C questions in placement tests and university exams are rarely about syntax alone. Most of what you will find here is output prediction, pointer arithmetic, arrays and strings, structures and unions, storage classes, recursion, and file handling. The explanations walk through memory behaviour step by step, which is usually where the confusion sits rather than in the code itself.
In a structure with flexible array members, which statement is INCORRECT? struct FlexArray { int len; int arr[]; };
Answer: B
Flexible array members require dynamic allocation; the structure itself cannot be allocated on the stack with a defined size for the array.
Q.122Hard
What is the output of the following? struct S { int a:3; int b:3; int c:3; }; printf("%zu", sizeof(struct S));
Answer: D
Bit field packing is implementation-defined. Size depends on compiler's bit field allocation strategy.
Q.123Hard
In competitive programming, when should you use union instead of struct?
Answer: B
Unions are optimal for memory-constrained scenarios and hardware programming where type casting is needed.
Q.124Hard
What will be the output of this code? struct X { int a; float b; }; struct Y { struct X x; int c; }; printf("%zu", sizeof(struct Y));
Answer: C
Nested struct X: int(4) + float(4) = 8 bytes. Y: struct X(8) + padding(0) + int(4) = 12, but with overall alignment it becomes 16 bytes.
Q.125Hard
In a structure, what does the #pragma pack(1) directive do?
Answer: A
#pragma pack(1) removes padding, making members occupy consecutive memory locations for compact storage.
Q.126Hard
If a structure contains a flexible array member, where must it be declared?
Answer: B
A flexible array member must be the last member of a structure. This is a C99 feature used for dynamic-sized arrays.
Q.127Hard
In competitive programming, when implementing a graph using adjacency lists, which data structure is most suitable?
Answer: B
An array of structures, where each structure contains a linked list (using self-referential pointers), is ideal for adjacency list representation.
Q.128Hard
If you have a structure with bit fields, which statement about their behavior is true?
Answer: A
Bit fields save memory by storing multiple values in single bytes, but behavior varies by compiler/platform, and you cannot take addresses of bit field members.
Q.129Hard
What is the output of this complex code? struct S { char c; int i; } s; printf("%lu", sizeof(s));
Answer: B
char(1 byte) + padding(3 bytes) + int(4 bytes) = 8 bytes due to alignment requirements.
Q.130Hard
In competitive programming, when you need a structure that can hold either an integer or a floating-point number (but not both simultaneously) in the most memory-efficient way, which should you use?
Answer: B
Union is memory-efficient as members share space. Only one can be accessed at a time, which suits the requirement.
Q.131Hard
What will be the output of the following code? union Test { int a; char b; }; union Test t; t.a = 65; printf("%d %c", t.a, t.b);
Answer: D
Since union members share memory, the output depends on the system's endianness. On little-endian systems, t.b would be 65 (or 'A'), but on big-endian systems, the result differs.
Q.132Hard
In competitive programming for GATE/ISRO 2025, which scenario would union be preferable over structure?
Answer: B
Unions are preferred when you have mutually exclusive data (only one member is used at a time) and memory is critical. A classic example is storing different data types in a tagged union pattern used in compilers and interpreters.
Q.133Hard
Write a code snippet to read 100 bytes from a file. Which is correct?
Answer: A
fread() syntax is fread(ptr, size, count, stream). To read 100 bytes, size=1 and count=100. Option B reads 100 items of 1 byte each, which is equivalent but less clear.
Q.134Hard
Which of the following correctly implements appending to a file?
Answer: A
Mode "a" opens a file for appending. New data is written at the end of the file. Mode "w" truncates the file, "r+" requires file to exist, "ab+" is appending in binary with both read/write.
Q.135Hard
In a program reading a binary file of 1000 integers, fread() is called as: fread(arr, sizeof(int), 1000, fp). If only 500 integers are present, what will fread() return?
Answer: A
fread() returns the number of complete items successfully read, not the number of bytes. It will return 500 even if 1000 were requested.
Q.136Hard
Consider using fseek() on a file opened in text mode with SEEK_END and a non-zero offset. What is the standard behavior?
Answer: B
In text mode, fseek() with non-zero offset relative to SEEK_END is undefined behavior. Use fseek(fp, 0, SEEK_END) for reliable behavior.
Q.137Hard
A program writes data using fprintf() and later attempts to read it back. However, the read operation fails intermittently. What could be the most likely cause?
Answer: B
fprintf() buffers output. Without flushing (fflush() or fclose()), subsequent read operations may see incomplete data. This is a common source of bugs.
Q.138Hard
What is the purpose of using fflush() in file handling, and when is it critical to use it?
Answer: B
fflush() writes any buffered data to the underlying file. It's critical when you need to ensure data is written to disk before continuing, especially before reading the same file.
Q.139Hard
A program writes binary data using fwrite() but reads it back with fprintf(). What will happen?
Answer: B
Mixing write (fwrite - binary) and read (fprintf - formatted text) functions on same data causes mismatch. Binary data won't have format specifiers; misinterpretation results.
Q.140Hard
A program uses fgetc() to read 100,000 characters from a file sequentially. Which alternative would be more efficient?
Answer: B
fread() with buffer reads multiple bytes per call, reducing function call overhead. fgetc() makes 100,000 separate calls. fgets() is limited to reading up to newline.
