Types of Errors in C

Errors in C programming are discrepancies that can cause a program to malfunction, leading to failures in compiling, halting execution, or generating incorrect results. There are five primary error types: Syntax, Runtime, Logical, Semantic, and Linker. For instance, using 'print' instead of 'printf' for displaying today's date results in an error, as the compiler fails to recognize 'print'. This issue illustrates how errors can disrupt a program's expected behavior, whether by yielding wrong outputs, stopping execution, or impeding compilation. Addressing these errors through debugging is essential for ensuring program accuracy and functionality.

How to Read an Error in C?

In order to resolve an error, we must figure out how and why did the error occur. Whenever we encounter an error in our code, the compiler stops the code compilation if it is a syntax error or it either stops the program's execution or generates a garbage value if it is a run time error.

Syntax errors are easy to figure out because the compiler highlights the line of code that caused the error. Generally, we can find the error's root cause on the highlighted line or above the highlighted line.

For example:

Output:

As we can see, the compiler shows an error on line 4 of the code. So, in order to figure out the problem, we will go through line 4 and a few lines above it. Once we do that, we can quickly determine that we are missing a semi-colon (;) in line 4. The compiler also suggested the same thing.

Other than the syntax errors, run time errors are often encountered while coding. These errors are the ones that occur while the code is being executed.

Let us now see an example of a run time error:

Output:

As we can see, the compiler generated a warning at line 6 because we are dividing a number by zero.

Sometimes, the compiler does not throw a run time error. Instead, it returns a garbage value. In situations like these, we have to figure out why did we get an incorrect output by comparing the output with the expected output. In other cases, the compiler does not display any error at all. The program execution just ends abruptly in cases like these.

Let us take another example to understand this kind of run time error:

Output:

In the above code, we are trying to access the 10000th element but the size of array is only 1 therefore there is no space allocated to the 10000th element, this is known as segmentation fault.

Types of Errors in C

There are five different types of errors in C.

1. Syntax Error
2. Run Time Error
3. Logical Error
4. Semantic Error
5. Linker Error

1. Syntax Error

Syntax errors occur when a programmer makes mistakes in typing the code's syntax correctly or makes typos. In other words, syntax errors occur when a programmer does not follow the set of rules defined for the syntax of C language.

Syntax errors are sometimes also called compilation errors because they are always detected by the compiler. Generally, these errors can be easily identified and rectified by programmers.

The most commonly occurring syntax errors in C language are:

• Missing semi-colon (;)
• Missing parenthesis ({})
• Assigning value to a variable without declaring it

Let us take an example to understand syntax errors:

Output:

If the user assigns any value to a variable without defining the data type of the variable, the compiler throws a syntax error.

Let's see another example:

Output:

A for loop needs 3 arguments to run. Since we entered only one argument, the compiler threw a syntax error.

2. Runtime Error

Errors that occur during the execution (or running) of a program are called RunTime Errors. These errors occur after the program has been compiled successfully. When a program is running, and it is not able to perform any particular operation, it means that we have encountered a run time error. For example, while a certain program is running, if it encounters the square root of -1 in the code, the program will not be able to generate an output because calculating the square root of -1 is not possible. Hence, the program will produce an error.

Runtime errors can be a little tricky to identify because the compiler can not detect these errors. They can only be identified once the program is running. Some of the most common run time errors are: number not divisible by zero, array index out of bounds, string index out of bounds, etc.

Runtime errors can occur because of various reasons. Some of the reasons are:

1. Mistakes in the Code: Let us say during the execution of a while loop, the programmer forgets to enter a break statement. This will lead the program to run infinite times, hence resulting in a run time error.
2. Memory Leaks: If a programmer creates an array in the heap but forgets to delete the array's data, the program might start leaking memory, resulting in a run time error.
3. Mathematically Incorrect Operations: Dividing a number by zero, or calculating the square root of -1 will also result in a run time error.
4. Undefined Variables: If a programmer forgets to define a variable in the code, the program will generate a run time error.

Example 1:

Output:

**In some compilers, you may also see this output: **

In the above example, we used a for loop to calculate the square root of six integers. But because we also tried calculating the square root of two negative numbers, the program generated two errors (the IND written above stands for "Indeterminate"). These errors are the run time errors. -nan is similar to IND.

Example 2:

Output:

This is an integer overflow error. The maximum value an integer can hold in C is 2147483647. Since in the above example, we assigned 2147483649 to the variable var, the variable overflows, and we get -2147483647 as the output (because of the circular property).

3. Logical Error

Sometimes, we do not get the output we expected after the compilation and execution of a program. Even though the code seems error free, the output generated is different from the expected one. These types of errors are called Logical Errors. Logical errors are those errors in which we think that our code is correct, the code compiles without any error and gives no error while it is running, but the output we get is different from the output we expected.

In 1999, NASA lost a spacecraft due to a logical error. This happened because of some miscalculations between the English and the American Units. The software was coded to work for one system but was used with the other.

For Example:

Output:

INF signifies a division by zero error. In the above example, at line 8, we wanted to check whether the variable b was equal to zero. But instead of using the equal to comparison operator (==), we use the assignment operator (=). Because of this, the if statement became false and the value of b became 0. Finally, the else clause got executed.

4. Semantic Error

Errors that occur because the compiler is unable to understand the written code are called Semantic Errors. A semantic error will be generated if the code makes no sense to the compiler, even though it is syntactically correct. It is like using the wrong word in the wrong place in the English language. For example, adding a string to an integer will generate a semantic error.

Semantic errors are different from syntax errors, as syntax errors signify that the structure of a program is incorrect without considering its meaning. On the other hand, semantic errors signify the incorrect implementation of a program by considering the meaning of the program.

The most commonly occurring semantic errors are: use of un-initialized variables, type compatibility, and array index out of bounds.

Example 1:

Output:

When we have an expression on the left-hand side of an assignment operator (=), the program generates a semantic error. Even though the code is syntactically correct, the compiler does not understand the code.

Example 2:

Output:

In the above example, we printed six elements while the array arr only had five. Because we tried to access the sixth element of the array, we got a semantic error and hence, the program generated a garbage value.

5. Linker Error

Linker is a program that takes the object files generated by the compiler and combines them into a single executable file. Linker errors are the errors encountered when the executable file of the code can not be generated even though the code gets compiled successfully. This error is generated when a different object file is unable to link with the main object file. We can run into a linked error if we have imported an incorrect header file in the code, we have a wrong function declaration, etc.

For Example:

Output:

In the above code, as we wrote Main() instead of main(), the program generated a linker error. This happens because every file in the C language must have a main() function. As in the above program, we did not have a main() function, the program was unable to run the code, and we got an error. This is one of the most common type of linker error.