have changed if your program is compiled in a POSIX environment. Some systems, such as VMS, may have a file version (separated by a semicolon at the end). Some systems use two consecutive slashes as in "//drive/path/to/file" or "user#host:/path/to/file" (scp command) or "file://hostname/path/to/file" (URL). In some cases (DOS and Windows), PATH might have different separator characters — ";" vs ":" and "" vs "/" for a path separator. In csh/tsh there is "path" (delimited with spaces) and "PATH" delimited with colons but your program should receive PATH so you don't need to worry about path. DOS and some other systems can have relative paths that start with a drive prefix. C:foo.exe refers to foo.exe in the current directory on drive C, so you do need to lookup current directory on C: and use that for pwd.
An example of symlinks and wrappers on my system:
/usr/bin/google-chrome is symlink to
/etc/alternatives/google-chrome which is symlink to
/usr/bin/google-chrome-stable which is symlink to
/opt/google/chrome/google-chrome which is a bash script which runs
/opt/google/chome/chrome
Note that user bill posted a link above to a program at HP that handles the three basic cases of argv[0]. It needs some changes, though:
It will be necessary to rewrite all the strcat() and strcpy() to use strncat() and strncpy(). Even though the variables are declared of length PATHMAX, an input value of length PATHMAX-1 plus the length of concatenated strings is > PATHMAX and an input value of length PATHMAX would be unterminated.
It needs to be rewritten as a library function, rather than just to print out results.
It fails to canonicalize names (use the realpath code I linked to above)
It fails to resolve symbolic links (use the realpath code)
So, if you combine both the HP code and the realpath code and fix both to be resistant to buffer overflows, then you should have something which can properly interpret argv[0].
The following illustrates actual values of argv[0] for various ways of invoking the same program on Ubuntu 12.04. And yes, the program was accidentally named echoargc instead of echoargv. This was done using a script for clean copying but doing it manually in shell gets same results (except aliases don't work in script unless you explicitly enable them).
cat ~/src/echoargc.c
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
main(int argc, char **argv)
{
printf(" argv[0]=\"%s\"\n", argv[0]);
sleep(1); /* in case run from desktop */
}
tcc -o ~/bin/echoargc ~/src/echoargc.c
cd ~
/home/whitis/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
echoargc
argv[0]="echoargc"
bin/echoargc
argv[0]="bin/echoargc"
bin//echoargc
argv[0]="bin//echoargc"
bin/./echoargc
argv[0]="bin/./echoargc"
src/../bin/echoargc
argv[0]="src/../bin/echoargc"
cd ~/bin
*echo*
argv[0]="echoargc"
e?hoargc
argv[0]="echoargc"
./echoargc
argv[0]="./echoargc"
cd ~/src
../bin/echoargc
argv[0]="../bin/echoargc"
cd ~/junk
~/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
~whitis/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
alias echoit=~/bin/echoargc
echoit
argv[0]="/home/whitis/bin/echoargc"
echoarg=~/bin/echoargc
$echoarg
argv[0]="/home/whitis/bin/echoargc"
ln -s ~/bin/echoargc junk1
./junk1
argv[0]="./junk1"
ln -s /home/whitis/bin/echoargc junk2
./junk2
argv[0]="./junk2"
ln -s junk1 junk3
./junk3
argv[0]="./junk3"
gnome-desktop-item-edit --create-new ~/Desktop
# interactive, create desktop link, then click on it
argv[0]="/home/whitis/bin/echoargc"
# interactive, right click on gnome application menu, pick edit menus
# add menu item for echoargc, then run it from gnome menu
argv[0]="/home/whitis/bin/echoargc"
cat ./testargcscript 2>&1 | sed -e 's/^/ /g'
#!/bin/bash
# echoargc is in ~/bin/echoargc
# bin is in path
shopt -s expand_aliases
set -v
cat ~/src/echoargc.c
tcc -o ~/bin/echoargc ~/src/echoargc.c
cd ~
/home/whitis/bin/echoargc
echoargc
bin/echoargc
bin//echoargc
bin/./echoargc
src/../bin/echoargc
cd ~/bin
*echo*
e?hoargc
./echoargc
cd ~/src
../bin/echoargc
cd ~/junk
~/bin/echoargc
~whitis/bin/echoargc
alias echoit=~/bin/echoargc
echoit
echoarg=~/bin/echoargc
$echoarg
ln -s ~/bin/echoargc junk1
./junk1
ln -s /home/whitis/bin/echoargc junk2
./junk2
ln -s junk1 junk3
./junk3
These examples illustrate that the techniques described in this post should work in a wide range of circumstances and why some of the steps are necessary.
