I have written a C++ program and complied it to produce a.out file. However, whenever I try to run it, I get Permission Denied. I read that we can use sudo, but I can't quite get it to work. I use something like, sudo "./a.out" but that too doesn't work.
Edit:
Here is the message I get when I try "./a.out".
bash: ./a.out: Permission denied
Usually, g++ gives the created file execute permissions. If you do not pass the -o option, the file will be named a.out.
Two possible reasons why your file does not have the execute bit set, with their solutions:
The umask value is set to a value like 0133, thereby preventing the execute bit from being set. Solution: set the permissions explicitly:
chmod 755 a.out
fmask=0022 or umask=0022 (omitting fmask). See the Mount options for fat section on the manual page of mount for more details.For bash scripts which do not have the execute bit set, you could run bash file.sh. Such a feature exists for all files with executable content (compiled files and files with a shebang line #!/path/to/interpreter set). To execute files without the execute bit set, use the special file /lib/ld-linux.so.2 (or /lib/ld-linux-x86-64.so.2 for 64-bit applications) to run such a program:
/lib/ld-linux-x86-64.so.2 a.out
Just copy the folder to your home folder and it will work. You are probably trying to run it on an external drive or something.
.out is an unusual extension. Usually this would normally signify a "trace output" file.
Check your syntax that you are using to compile
e.g.
gcc myfile.c /usr/lib/libsomelibrary.a -o outputfilename
or maybe
g++ myfile.cpp -lm -o outputfilename
You can should examine to see if the executable bit is set on the file
ls -l a.out
or you can just force the executable bit
chmod +x a.out
then you can run your file
./a.out
or simply
a.out
You should also perhaps check that the output file has been written correctly as a binary
i.e.
file a.out
This will report what format the file is - either a script or a binary
You rarely need to execute as root unless you have restricted who should be able to run the executable.
If you have compiled as root (e.g. sudo make), or have a Makefile that installed the executable as root then can I suggest you regain the permission as the user logged in
i.e.
sudo chown fred:fred a.out
i.e. replace "fred" with your user id.
on Zorin/Ubuntu I get this..
$ g++ --std=c++17 -Werror -Wall main.cpp -o main.out
$ sudo chmod +x main.out
$ ./main.out
bash: ./main.out: Permission denied
I had to move my whole REPO from the USB drive to the SSD.
On USB, I could not get it to be executable. Now works without the chmod.
The workaround for FAT-filesystems in the first answer
"This could be the case if you're putting files on a FAT32-formatted flash drive. Solution: (...) mount the drive with fmask=0022 or umask=0022 (omitting fmask)."
normally does not work - the default for umask is mostly 0022 anyway, so this does not change anything.
Another mount parameter's default, however, effectively disallows execution of binaries, especially if the FAT-filesystem is mounted as non-root-user: noexec
So just mount FAT-formatted drives with the option exec like so:
sudo mount -o exec /dev/sd.. /mountpoint
(this must normally be done as root, hence the "sudo") and you should be able to execute binaries directly from there.
I'd wager that your program does not have a 'main()' function, as if it did, your compiler would have made a.out executable. Right now it is just an object file full of code, but there's no entry point. main() is a special function name in C and C++ that tells the compiler to create a program rather than just object files which can be linked to a program or library.
I'd be interested to know what command line you used to produce this file, as GNU GCC's c++ compiler, g++, will not let me create a simple program w/o a main function:
#include <iostream>
using namespace std;
void no_main()
{
cout << "Hello World" << endl;
}
$ g++ hello.cc
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 0 has invalid symbol index 11
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 1 has invalid symbol index 12
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 2 has invalid symbol index 2
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 3 has invalid symbol index 2
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 4 has invalid symbol index 11
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 5 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 6 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 7 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 8 has invalid symbol index 2
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 9 has invalid symbol index 2
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 10 has invalid symbol index 12
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 11 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 12 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 13 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 14 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 15 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 16 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 17 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 18 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 19 has invalid symbol index 13
/usr/bin/ld.bfd.real: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 20 has invalid symbol index 21
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/../../../crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status
However if I make change 'void no_main' to 'int main' it works:
$ g++ hello.cc
$ ./a.out
Hello World
