<!--
Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.

SPDX-License-Identifier: curl
-->

# curl C code style

Source code that has a common style is easier to read than code that uses
different styles in different places. It helps making the code feel like one
single code base. Easy-to-read is an important property of code and helps
making it easier to review when new things are added and it helps debugging
code when developers are trying to figure out why things go wrong. A unified
style is more important than individual contributors having their own personal
tastes satisfied.

Our C code has a few style rules. Most of them are verified and upheld by the
`scripts/checksrc.pl` script. Invoked with `make checksrc` or even by default
by the build system when built after `./configure --enable-debug` has been
used.

It is normally not a problem for anyone to follow the guidelines, copy
the style already used in the source code and there are no particularly
unusual rules in our set of rules.

We also work hard on writing code that are warning-free on all the major
platforms and in general on as many platforms as possible. Code that causes
warnings is not accepted as-is.

## Readability

A primary characteristic for code is readability. The intent and meaning of
the code should be visible to the reader. Being clear and unambiguous beats
being clever and saving two lines of code. Write simple code. You and others
who come back to this code over the coming decades want to be able to quickly
understand it when debugging.

## Naming

Try using a non-confusing naming scheme for your new functions and variable
names. It does not necessarily have to mean that you should use the same as in
other places of the code, only that the names should be logical,
understandable and be named according to what they are used for. File-local
functions should be made static. We like lower case names.

See the [INTERNALS](https://curl.se/dev/internals.html#symbols) document on
how we name non-exported library-global symbols.

## Indenting

We use only spaces for indentation, never TABs. We use two spaces for each new
open brace.

```c
if(something_is_true) {
  while(second_statement == fine) {
    moo();
  }
}
```

## Comments

Since we write C89 code, **//** comments are not allowed. They were not
introduced in the C standard until C99. We use only __/* comments */__.

```c
/* this is a comment */
```

## Long lines

Source code in curl may never be wider than 79 columns and there are two
reasons for maintaining this even in the modern era of large and high
resolution screens:

1. Narrower columns are easier to read than wide ones. There is a reason
   newspapers have used columns for decades or centuries.

2. Narrower columns allow developers to easier show multiple pieces of code
   next to each other in different windows. It allows two or three source
   code windows next to each other on the same screen - as well as multiple
   terminal and debugging windows.

## Braces

In if/while/do/for expressions, we write the open brace on the same line as
the keyword and we then set the closing brace on the same indentation level as
the initial keyword. Like this:

```c
if(age < 40) {
  /* clearly a youngster */
}
```

You may omit the braces if they would contain only a one-line statement:

```c
if(!x)
  continue;
```

For functions the opening brace should be on a separate line:

```c
int main(int argc, char **argv)
{
  return 1;
}
```

## 'else' on the following line

When adding an **else** clause to a conditional expression using braces, we
add it on a new line after the closing brace. Like this:

```c
if(age < 40) {
  /* clearly a youngster */
}
else {
  /* probably grumpy */
}
```

## No space before parentheses

When writing expressions using if/while/do/for, there shall be no space
between the keyword and the open parenthesis. Like this:

```c
while(1) {
  /* loop forever */
}
```

## Use boolean conditions

Rather than test a conditional value such as a bool against TRUE or FALSE, a
pointer against NULL or != NULL and an int against zero or not zero in
if/while conditions we prefer:

```c
result = do_something();
if(!result) {
  /* something went wrong */
  return result;
}
```

## No assignments in conditions

To increase readability and reduce complexity of conditionals, we avoid
assigning variables within if/while conditions. We frown upon this style:

```c
if((ptr = malloc(100)) == NULL)
  return NULL;
```

and instead we encourage the above version to be spelled out more clearly:

```c
ptr = malloc(100);
if(!ptr)
  return NULL;
```

## New block on a new line

We never write multiple statements on the same source line, even for short
if() conditions.

```c
if(a)
  return TRUE;
else if(b)
  return FALSE;
```

and NEVER:

```c
if(a) return TRUE;
else if(b) return FALSE;
```

## Space around operators

Please use spaces on both sides of operators in C expressions. Postfix **(),
[], ->, ., ++, --** and Unary **+, -, !, ~, &** operators excluded they should
have no space.