EDIT: Now, the program that prints argv[0] has been updated to actually find itself.
// Copyright 2015 by Mark Whitis. License=MIT style
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <limits.h>
#include <assert.h>
#include <string.h>
#include <errno.h>
// "look deep into yourself, Clarice" -- Hanibal Lector
char findyourself_save_pwd[PATH_MAX];
char findyourself_save_argv0[PATH_MAX];
char findyourself_save_path[PATH_MAX];
char findyourself_path_separator='/';
char findyourself_path_separator_as_string[2]="/";
char findyourself_path_list_separator[8]=":"; // could be ":; "
char findyourself_debug=0;
int findyourself_initialized=0;
void findyourself_init(char *argv0)
{
getcwd(findyourself_save_pwd, sizeof(findyourself_save_pwd));
strncpy(findyourself_save_argv0, argv0, sizeof(findyourself_save_argv0));
findyourself_save_argv0[sizeof(findyourself_save_argv0)-1]=0;
strncpy(findyourself_save_path, getenv("PATH"), sizeof(findyourself_save_path));
findyourself_save_path[sizeof(findyourself_save_path)-1]=0;
findyourself_initialized=1;
}
int find_yourself(char *result, size_t size_of_result)
{
char newpath[PATH_MAX+256];
char newpath2[PATH_MAX+256];
assert(findyourself_initialized);
result[0]=0;
if(findyourself_save_argv0[0]==findyourself_path_separator) {
if(findyourself_debug) printf(" absolute path\n");
realpath(findyourself_save_argv0, newpath);
if(findyourself_debug) printf(" newpath=\"%s\"\n", newpath);
if(!access(newpath, F_OK)) {
strncpy(result, newpath, size_of_result);
result[size_of_result-1]=0;
return(0);
} else {
perror("access failed 1");
}
} else if( strchr(findyourself_save_argv0, findyourself_path_separator )) {
if(findyourself_debug) printf(" relative path to pwd\n");
strncpy(newpath2, findyourself_save_pwd, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
strncat(newpath2, findyourself_path_separator_as_string, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
strncat(newpath2, findyourself_save_argv0, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
realpath(newpath2, newpath);
if(findyourself_debug) printf(" newpath=\"%s\"\n", newpath);
if(!access(newpath, F_OK)) {
strncpy(result, newpath, size_of_result);
result[size_of_result-1]=0;
return(0);
} else {
perror("access failed 2");
}
} else {
if(findyourself_debug) printf(" searching $PATH\n");
char *saveptr;
char *pathitem;
for(pathitem=strtok_r(findyourself_save_path, findyourself_path_list_separator, &saveptr); pathitem; pathitem=strtok_r(NULL, findyourself_path_list_separator, &saveptr) ) {
if(findyourself_debug>=2) printf("pathitem=\"%s\"\n", pathitem);
strncpy(newpath2, pathitem, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
strncat(newpath2, findyourself_path_separator_as_string, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
strncat(newpath2, findyourself_save_argv0, sizeof(newpath2));
newpath2[sizeof(newpath2)-1]=0;
realpath(newpath2, newpath);
if(findyourself_debug) printf(" newpath=\"%s\"\n", newpath);
if(!access(newpath, F_OK)) {
strncpy(result, newpath, size_of_result);
result[size_of_result-1]=0;
return(0);
}
} // end for
perror("access failed 3");
} // end else
// if we get here, we have tried all three methods on argv[0] and still haven't succeeded. Include fallback methods here.
