CODEWITHSUNDEEP
c
Roopesh Majeti
Roopesh Majeti

Reputation: 554

Mechanism for returning a string from C main() program

I know that C program generally ends with return, where we return the status of the program. However, I want to return a string. The reason is that, I will be calling the C-executable from a shell script and printing the returned string. Is there any mechanism for the same ?

Upvotes: 4

Views: 4248

Answers (7)

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215517

You have to print the string to stdout and use the shell's command substitution to read the result:

"`command`"

or:

"$(command)"

Either way, you should use the enclosing double-quotes as above. Note that this method has a potentially-serious problem that it removes all training newlines from the command's output. If you need to preserve the exact output, try:

output="$(command ; echo x)"
output="${output%x}"

Since OP mentioned that this is for a password, another piece of advice: once you've read a password into a shell variable, never pass it on the command line to other programs. Command lines are usually world-readable. Instead of something like

wget "http://$user:$pass@host/foo"

try something like

wget -i - << EOF
http://$user:$pass@host/foo
EOF

Here-files as in this example are useful with a number of programs which need to take passwords from scripts.

Upvotes: 0

winwaed
winwaed

Reputation: 7829

You cannot - you can only return an integer.

Remember that C was developed alongside Unix. Unix programs return an integer value which is intended as a status code. If a program was to return string(s) it would write them to stdout, and then the user (or a script) would pipe it into something useful.

MS DOS also had number-only status values.

Upvotes: 0

Ed Swangren
Ed Swangren

Reputation: 124760

You can't, but you don't need to either. You return a stat code from main, but you can always re-direct the output and capture it in your shell script.

Upvotes: 1

cdhowie
cdhowie

Reputation: 169318

Just output the string you want to return to standard output with printf. Then in your script, do something like this:

SOMESTRING="`./yourprogram`"

The backticks will capture the output of the program, which will be the string you printed.

Upvotes: 4

icecrime
icecrime

Reputation: 76815

There is not way to return a string from main(). Maybe the program itself should print the string ?

Upvotes: 5

peoro
peoro

Reputation: 26060

You cannot.

The best thing you could do is writing the string somewhere (on standard output, standard error or some file); then the shellscript will get your string from there.

Standard output (thus using just a printf) is probably the best solution, since it will be very easy from your C program print the string, and very easy for the shellscript getting that data:

From shell script:

STRING="$( ./your_program argv1 argv2 )"

Upvotes: 5

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799240

There is no such mechanism; the return code is expected to be a byte. If you want to output a string from your program then use something like printf() and command substitution in the shell script to capture it.

Upvotes: 10

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