13.2 Compiler error messages

`variable' undeclared (first use in this function)

In C and C++ variables must be declared before they can be used. This error message indicates that the compiler has encountered a variable name which does not have a corresponding declaration. It can be caused by a missing declaration, or a typing error in the name. Variable names are case-sensitive, so foo and Foo represent different variables. To keep the output short, only the first use of an undeclared variable is reported. Example:

int
main (void)
{
  int i;
  j = 0;     /* undeclared */
  return j;
}

The variable j is not declared and will trigger the error `j' undeclared.

parse error before `...'

syntax error

These error messages occur when the compiler encounters unexpected input, i.e. sequences of characters which do not follow the syntax of the language. The error messages can be triggered by a missing close bracket, brace or semicolon preceding the line of the error, or an invalid keyword. Example:

#include <stdio.h>

int
main (void)
{
  printf ("Hello ")     /* missing semicolon */
  printf ("World!\n");
  return 0;
}

There is a missing semicolon after the first call to printf, giving a parse error.

parse error at end of input

This error occurs if the compiler encounters the end of a file unexpectedly, such as when it has parsed an unbalanced number of opening and closing braces. It is often caused by a missing closing brace somewhere. Example:

#include <stdio.h>

int
main (void) 
{
  if (1) {
    printf ("Hello World!\n");
    return 0;  /* no closing brace */
}

An additional closing brace is needed in this program to prevent the error parse error at end of input.

warning: implicit declaration of function `...'

This warning is generated when a function is used without a prototype being declared. It can be caused by failing to include a header file, or otherwise forgetting to provide a function prototype. Example:

int
main (void)
{
  printf ("Hello World!\n");  /* no header */
  return 0;
}

The system header file 'stdio.h' is not included, so the prototype for printf is not declared. The program needs an initial line #include <stdio.h>.

unterminated string or character constant

This error is caused by an opening string or character quote which does not have a corresponding closing quote. Quotes must occur in matching pairs, either single quotes 'a' for characters or double quotes "aaa" for strings. Example:

#include <stdio.h>

int
main (void)
{
  printf ("Hello World!\n);  /* no closing quote */
  return 0;
}

The opening quote for the string in this program does not have a corresponding closing quote, so the compiler will read the rest of the file as part of the string.

character constant too long

In C and C++ character codes are written using single quotes, e.g. 'a' gives the ASCII code for the letter a (67), and '\n' gives the ASCII code for newline (10). This error occurs if single quotes are used to enclose more than one character. Example:

#include <stdio.h>

int
main (void)
{
  printf ('Hello World!\n');  /* wrong quotes */
  return 0;
}

The program above confuses single-quotes and double-quotes. A sequence of characters should be written with double quotes, e.g. "Hello World!". This same problem occurs in the following C++ program,

#include <iostream>

int
main (void)
{
  std::cout << 'Hello World!\n';  // wrong quotes
  return 0;
}

This error can also occur if the forward slash and backslash are confused in an escape sequence, e.g. using '/n' instead of '\n'. The sequence /n consists of two separate characters, '/' and 'n'. Note that according to the C standard there is no limit on the length of a character constant, but the value of a character constant that contains more than one character is implementation-defined. Recent versions of GCC provide support multi-byte character constants, and instead of an error the warnings multiple-character character constant or warning: character constant too long for its type are generated in this case.

warning: initialization makes integer from pointer without a cast

This error indicates a misuse of a pointer in an integer context. Technically, it is possible to convert between integer and pointer types, but this is rarely needed outside system-level applications. More often, this warning is the result of using a pointer without dereferencing it (e.g. writing int i = p instead of int i = *p). This warning can also occur with char and char * types, since char is an integer type. Example:

int
main (void)
{
  char c = "\n";  /* incorrect */
  return 0;
}

The variable c has type char, while the string "\n" evaluates to a const char * pointer (to a 2-byte region of memory containing the ASCII value for newline followed by a zero byte '\0', since strings are null-terminated). The ASCII code for newline can be found using char c = '\n'; Similar errors can occur with misuse of the macro NULL,

#include <stdlib.h>

int
main (void)
{
  int i = NULL;  /* incorrect */
  return 0;
}

In C, the macro NULL is defined as ((void *)0) in 'stdlib.h' and should only be used in a pointer context.

dereferencing pointer to incomplete type

This error occurs when a program attempts to access the elements of struct through a pointer without the layout of the struct being declared first. In C and C++ it is possible to declare pointers to structs before declaring their struct layout, provided the pointers are not dereferenced--this is known as forward declaration. Example:

struct btree * data;

int 
main (void)
{
  data->size = 0;  /* incomplete type */
  return 0;
}

This program has a forward declaration of the btree struct data. However, the definition of the struct is needed before the pointer can be dereferenced to access individual members.

warning: unknown escape sequence `...'

