GCC X86-64 Calling Conventions Explained

The x86-64 architecture on Linux follows the System V AMD64 ABI calling convention. Understanding this is essential when writing assembly, optimizing code, debugging, or interfacing with C libraries.

Register usage for arguments

Integer and pointer arguments use these registers in order:

RDI — first argument
RSI — second argument
RDX — third argument
RCX — fourth argument
R8 — fifth argument
R9 — sixth argument

Floating-point arguments use XMM registers:

XMM0 through XMM7 — floating-point arguments

If a function takes both integer and floating-point arguments, they occupy their respective registers independently. The first integer argument still uses RDI even if floating-point arguments are present.

Additional arguments beyond the sixth integer or eighth floating-point argument are passed on the stack, in right-to-left order (the caller pushes them).

Return values

RAX — return value (also RDX:RAX for 128-bit returns)
XMM0 and XMM1 — floating-point return values

System calls

System calls follow a slightly different convention. The syscall number goes in RAX, and arguments use the same registers as normal calls except R10 is used instead of RCX for the fourth argument. This avoids clobbering RCX, which the syscall instruction uses internally.

Caller and callee saved registers

Caller-saved (volatile): RAX, RCX, RDX, RSI, RDI, R8, R9, R10, R11, and XMM0-XMM15
Callee-saved (non-volatile): RBX, RSP, RBP, R12, R13, R14, R15

The callee must preserve these registers if it uses them. The caller cannot assume them to survive a function call.

Stack alignment and red zone

The stack must be 16-byte aligned before a call instruction. Since call pushes an 8-byte return address, the stack is 8 bytes misaligned at function entry. Most code compensates by immediately pushing another 8-byte value (like rbp) to restore alignment.

The 128 bytes below RSP form a “red zone” that is safe from signal handlers. Functions can use this space for temporary data without allocating stack space, though signal handlers will overwrite it.

Practical example

#include <stdio.h>

long add_five(long a, long b, long c, long d, long e, long f, long g) {
    return a + b + c + d + e + f + g;
}

int main() {
    long result = add_five(1, 2, 3, 4, 5, 6, 7);
    printf("%ld\n", result);
    return 0;
}

In this function:

a through f arrive in RDI, RSI, RDX, RCX, R8, R9
g (the seventh argument) is passed on the stack
The result is returned in RAX

Examine the calling convention in practice with objdump -d or gdb to verify how arguments are passed.

Variadic functions

Functions with variable arguments (like printf) are more complex. They must track how many floating-point arguments were passed in XMM registers via RAX. The System V ABI specifies that RAX should contain the number of vector registers used for floating-point arguments (0-8). This allows variadic functions to know whether to retrieve arguments from registers or the stack.

References

2026 Comprehensive Guide: Best Practices

This extended guide covers GCC x86-64 Calling Conventions Explained with advanced techniques and troubleshooting tips for 2026. Following modern best practices ensures reliable, maintainable, and secure systems.

Advanced Implementation Strategies

For complex deployments, consider these approaches: Infrastructure as Code for reproducible environments, container-based isolation for dependency management, and CI/CD pipelines for automated testing and deployment. Always document your custom configurations and maintain separate development, staging, and production environments.

Security and Hardening

Security is foundational to all system administration. Implement layered defense: network segmentation, host-based firewalls, intrusion detection, and regular security audits. Use SSH key-based authentication instead of passwords. Encrypt sensitive data at rest and in transit. Follow the principle of least privilege for access controls.

Performance Optimization

Monitor resources continuously with tools like top, htop, iotop
Profile application performance before and after optimizations
Use caching strategically: application caches, database query caching, CDN for static assets
Optimize database queries with proper indexing and query analysis
Implement connection pooling for network services

Troubleshooting Methodology

Follow a systematic approach to debugging: reproduce the issue, isolate variables, check logs, test fixes. Keep detailed logs and document solutions found. For intermittent issues, add monitoring and alerting. Use verbose modes and debug flags when needed.

Related Tools and Utilities

These tools complement the techniques covered in this article:

System monitoring: htop, vmstat, iostat, dstat for resource tracking
Network analysis: tcpdump, wireshark, netstat, ss for connectivity debugging
Log management: journalctl, tail, less for log analysis
File operations: find, locate, fd, tree for efficient searching
Package management: dnf, apt, rpm, zypper for package operations

Integration with Modern Workflows

Modern operations emphasize automation, observability, and version control. Use orchestration tools like Ansible, Terraform, or Kubernetes for infrastructure. Implement centralized logging and metrics. Maintain comprehensive documentation for all systems and processes.

Quick Reference Summary

This comprehensive guide provides extended knowledge for GCC x86-64 Calling Conventions Explained. For specialized requirements, refer to official documentation. Practice in test environments before production deployment. Keep backups of critical configurations and data.