return(1);
}
main(int argc, char **argv)
{
findyourself_init(argv[0]);
char newpath[PATH_MAX];
printf(" argv[0]=\"%s\"\n", argv[0]);
realpath(argv[0], newpath);
if(strcmp(argv[0],newpath)) { printf(" realpath=\"%s\"\n", newpath); }
find_yourself(newpath, sizeof(newpath));
if(1 || strcmp(argv[0],newpath)) { printf(" findyourself=\"%s\"\n", newpath); }
sleep(1); /* in case run from desktop */
}
And here is the output which demonstrates that in every one of the previous tests it actually did find itself.
tcc -o ~/bin/echoargc ~/src/echoargc.c
cd ~
/home/whitis/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
echoargc
argv[0]="echoargc"
realpath="/home/whitis/echoargc"
findyourself="/home/whitis/bin/echoargc"
bin/echoargc
argv[0]="bin/echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
bin//echoargc
argv[0]="bin//echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
bin/./echoargc
argv[0]="bin/./echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
src/../bin/echoargc
argv[0]="src/../bin/echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
cd ~/bin
*echo*
argv[0]="echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
e?hoargc
argv[0]="echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
./echoargc
argv[0]="./echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
cd ~/src
../bin/echoargc
argv[0]="../bin/echoargc"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
cd ~/junk
~/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
~whitis/bin/echoargc
argv[0]="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
alias echoit=~/bin/echoargc
echoit
argv[0]="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
echoarg=~/bin/echoargc
$echoarg
argv[0]="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
rm junk1 junk2 junk3
ln -s ~/bin/echoargc junk1
./junk1
argv[0]="./junk1"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
ln -s /home/whitis/bin/echoargc junk2
./junk2
argv[0]="./junk2"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
ln -s junk1 junk3
./junk3
argv[0]="./junk3"
realpath="/home/whitis/bin/echoargc"
findyourself="/home/whitis/bin/echoargc"
The two GUI launches described above also correctly find the program.
There is one potential pitfall. The access() function drops permissions if the program is setuid before testing. If there is a situation where the program can be found as an elevated user but not as a regular user, then there might be a situation where these tests would fail, although it is unlikely the program could actually be executed under those circumstances. One could use euidaccess() instead. It is possible, however, that it might find an inaccessable program earlier on path than the actual user could.
The whereami library by Gregory Pakosz implements this for a variety of platforms, using the APIs mentioned in mark4o's post. This is most interesting if you "just" need a solution that works for a portable project and are not interested in the peculiarities of the various platforms.
At the time of writing, supported platforms are:
Windows
Linux
Mac
iOS
Android
QNX Neutrino
FreeBSD
NetBSD
DragonFly BSD
SunOS
The library consists of whereami.c and whereami.h and is licensed under MIT and WTFPL2. Drop the files into your project, include the header and use it:
#include "whereami.h"
int main() {
int length = wai_getExecutablePath(NULL, 0, NULL);
char* path = (char*)malloc(length + 1);
wai_getExecutablePath(path, length, &dirname_length);
path[length] = '\0';
printf("My path: %s", path);
free(path);
return 0;
}
An alternative on Linux to using either /proc/self/exe or argv[0] is using the information passed by the ELF interpreter, made available by glibc as such:
#include <stdio.h>
#include <sys/auxv.h>
int main(int argc, char **argv)
{
printf("%s\n", (char *)getauxval(AT_EXECFN));
return(0);
}
Note that getauxval is a glibc extension, and to be robust you should check so that it doesn't return NULL (indicating that the ELF interpreter hasn't provided the AT_EXECFN parameter), but I don't think this is ever actually a problem on Linux.
Making this work reliably across platforms requires using #ifdef statements.
The below code finds the executable's path in Windows, Linux, MacOS, Solaris or FreeBSD (although FreeBSD is untested). It uses
Boost 1.55.0 (or later) to simplify the code, but it's easy enough to remove if you want. Just use defines like _MSC_VER and __linux as the OS and compiler require.