Examples:

```c
bla = func();
who = name[0];
age += 1;
true = !false;
size += -2 + 3 * (a + b);
ptr->member = a++;
struct.field = b--;
ptr = &address;
contents = *pointer;
complement = ~bits;
empty = (!*string) ? TRUE : FALSE;
```

## No parentheses for return values

We use the 'return' statement without extra parentheses around the value:

```c
int works(void)
{
  return TRUE;
}
```

## Parentheses for sizeof arguments

When using the sizeof operator in code, we prefer it to be written with
parentheses around its argument:

```c
int size = sizeof(int);
```

## Column alignment

Some statements cannot be completed on a single line because the line would be
too long, the statement too hard to read, or due to other style guidelines
above. In such a case the statement spans multiple lines.

If a continuation line is part of an expression or sub-expression then you
should align on the appropriate column so that it is easy to tell what part of
the statement it is. Operators should not start continuation lines. In other
cases follow the 2-space indent guideline. Here are some examples from
libcurl:

```c
if(Curl_pipeline_wanted(handle->multi, CURLPIPE_HTTP1) &&
   (handle->set.httpversion != CURL_HTTP_VERSION_1_0) &&
   (handle->set.httpreq == HTTPREQ_GET ||
    handle->set.httpreq == HTTPREQ_HEAD))
  /* did not ask for HTTP/1.0 and a GET or HEAD */
  return TRUE;
```

If no parenthesis, use the default indent:

```c
data->set.http_disable_hostname_check_before_authentication =
  va_arg(param, long) ? TRUE : FALSE;
```

Function invoke with an open parenthesis:

```c
if(option) {
  result = parse_login_details(option, strlen(option),
                               (userp ? &user : NULL),
                               (passwdp ? &passwd : NULL),
                               NULL);
}
```

Align with the "current open" parenthesis:

```c
DEBUGF(infof(data, "Curl_pp_readresp_ %d bytes of trailing "
             "server response left\n",
             (int)clipamount));
```

## Platform dependent code

Use **#ifdef HAVE_FEATURE** to do conditional code. We avoid checking for
particular operating systems or hardware in the #ifdef lines. The HAVE_FEATURE
shall be generated by the configure script for Unix-like systems and they are
hard-coded in the `config-[system].h` files for the others.

We also encourage use of macros/functions that possibly are empty or defined
to constants when libcurl is built without that feature, to make the code
seamless. Like this example where the **magic()** function works differently
depending on a build-time conditional:

```c
#ifdef HAVE_MAGIC
void magic(int a)
{
  return a + 2;
}
#else
#define magic(x) 1
#endif

int content = magic(3);
```

## No typedefed structs

Use structs by all means, but do not typedef them. Use the `struct name` way
of identifying them:

```c
struct something {
   void *valid;
   size_t way_to_write;
};
struct something instance;
```

**Not okay**:

```c
typedef struct {
   void *wrong;
   size_t way_to_write;
} something;
something instance;
```

## Banned functions

To avoid footguns and unintended consequences we forbid the use of a number of
C functions. The `checksrc` script finds and yells about them if used. This
makes us write better code.

This is the full list of functions generally banned.

    _access
    _fstati64
    _lseeki64
    _mbscat
    _mbsncat
    _open
    _tcscat
    _tcsdup
    _tcsncat
    _tcsncpy
    _waccess
    _wcscat
    _wcsdup
    _wcsncat
    _wfopen
    _wfreopen
    _wopen
    accept
    accept4
    access
    aprintf
    atoi
    atol
    calloc
    close
    CreateFile
    CreateFileA
    CreateFileW
    fclose
    fdopen
    fopen
    fprintf
    free
    freeaddrinfo
    freopen
    fstat
    getaddrinfo
    gets
    gmtime
    llseek
    LoadLibrary
    LoadLibraryA
    LoadLibraryEx
    LoadLibraryExA
    LoadLibraryExW
    LoadLibraryW
    localtime
    lseek
    malloc
    mbstowcs
    MoveFileEx
    MoveFileExA
    MoveFileExW
    msnprintf
    mvsnprintf
    open
    printf
    realloc
    recv
    rename
    send
    snprintf
    socket
    socketpair
    sprintf
    sscanf
    stat
    strcat
    strcpy
    strdup
    strerror
    strncat
    strncpy
    strtok
    strtok_r
    strtol
    strtoul
    vaprintf
    vfprintf
    vprintf
    vsnprintf
    vsprintf
    wcscpy
    wcsdup
    wcsncpy
    wcstombs
    WSASocket
    WSASocketA
    WSASocketW