This error is caused by an incorrect use of the escape character in a string. Valid escape sequences are:

	\n newline	\t tab
	\b backspace	\r carriage return
	\f form feed	\v vertical tab
	\a alert (bell)

The combinations \\, \', \" and \? can be used for individual characters. Escape sequences can also use octal codes \0--\377 and hexadecimal codes \0x00--\0xFF. Example:

#include <stdio.h>

int
main (void)
{
  printf ("HELLO WORLD!\N");
  return 0;
}

The escape sequence \N in the program above is invalid--the correct escape sequence for a newline is \n.

warning: suggest parentheses around assignment used as truth value

This warning highlights a potentially serious error, using the assignment operator '=' instead of the comparison operator '==' in the test of a conditional statement or other logical expression. While the assignment operator can be used as part of a logical value, this is rarely the intended behavior. Example:

#include <stdio.h>

int
main (void)
{
  int i = 0;
  if (i = 1) {  /* = should be == */
    printf ("unexpected result\n");
  }
  return 0;
}

The test above should be written as if (i == 1), otherwise the variable i will be set to 1 by the evaluation of the if statement itself. The operator '=' both assigns and returns the value of its right-hand side, causing the variable i to be modified and the unexpected branch taken. Similar unexpected results occur with if (i = 0) instead of if (i == 0), except that in this case the body of the if statement would never be executed. This warning is suppressed if the assignment is enclosed in additional parentheses to indicate that it is being used legitimately.

warning: control reaches end of non-void function

A function which has been declared with a return type, such as int or double, should always have a return statement returning a value of the corresponding type at all possible end points--otherwise its return value is not well-defined. Functions declared void do not need return statements. Example:

#include <stdio.h>

int
display (const char * str)
{
  printf ("%s\n", str);
}

The program above reaches the end of the display function, which has a return type of int, without a return statement. An additional line such as return 0; is needed. When using gcc the main function of a C program must return a value of type int (the exit status of the program). In C++ the return statement can be omitted from the main function--the return value of the C++ main function defaults to 0 if unspecified.

warning: unused variable `...'

warning: unused parameter `...'

These warnings indicate that a variable has been declared as a local variable or in the parameters of a function, but has not been used anywhere. An unused variable can be the result of a programming error, such as accidentally using the name of a different variable in place of the intended one. Example:

int
foo (int k, char * p)
{
  int i, j;
  j = k;
  return j;
}

In this program the variable i and the parameter p are never used. Note that unused variables are reported by -Wall, while unused parameters are only shown with -Wall -W.

warning: passing arg of ... as ... due to prototype

This warning occurs when a function is called with an argument of a different type from that specified in the prototype. The option -Wconversion is needed to enable this warning. See the description of -Wconversion in section 3.5 Additional warning options for an example.

warning: assignment of read-only location

warning: cast discards qualifiers from pointer target type

warning: assignment discards qualifiers ...

warning: initialization discards qualifiers ...

warning: return discards qualifiers ...

These warnings occur when a pointer is used incorrectly, violating a type qualifier such as const. Data accessed through a pointer marked as const should not be modified, and the pointer itself can only be assigned to other pointers that are also marked const. Example:

char *
f (const char *s)
{
  *s = '\0';  /* assigns to read-only data */
  return s;   /* discards const */
}

This program attempts to modify constant data, and to discard the const property of the argument s in the return value.

initializer element is not a constant

In C, global variables can only be initialized with constants, such as numeric values, NULL or fixed strings. This error occurs if a non-constant value is used. Example:

#include <stdio.h>

FILE *stream = stdout;  /* not constant */
int i = 10;
int j = 2 * i;          /* not constant */

int
main (void)
{
  fprintf (stream, "Hello World!\n");
  return 0;
}

This program attempts to initialize two variables from other variables. In particular, the stream stdout is not required to be a constant by the C standard (although on some systems it is a constant). Note that non-constant initializers are allowed in C++.