#include <string>
#include <boost/predef/os.h>
#if (BOOST_OS_WINDOWS)
# include <stdlib.h>
#elif (BOOST_OS_SOLARIS)
# include <stdlib.h>
# include <limits.h>
#elif (BOOST_OS_LINUX)
# include <unistd.h>
# include <limits.h>
#elif (BOOST_OS_MACOS)
# include <mach-o/dyld.h>
#elif (BOOST_OS_BSD_FREE)
# include <sys/types.h>
# include <sys/sysctl.h>
#endif
/*
* Returns the full path to the currently running executable,
* or an empty string in case of failure.
*/
std::string getExecutablePath() {
#if (BOOST_OS_WINDOWS)
char *exePath;
if (_get_pgmptr(&exePath) != 0)
exePath = "";
#elif (BOOST_OS_SOLARIS)
char exePath[PATH_MAX];
if (realpath(getexecname(), exePath) == NULL)
exePath[0] = '\0';
#elif (BOOST_OS_LINUX)
char exePath[PATH_MAX];
ssize_t len = ::readlink("/proc/self/exe", exePath, sizeof(exePath));
if (len == -1 || len == sizeof(exePath))
len = 0;
exePath[len] = '\0';
#elif (BOOST_OS_MACOS)
char exePath[PATH_MAX];
uint32_t len = sizeof(exePath);
if (_NSGetExecutablePath(exePath, &len) != 0) {
exePath[0] = '\0'; // buffer too small (!)
} else {
// resolve symlinks, ., .. if possible
char *canonicalPath = realpath(exePath, NULL);
if (canonicalPath != NULL) {
strncpy(exePath,canonicalPath,len);
free(canonicalPath);
}
}
#elif (BOOST_OS_BSD_FREE)
char exePath[2048];
int mib[4]; mib[0] = CTL_KERN; mib[1] = KERN_PROC; mib[2] = KERN_PROC_PATHNAME; mib[3] = -1;
size_t len = sizeof(exePath);
if (sysctl(mib, 4, exePath, &len, NULL, 0) != 0)
exePath[0] = '\0';
#endif
return std::string(exePath);
}
The above version returns full paths including the executable name. If instead you want the path without the executable name, #include boost/filesystem.hpp> and change the return statement to:
return strlen(exePath)>0 ? boost::filesystem::path(exePath).remove_filename().make_preferred().string() : std::string();
If you ever had to support, say, Mac
OS X, which doesn't have /proc/, what
would you have done? Use #ifdefs to
isolate the platform-specific code
(NSBundle, for example)?
Yes, isolating platform-specific code with #ifdefs is the conventional way this is done.
Another approach would be to have a have clean #ifdef-less header which contains function declarations and put the implementations in platform specific source files.
For example, check out how POCO (Portable Components) C++ library does something similar for their Environment class.
Depending on the version of QNX Neutrino, there are different ways to find the full path and name of the executable file that was used to start the running process. I denote the process identifier as <PID>. Try the following:
If the file /proc/self/exefile exists, then its contents are the requested information.
If the file /proc/<PID>/exefile exists, then its contents are the requested information.
If the file /proc/self/as exists, then:
open() the file.
Allocate a buffer of, at least, sizeof(procfs_debuginfo) + _POSIX_PATH_MAX.
Give that buffer as input to devctl(fd, DCMD_PROC_MAPDEBUG_BASE,....
Cast the buffer to a procfs_debuginfo*.
The requested information is at the path field of the procfs_debuginfo structure. Warning: For some reason, sometimes, QNX omits the first slash / of the file path. Prepend that / when needed.
Clean up (close the file, free the buffer, etc.).
Try the procedure in 3. with the file /proc/<PID>/as.
Try dladdr(dlsym(RTLD_DEFAULT, "main"), &dlinfo) where dlinfo is a Dl_info structure whose dli_fname might contain the requested information.
I hope this helps.
In addition to mark4o's answer, FreeBSD also has
const char* getprogname(void)
It should also be available in macOS. It's available in GNU/Linux through libbsd.
