asprintf - One True asprintf, vasprintf! 🛠️

Robust, portable implementation of asprintf(), vasprintf(). Thoroughly tested.

Highlights

• 🚀 Fast - performance akin other implementations
• 📦 Simple - zero dependencies, lightweight (37 lines, 566 bytes) and ISO C99 compatible.
• 🔒 Robust - safety-first, with substantial unit testing.
• ⚙️ Compatible - interop across different systems, drop-in asprintf, vasprintf replacement for glibc, *BSD libc, musl libc and many more.

Motivation

Wanted to have a version of asprintf that is simple, robust and just works everywhere. Needed it to be backward-compatible with existing libc libraries, consistent across diverse platforms and systems. It should also focus on correctness, minimize unexpected behavior, be thoroughly tested, be easy to read and stick to both C99 and POSIX.1-2024.

This process also gave me the opportunity to review existing implementations and deepen my understanding of pointers and systems programming.

Getting Started

Important

asprintf() and vasprintf() are now included in POSIX.1-2024, following Austin Group Bug #1496. You can read the latest version at posix-asprintf. For systems supporting POSIX.1-2024 or later, Neved4/asprintf is no longer necessary. For systems limited to POSIX.1-2017 or under, it offers a practical solution.

Prerequisites

If you are building asprintf, ensure you have:

• A C compiler that supports C99

Installing

If you have clib installed, run:

$ clib install Neved4/asprintf

If you don't have the above, start by cloning the repository:

$ git clone https://github.com/Neved4/asprintf

Once you've cloned the repository, build by executing:

$ make asprintf

Alternatively, if you have zig:

$ zig cc asprintf.c -t asprintf

Usage

// asprintf, vasprintf - print to allocated string

#include "asprintf.h"

int asprintf(char **strp, const char *fmt, ...);

int vasprintf(char **strp, const char *fmt, va_list ap);

Examples

Consider a getconf() function to retrieve a config specified path, that supports both XDG_CONFIG_HOME and fallbacks:

char *getconf() {
    const char *file = "tz.conf", *home = getenv("HOME"),
        *xdg_config_home = getenv("XDG_CONFIG_HOME");
    char *config = NULL;

    // Path building logic

    return config;
}

After which we'll have to lay down our path building logic.

Before asprintf

if (access("tz.conf", F_OK) != -1) {
    config = strdup("tz.conf");
} else if (xdg_config_home) {
    size_t len = strlen(xdg_config_home) + strlen("twc") + strlen(file) + 3;
    config = (char *)malloc(len);
    if (config != NULL) {
        snprintf(config, len, "%s/%s/%s", xdg_config_home, "twc", file);
    }
} else if (home) {
    size_t len = strlen(home) + strlen(".config/twc") + strlen(file) + 3;
    config = (char *)malloc(len);
    if (config != NULL) {
        snprintf(config, len, "%s/%s/%s", home, ".config/twc", file);
    }
}

After asprintf

if (access("tz.conf", F_OK) != -1) {
    config = strdup("tz.conf");
} else if (xdg_config_home) {
    asprintf(&config, "%s/%s/%s", xdg_config_home, "twc", file);
} else if (home) {
    asprintf(&config, "%s/%s/%s", home, ".config/twc", file);
}

Testing

To run all the tests against asprintf execute the following command:

cc test.c asprintf.c -o test && ./test

To use your system's default asprintf(3), run this instead:

cc test.c -o test && ./test

To link it against other asprintf implementations, run:

cc test.c /path/to/asprintf.c -o test && ./test

Any of the above will output something like:

 Ok "Memory allocation"
 Ok "Empty string as input"
 Ok "String formatting variations"
 Ok "Special characters in format string"
Err "Boundary cases for integers"

// More tests

-----------
Passing: 4
 Failed: 1
  Total: 5

Docker

To compile the binary inside a Docker image, run:

docker build .

Compatibility

Runs on Linux, macOS and *BSD systems on both x86_64 and arm64. Builds with clang, gcc, tcc, zig and any other compiler that supports C99 or later.

Should be compatible with glibc, GLib, FreeBSD libc, musl libc asprintf.

Standards

asprintf is compatible with both POSIX.1-2024,¹ and C99.²

License

asprintf is licensed under the terms of the MIT License.

See the LICENSE file for details.

See Also

Reference

• ISO/IEC 9899
• ISO/IEC DIS 9945
• ISO/IEC TS 17961:2013
• MISRA C:2012
• SEI CERT C .pdf

System implementations

Source	Docs
Debian asprintf	manpages.debian.org/asprintf
FreeBSD asprintf	man.freebsd.org/asprintf
OpenBSD asprintf	man.openbsd.org/asprintf
NetBSD asprintf	man.netbsd.org/asprintf.3
Solaris asprintf	docs.oracle.com/asprintf-3c
z/OS asprintf	ibm.com/docs/zos/asprintf
sortix asprintf

Other libc implementations

• cosmopolitan/asprintf.c
• dietlibc
• glibc/asprintf.c
• musl/asprintf.c
• newlib/asprintf.c
• picolibc/asprintf.c
• uclibc/asprintf.c

Other asprintf implementations

• akharrou/ft_asprintf
• andrelmbackman/ft_printf
• d4wae89d498/ft_asprintf
• dragonblocks/asprintf
• eiszapfen2000/asprintf
• finwo/c-asprintf
• jkaivo/asprintf
• jwerle/asprintf.c
• libressl/portable
• lordmulder/asprintf-msvc
• n-nino/vc6-asprintf
• naev/naev/asprintf
• tap3edit/mp/mpasprintf
• timbaker/asprintf
• weiss/c99-snprintf

Further reading

• Chris Wellons: How to Write Portable C Without Complicating Your Build
• Fedora Defensive Coding Guide: The C Programming Language
• FreeBSD Developers' Handbook: Chapter 3. Secure Programming
• GNU Autoconf: Portability of C Functions
• GNU Autoconf: Portable C and C++ Programming
• Oracle Developer's Guide to Solaris: Security Considerations When Using C Functions

Footnotes

1. IEEE Std 1003.1-2024: Standard for Information Technology — Portable Operating System Interface (POSIX®),
   ISO/IEC DIS 9945. URL: https://pubs.opengroup.org/onlinepubs/9799919799/ ↩

2. ISO/IEC 9899: Standard for Information Technology — Programming languages — C, ISO/IEC 9899:2023.
   URL: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf ↩