Table of Contents

1 Overview

The function runcap runs an external command, and waits for its termination, optionally capturing its standard output and standard error streams, and piping data to its standard input.

Upon return from the function, the caller can obtain the termination status of the program and access the captured output data.

2 Usage

The function runcap is defined as follows:

int runcap(struct runcap *rc, int flags);

The rc argument points to the structure that controls the execution of the program. It contains the input and output members. The only member of this structure that must be initialized on input is rc_argv, which points to the array of pointers to null-terminated strings that represent the command name and the argument list to that program. Initialization of the rest of input members is optional. For each input member, there is a corresponding flag in flags which must be set, if the member is initialized. For example, if the rc_timeout member is set (indicating maximum time the program execution is allowed to take), then the RCF_TIMEOUT flag must be set.

Upon return, the function returns the execution status of the program, and initializes the output members of rc to hold the standard output and standard error streams captured from the command. Special functions are provided to read these streams.

For a detailed description of runcap and accompanying functions, please see the runcap(3) manual. In this chapter we will illustrate the usage of the runcap library by examples.

The example below defines the function runcom with the following prototype:

int runcom(char *cmd, char *in, char **out, char **err);

The function runs the command line given in the cmd argument using /bin/sh and returns the data it printed on its standard output and error streams in the memory locations pointed to by the arguments out and err, correspondingly. If any of these arguments is NULL, capturing of the corresponding stream will be disabled.

The function returns program exit status on success, -1 if the program terminated on signal and -2 on error. This example implements only rudimentary error handling, in order to minimize the amount of irrelevant code.

int runcom(char *cmd, char *in, char **out, char **err)
    int status;
    char *p;
    char c;
    char *argv[] = { "/bin/sh", "-c", cmd, NULL };
    struct runcap rc;
    int rcflags = RCF_TIMEOUT;

    /* Declare the command line to be run.  The rc_argv filed is the
     * only field that must be initialized on input.
    rc.rc_argv = argv;

    /* Set maximum execution timeout.  The presense of this setting is
     * indicated by the RCF_TIMEOUT flag in rcflags. 
    rc.rc_timeout = 10;

    /* If the input string is supplied, initialize the input stream and
     * raise the RCF_STDIN flag to indicate that it has been initialized.
    if (in) {
        rc.rc_cap[RUNCAP_STDIN].sc_base = in;
        rc.rc_cap[RUNCAP_STDIN].sc_size = strlen(in);
        rc.rc_cap[RUNCAP_STDIN].sc_fd = -1;
        rcflags |= RCF_STDIN;

    /* If out argument is NULL, disable capturing program's stdout.  To
     * disable capturing a stream, it suffices to initialize its sc_size
     * field to zero and raise the corresponding RCF_*_SIZE bit in flags.
    if (!out) {
        rc.rc_cap[RUNCAP_STDOUT].sc_size = 0;
        rcflags |= RCF_STDOUT_SIZE;

    /* Same for the stderr: */
    if (!err) {
        rc.rc_cap[RUNCAP_STDERR].sc_size = 0;
        rcflags |= RCF_STDERR_SIZE;

    /* Run the command.  The runcap function returns 0 on success.  On
     * error, it returns -1 and sets the errno variable.  Its value is 
     * also duplicated in the rc_errno member of struct runcap.
    if (runcap(&rc, rcflags)) {
        return -2;

    /* Upon return, the sc_leng member of the capturing structure for 
     * stdout and stderr contains total amount of bytes in the corresponding
     * stream. The stream can be read using the runcap_getc and 
     *  runcap_getline functions.
    if (rc.rc_cap[RUNCAP_STDOUT].sc_leng) {
        p = malloc(rc.rc_cap[RUNCAP_STDOUT].sc_leng + 1);
        assert(p != NULL);
        *out = p;
        while (runcap_getc(&rc, RUNCAP_STDOUT, &c))
            *p++ = c;
        *p = 0;
    } else
        *out = NULL;

    if (rc.rc_cap[RUNCAP_STDERR].sc_leng) {
        p = malloc(rc.rc_cap[RUNCAP_STDERR].sc_leng + 1);
        assert(p != NULL);
        *err = p;
        while (runcap_getc(&rc, RUNCAP_STDERR, &c))
            *p++ = c;
        *p = 0;
    } else
        *err = NULL;

    /* Analyze the exit status of the command */
    if (WIFEXITED(rc.rc_status)) {
        status = WEXITSTATUS(rc.rc_status);
    } else {
        status = -1;

        if (WIFSIGNALED(rc.rc_status)) {
            fprintf(stderr, "%s terminated on signal %d\n",
                    argv[0], WTERMSIG(rc.rc_status));
        } else {
            fprintf(stderr, "%s terminated with unrecognized status: %d\n",
                    argv[0], rc.rc_status);
    return status;

3 Downloading

To clone the project from the repository, run

git clone git://

4 Building

The project can be used either a standalone library, or as a shared or static convenience library embedded in another project. If you cloned the project from the git repository, you will need to bootstrap it first. To do so, change to the runcap directory and run

autoreconf -I. -f -i -s

Use the RUNCAP_BUILD environment variable to indicate the type of the build you need. Allowed values are:

Build standalone installable library (default).
Build shared convenience library.
Build static convenience library.

Once bootstrapped, the project can be built with the usual sequence of commands:

  1. Configure the package
  2. Build it
  3. If building installable library, install it (normally run as root).
    make install

    This will install the files and libruncap.a to the system library directory, and the header file runcap.h to the system include directory.

5 Incorporating as a submodule

To incorporate runcap to your project as a submodule, follow these steps:

  1. Change to your project's toplevel directory.
  2. Clone the project.
    git submodule add git://
    git submodule init
  3. Add it to git index:
    git add runcap
  4. Add it to your toplevel
    ACLOCAL_AMFLAGS = -I runcap
    SUBDIRS = runcap
  5. Edit your Add the following line:
  6. Add the following to the file which builds the target that uses on the runcap library:

6 RUNCAP_SETUP autoconf macro

The RUNCAP_SETUP macro initializes the runcap library. It should be used in the file or in one of the files included to it. The macro invocation syntax is:


Both arguments are optional:

Name of the subdirectory where the runcap sources reside. If omitted, runcap is assumed. When building runcap as a standalone library, it is set to . (a dot).
Build type: install, shared, or static. Defaults to static.

This macro defines the following make variables:

cpp options to access the runcap.h include file. Use it in the convenient _CPPFLAGS make variable.
Lists the pathname of the runcap library. Use it in the LDADD or prog_LDADD make variable.
Type of the build.

7 Copyright

Copyright (C) 2017-2019 Sergey Poznyakoff

Permission is granted to anyone to make or distribute verbatim copies of this document as received, in any medium, provided that the copyright notice and this permission notice are preserved, thus giving the recipient permission to redistribute in turn.

Permission is granted to distribute modified versions of this document, or of portions of it, under the above conditions, provided also that they carry prominent notices stating who last changed them.

Date: 2019-08-14T17:21+0300

Author: Sergey Poznyakoff

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