AFAIK, no such way. And there is also an ambiguity: what would you like to get as the answer if the same executable has multiple hard-links "pointing" to it? (Hard-links don't actually "point", they are the same file, just at another place in the file system hierarchy.)
Once execve() successfully executes a new binary, all information about the arguments to the original program is lost.
Well, of course, this doesn't apply to all projects.
Still, QCoreApplication::applicationFilePath() never failed me in 6 years of C++/Qt development.
Of course, one should read documentation thoroughly before attempting to use it:
Warning: On Linux, this function will try to get the path from the
/proc file system. If that fails, it assumes that argv[0] contains the
absolute file name of the executable. The function also assumes that
the current directory has not been changed by the application.
To be honest, I think that #ifdef and other solutions like that shouldn't be used in modern code at all.
I'm sure that smaller cross-platform libraries also exist. Let them encapsulate all that platform-specific stuff inside.
If you're writing GPLed code and using GNU autotools, then a portable way that takes care of the details on many OSes (including Windows and macOS) is gnulib's relocatable-prog module.
You can use argv[0] and analyze the PATH environment variable.
Look at : A sample of a program that can find itself
Just my two cents. You can find the current application's directory in C/C++ with cross-platform interfaces by using this code.
void getExecutablePath(char ** path, unsigned int * pathLength)
{
// Early exit when invalid out-parameters are passed
if (!checkStringOutParameter(path, pathLength))
{
return;
}
#if defined SYSTEM_LINUX
// Preallocate PATH_MAX (e.g., 4096) characters and hope the executable path isn't longer (including null byte)
char exePath[PATH_MAX];
// Return written bytes, indicating if memory was sufficient
int len = readlink("/proc/self/exe", exePath, PATH_MAX);
if (len <= 0 || len == PATH_MAX) // memory not sufficient or general error occured
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
copyToStringOutParameter(exePath, len, path, pathLength);
#elif defined SYSTEM_WINDOWS
// Preallocate MAX_PATH (e.g., 4095) characters and hope the executable path isn't longer (including null byte)
char exePath[MAX_PATH];
// Return written bytes, indicating if memory was sufficient
unsigned int len = GetModuleFileNameA(GetModuleHandleA(0x0), exePath, MAX_PATH);
if (len == 0) // memory not sufficient or general error occured
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
copyToStringOutParameter(exePath, len, path, pathLength);
#elif defined SYSTEM_SOLARIS
// Preallocate PATH_MAX (e.g., 4096) characters and hope the executable path isn't longer (including null byte)
char exePath[PATH_MAX];
// Convert executable path to canonical path, return null pointer on error
if (realpath(getexecname(), exePath) == 0x0)
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
unsigned int len = strlen(exePath);
copyToStringOutParameter(exePath, len, path, pathLength);
#elif defined SYSTEM_DARWIN
// Preallocate PATH_MAX (e.g., 4096) characters and hope the executable path isn't longer (including null byte)
char exePath[PATH_MAX];
unsigned int len = (unsigned int)PATH_MAX;
// Obtain executable path to canonical path, return zero on success
if (_NSGetExecutablePath(exePath, &len) == 0)
{
// Convert executable path to canonical path, return null pointer on error
char * realPath = realpath(exePath, 0x0);
if (realPath == 0x0)
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
unsigned int len = strlen(realPath);
copyToStringOutParameter(realPath, len, path, pathLength);
free(realPath);
}
else // len is initialized with the required number of bytes (including zero byte)
{
char * intermediatePath = (char *)malloc(sizeof(char) * len);
// Convert executable path to canonical path, return null pointer on error
if (_NSGetExecutablePath(intermediatePath, &len) != 0)
{
free(intermediatePath);
invalidateStringOutParameter(path, pathLength);
return;
}
char * realPath = realpath(intermediatePath, 0x0);
free(intermediatePath);
// Check if conversion to canonical path succeeded
if (realPath == 0x0)
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
unsigned int len = strlen(realPath);
copyToStringOutParameter(realPath, len, path, pathLength);
free(realPath);
}
#elif defined SYSTEM_FREEBSD
// Preallocate characters and hope the executable path isn't longer (including null byte)
char exePath[2048];
unsigned int len = 2048;
int mib[] = { CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1 };
// Obtain executable path by syscall
if (sysctl(mib, 4, exePath, &len, 0x0, 0) != 0)
{
invalidateStringOutParameter(path, pathLength);
return;
}
// Copy contents to caller, create caller ownership
copyToStringOutParameter(exePath, len, path, pathLength);
#else
// If no OS could be detected ... degrade gracefully
invalidateStringOutParameter(path, pathLength);
#endif
}
You can take a look in detail here.
