Sun Studio 12: C User's Guide

4.6 lint Reference and Examples

This section provides reference information on lint, including checks performed by lint, lint libraries, and lint filters.

4.6.1 Diagnostics Performed by lint

lint-specific diagnostics are issued for three broad categories of conditions: inconsistent use, nonportable code, and questionable constructs. In this section, we review examples of lint’s behavior in each of these areas, and suggest possible responses to the issues they raise. Consistency Checks

Inconsistent use of variables, arguments, and functions is checked within files as well as across them. Generally speaking, the same checks are performed for prototype uses, declarations, and parameters as lint checks for old-style functions. If your program does not use function prototypes, lint checks the number and types of parameters in each call to a function more strictly than the compiler. lint also identifies mismatches of conversion specifications and arguments in [fs]printf() and [fs]scanf() control strings.

Examples: Portability Checks

Some nonportable code is flagged by lint in its default behavior, and a few more cases are diagnosed when lint is invoked with -p or -Xc. The latter causes lint to check for constructs that do not conform to the ISO C standard. For the messages issued under -p and -Xc, see 4.6.2 lint Libraries.


char c;
c = getchar();
if (c == EOF) ...

where EOF has the value -1, always fails on machines where character variables take on nonnegative values. lint invoked with -p checks all comparisons that imply a plain char may have a negative value. However, declaring c as a signed char in the above example eliminates the diagnostic, not the problem. That’s because getchar() must return all possible characters and a distinct EOF value, so a char cannot store its value. We cite this example, perhaps the most common one arising from implementation-defined sign-extension, to show how a thoughtful application of lint’s portability option can help you discover bugs not related to portability. In any case, declare c as an int.

short s;
long l;
s = l;

lint flags all such assignments by default; the diagnostic can be suppressed by invoking the -a option. Bear in mind that you may be suppressing other diagnostics when you invoke lint with this or any other option. Check the list in 4.6.2 lint Libraries for the options that suppress more than one diagnostic.

int *fun(y)
char *y;
    return(int *)y;

because, on most machines, an int cannot start on an arbitrary byte boundary, whereas a char can. You can suppress the diagnostic by invoking lint with -h, although, again, you may be disabling other messages. Better still, eliminate the problem by using the generic pointer void *.

int a[10];
    int i = 1;
    a[i++] = i;

In this example, the value of a[1] may be 1 if one compiler is used, 2 if another. The bitwise logical operator & can give rise to this diagnostic when it is mistakenly used in place of the logical operator &&:

if ((c = getchar()) != EOF & c != ’0’) Questionable Constructs

lint flags a miscellany of legal constructs that may not represent what the programmer intended. Examples:

unsigned x;
if (x < 0) ...

always fails. The test:

unsigned x;
if (x > 0) ...

is equivalent to:

if (x != 0) ...

This may not be the intended action. lint flags questionable comparisons of unsigned variables with negative constants or 0. To compare an unsigned variable to the bit pattern of a negative number, cast it to unsigned:

if (u == (unsigned) -1) ...

Or use the U suffix:

if (u == -1U) ...

int fun()
    int a, b, x, y;
    (a = x) && (b == y);

if (x & a == 0) ...

is evaluated as:

if (x & (a == 0)) ...

which is most likely not what you intended. Invoking lint with -h disables the diagnostic.

4.6.2 lint Libraries

You can use lint libraries to check your program for compatibility with the library functions you have called in it—the declaration of the function return type, the number and types of arguments the function expects, and so on. The standard lint libraries correspond to libraries supplied by the C compilation system, and generally are stored in a standard place on your system. By convention, lint libraries have names of the form llib-lx.ln.

The lint standard C library, llib-lc.ln, is appended to the lint command line by default; checks for compatibility with it can be suppressed by invoking the -n option. Other lint libraries are accessed as arguments to -l. That is:

% lint -lx file1.c file2.c

directs lint to check the usage of functions and variables in file1.c and file2.c for compatibility with the lint library llib-lx.ln. The library file, which consists only of definitions, is processed exactly as are ordinary source files and ordinary .ln files, except that functions and variables used inconsistently in the library file, or defined in the library file but not used in the source files, elicit no complaints.

To create your own lint library, insert the directive NOTE(LINTLIBRARY) at the head of a C source file, then invoke lint for that file with the -o option and the library name given to -l:

% lint -ox file1.c file2.c

causes only definitions in the source files headed by NOTE(LINTLIBRARY) to be written to the file llib-lx.ln. (Note the analogy of lint -o to cc -o.) A library can be created from a file of function prototype declarations in the same way, except that both NOTE(LINTLIBRARY) and NOTE(PROTOLIB(n))must be inserted at the head of the declarations file. If n is 1, prototype declarations are written to a library .ln file just as are old-style definitions. If n is 0, the default, the process is cancelled. Invoking lint with -y is another way of creating a lint library. The command line:

% lint -y -ox file1.c file2.c

causes each source file named on that line to be treated as if it begins with NOTE(LINTLIBRARY), and only its definitions to be written to llib-lx.ln.

By default, lint searches for lint libraries in the standard place. To direct lint to search for a lint library in a directory other than the standard place, specify the path of the directory with the -L option:

% lint -Ldir -lx file1.c file2.c

In enhanced mode, lint produces .ln files which store additional information than .ln files produced in basic mode. In enhanced mode, lint can read and understand all .ln files generated by either basic or enhanced lint modes. In basic mode, lint can read and understand .ln files generated only using basic lint mode.

By default, lint uses libraries from the /usr/lib directory. These libraries are in the basic lint format. You can run a makefile once, and create enhanced lint libraries in a new format, which will enable enhanced lint to work more effectively. To run the makefile and create the new libraries, enter the command:

% cd /opt/SUNWspro/prod/src/lintlib; make

where /opt/SUNWspro/prod is the installation directory. After the makefile is run, lint uses the new libraries in enhanced mode, instead of the libraries in the /usr/lib directory.

The specified directory is searched before the standard place.

4.6.3 lint Filters

A lint filter is a project-specific post-processor that typically uses an awk script or similar program to read the output of lint and discard messages that your project has deemed as not identifying real problems—string functions, for instance, returning values that are sometimes or always ignored. lint filters generate customized diagnostic reports when lint options and directives do not provide sufficient control over output.

Two options to lint are particularly useful in developing a filter: