Memory Checking with Valgrind#

Valgrind can find memory errors and memory leaks in C++ programs. It is most useful when a program compiles but crashes, behaves unpredictably, or uses dynamic memory.

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Compile first#

Compile with debugging information:

g++ -ggdb $CXXFLAGS main.cpp -o main

If the program has multiple source files:

g++ -ggdb $CXXFLAGS main.cpp helper.cpp -o main

Run Valgrind#

Run the program under Valgrind:

valgrind ./main

For a more detailed memory-leak report:

valgrind --leak-check=full --show-leak-kinds=all ./main

If your program uses command-line arguments, put them after the program name:

valgrind --leak-check=full ./main input.txt

What to look for#

Read the first error first. Later messages may be caused by the first problem.

Common Valgrind messages include:

Message	Meaning
`Invalid read`	The program read memory it should not read.
`Invalid write`	The program wrote memory it should not write.
`Use of uninitialised value`	The program used a value before assigning to it.
`definitely lost`	The program leaked memory.

For beginning programs, a clean run usually reports no memory errors and no definitely lost memory.

Good habits#

Fix compiler warnings before using Valgrind.
Use GDB first if the program crashes immediately and you need to find where.
Use Valgrind when the problem involves memory, pointers, arrays, or dynamic allocation.
Re-run Valgrind after each fix.

Valgrind output can be long. Save the command you ran and the first error message if you need help.