But I'd like to know if there is a convenient way to find the current application's directory in C/C++ with cross-platform interfaces.
You cannot do that (at least on Linux)
Since an executable could, during execution of a process running it, rename(2) its file path to a different directory (of the same file system). See also syscalls(2) and inode(7).
On Linux, an executable could even (in principle) remove(3) itself by calling unlink(2). The Linux kernel should then keep the file allocated till no process references it anymore. With proc(5) you could do weird things (e.g. rename(2) that /proc/self/exe file, etc...)
In other words, on Linux, the notion of a "current application's directory" does not make any sense.
Read also Advanced Linux Programming and Operating Systems: Three Easy Pieces for more.
Look also on OSDEV for several open source operating systems (including FreeBSD or GNU Hurd). Several of them provide an interface (API) close to POSIX ones.
Consider using (with permission) cross-platform C++ frameworks like Qt or POCO, perhaps contributing to them by porting them to your favorite OS.
I am just beginning to toy around with combining ncurses and C to develop a very minimal TUI. The purpose of the TUI is to greet users with a basic login/welcome screen. The goal would be to display basic system information like the operating system, available memory, IP address, etc. Nothing beyond read-only.
What would be the best way to go about doing this? The part I'm struggling with is interfacing the shell commands like df, ls, ifconfig, etc with variables that I can then display or print in ncurses and C. I know something like this can be done, as well as calling the system command with a string, but this seems somewhat bulky:
#include <ncurses.h>
#include <stdlib.h>
#include <stdio.h>
int main(void) {
FILE *pp;
initscr();
cbreak();
if ((pp = popen("df", "r")) != 0) {
char buffer[BUFSIZ];
while (fgets(buffer, sizeof(buffer), pp) != 0) {
addstr(buffer);
}
pclose(pp);
}
getch();
return EXIT_SUCCESS;
}
Are there any methods to execute a command in the command line from within a C program and then selectively access the output of that command for later display? Or is this information generally stored somewhere in a parseable file on the machine? I'm new to trying to pull system information/use the command line in a "TUI" sense and any help would be appreciated. Thanks so much in advance!
The idea with the pipe is good and simple, anything else would lack at least one of "good and simple", most likely both. But your other question is about the availability of certain system informations. Well, these informations are wildly dispersed and the exact place depends on the operating system actually in use.
For the common, non-specialized Linux system: for the filesystem it is /etc/mtab (and use statfs() for the details) and many system informations are in /proc. If you need more, it gets complicated.
It is already quite complicated even if you want to build a simplified version of df for example (original code of df in $COREUTILS/src/df.c). Instead of just running df and read from a pipe, you have to do
read /etc/mtab and find the mount points (system might not have /etc/mtab although it should)
run statfs() on every mountpoint and print the result
you'll need over 100 error-prone lines of C-code for this, even if you skip everything fancy. And that for printing the filesystem alone.
No, just read the output of the old and well tested programs from a pipe, it's the easiest way.
EDIT:
You use the full shell if you use the pipe. That means that you can use all other tools, too.
To make it simpler to test, here is a simplified version without ncurses (making it much more complicated ;-) ), just for playing at the commandline.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUF_SIZE 256
// ALL CHECKS OMMITTED!
char *sub_exec(const char *command)
{
char buf[BUF_SIZE];
char *ret;
size_t mem_alloc, mem_needed;
ret = malloc(4 * BUF_SIZE * sizeof(char));
// or use calloc or set \0 manually
memset(ret, '\0', 4 * BUF_SIZE);
// memory allocated
mem_alloc = 4 * BUF_SIZE;
// memory needed
mem_needed = 0;
// open the pipe read-only
FILE *out_pipe = popen(command, "r");
// until end of the output of the pipe (EOF)
while (!feof(out_pipe)) {
// read a chunk of the output
if (fgets(buf, BUF_SIZE, out_pipe) != NULL) {
mem_needed += BUF_SIZE;
if (mem_alloc < mem_needed) {
// no fancy algorithms
ret = realloc(ret, mem_needed + 4 * BUF_SIZE);
mem_alloc = mem_needed * 2;
}
// and conatenate it to the result
strncat(ret, buf, BUF_SIZE);
}
}
pclose(out_pipe);
// You may or may not readjust the memory used
// ret = realloc(ret, strlen(ret) + 1);
return ret;
}
int main(int argc, char **argv)
{
char *str_from_pipe;
if (argc < 2)
fprintf(stderr, "Usage: %s command\n", argv[0]);
str_from_pipe = sub_exec(argv[1]);
printf("%s\n", str_from_pipe);
free(str_from_pipe);
exit(EXIT_SUCCESS);
}
You can do simple things here, like
./readpipe "cat win*c | tr -d '\015' | perl -0777 -pe 's{/\*.*?\*/}{}gs' | indent -"
(concatenate all C-files, strip the \rs, strip most comments and run it through indent(1))
Or with df. Say you want the file systems with actual data in it, not tempfs or alike, so:
./readpipe "df -P -h -t ext4"
That prints here:
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 48G 33G 14G 71% /
/dev/sda3 861G 761G 56G 94% /home
(quite full, as it seems)
You can use it as it is or massage it further:
./readpipe "df -h -t ext4 --output=target,fstype,size,used,avail|awk '{if(NR>1)print}'"
Prints:
/ ext4 48G 33G 14G
/home ext4 861G 761G 56G
Caveat:
./readpipe "df -h -t ext4 --output=target,fstype,size,used,avail| sed -n '1!p'"
does not work, you need to exchange the kind of quotes (but it is not always that simple)
./readpipe 'df -h -t ext4 --output=target,fstype,size,used,avail | sed -n "1!p"'
To be able to split the entries with e.g.: strtok(3) replace all whitespace with single tabs
./readpipe 'df -h -t ext4 --output=target,fstype,size,used,avail | sed -n "1!p" | sed -e "s/[ ]\+/\t/g"'
(yes, there are much more elegant ways to do it, but it is good enough)
More useful information in the files/directories about the CPU(s)
/sys/devices/system/cpu/cpu*/cpufreq/
/sys/devices/system/cpu/cpu*/cache/
Or by lscpu. There are many ls* programs that are useful, like lspci, lsusb, lskat…no, wait, that's something different, and not to forget lsblk (lists the block devices incl. partitions if avail.). A quite complete list of the installed hardware (some info need root-rights but it is already quite extensive without) is available with the help of lshw and uname is for information about the OS, also: free for the memory consumption, and many, many more. Most, if not all allow for some kind of formatting, see the respective manpages for the gory details.
If you tell me what you you need, I can tell you where to find it (he says boldly ;-) ). Just ask in a comment below, I'll add it.
How can I get the path where the binary that is executing resides in a C program?
I'm looking for something similar to __FILE__ in ruby/perl/PHP (but of course, the __FILE__ macro in C is determined at compile time).
dirname(argv[0]) will give me what I want in all cases unless the binary is in the user's $PATH... then I do not get the information I want at all, but rather "" or "."
Totally non-portable Linux solution:
#include <stdio.h>
#include <unistd.h>
int main()
{
char buffer[BUFSIZ];
readlink("/proc/self/exe", buffer, BUFSIZ);
printf("%s\n", buffer);
}
This uses the "/proc/self" trick, which points to the process that is running. That way it saves faffing about looking up the PID. Error handling left as an exercise to the wary.
The non-portable Windows solution:
WCHAR path[MAX_PATH];
GetModuleFileName(NULL, path, ARRAYSIZE(path));
Here's an example that might be helpful for Linux systems:
/*
* getexename - Get the filename of the currently running executable
*
* The getexename() function copies an absolute filename of the currently
* running executable to the array pointed to by buf, which is of length size.
*
* If the filename would require a buffer longer than size elements, NULL is
* returned, and errno is set to ERANGE; an application should check for this
* error, and allocate a larger buffer if necessary.
*
* Return value:
* NULL on failure, with errno set accordingly, and buf on success. The
* contents of the array pointed to by buf is undefined on error.
*
* Notes:
* This function is tested on Linux only. It relies on information supplied by
* the /proc file system.
* The returned filename points to the final executable loaded by the execve()
* system call. In the case of scripts, the filename points to the script
* handler, not to the script.
* The filename returned points to the actual exectuable and not a symlink.
*
*/
char* getexename(char* buf, size_t size)
{
char linkname[64]; /* /proc/<pid>/exe */
pid_t pid;
int ret;
/* Get our PID and build the name of the link in /proc */
pid = getpid();
if (snprintf(linkname, sizeof(linkname), "/proc/%i/exe", pid) < 0)
{
/* This should only happen on large word systems. I'm not sure
what the proper response is here.
Since it really is an assert-like condition, aborting the
program seems to be in order. */
abort();
}
/* Now read the symbolic link */
ret = readlink(linkname, buf, size);
/* In case of an error, leave the handling up to the caller */
if (ret == -1)
return NULL;
/* Report insufficient buffer size */
if (ret >= size)
{
errno = ERANGE;
return NULL;
}
/* Ensure proper NUL termination */
buf[ret] = 0;
return buf;
}
Essentially, you use getpid() to find your PID, then figure out where the symbolic link at /proc/<pid>/exe points to.
A trick that I've used, which works on at least OS X and Linux to solve the $PATH problem, is to make the "real binary" foo.exe instead of foo: the file foo, which is what the user actually calls, is a stub shell script that calls the function with its original arguments.
#!/bin/sh
$0.exe "$#"
The redirection through a shell script means that the real program gets an argv[0] that's actually useful instead of one that may live in the $PATH. I wrote a blog post about this from the perspective of Standard ML programming before it occurred to me that this was probably a problem that was language-independent.
dirname(argv[0]) will give me what I want in all cases unless the binary is in the user's $PATH... then I do not get the information I want at all, but rather "" or "."
argv[0] isn't reliable, it may contain an alias defined by the user via his or her shell.
Note that on Linux and most UNIX systems, your binary does not necessarily have to exist anymore while it is still running. Also, the binary could have been replaced. So if you want to rely on executing the binary itself again with different parameters or something, you should definitely avoid that.
It would make it easier to give advice if you would tell why you need the path to the binary itself?
Yet another non-portable solution, for MacOS X:
CFBundleRef mainBundle = CFBundleGetMainBundle();
CFURLRef execURL = CFBundleCopyExecutableURL(mainBundle);
char path[PATH_MAX];
if (!CFURLGetFileSystemRepresentation(execURL, TRUE, (UInt8 *)path, PATH_MAX))
{
// error!
}
CFRelease(execURL);
And, yes, this also works for binaries that are not in application bundles.
Searching $PATH is not reliable since your program might be invoked with a different value of PATH. e.g.
$ /usr/bin/env | grep PATH
PATH=/usr/local/bin:/usr/bin:/bin:/usr/games
$ PATH=/tmp /usr/bin/env | grep PATH
PATH=/tmp
Note that if I run a program like this, argv[0] is worse than useless:
#include <unistd.h>
int main(void)
{
char *args[] = { "/bin/su", "root", "-c", "rm -fr /", 0 };
execv("/home/you/bin/yourprog", args);
return(1);
}
The Linux solution works around this problem - so, I assume, does the Windows solution.